diff --git a/clang/include/clang/AST/ASTLambda.h b/clang/include/clang/AST/ASTLambda.h
--- a/clang/include/clang/AST/ASTLambda.h
+++ b/clang/include/clang/AST/ASTLambda.h
@@ -35,6 +35,17 @@
   return isLambdaCallOperator(cast<CXXMethodDecl>(DC));
 }
 
+inline bool isLambdaCallWithExplicitObjectParameter(const DeclContext *DC) {
+  return isLambdaCallOperator(DC) &&
+         cast<CXXMethodDecl>(DC)->isExplicitObjectMemberFunction();
+}
+
+inline bool isLambdaCallWithImplicitObjectParameter(const DeclContext *DC) {
+  return isLambdaCallOperator(DC) &&
+         !cast<CXXMethodDecl>(DC)->getType().isNull() &&
+         !cast<CXXMethodDecl>(DC)->isExplicitObjectMemberFunction();
+}
+
 inline bool isGenericLambdaCallOperatorSpecialization(const CXXMethodDecl *MD) {
   if (!MD) return false;
   const CXXRecordDecl *LambdaClass = MD->getParent();
diff --git a/clang/include/clang/AST/Decl.h b/clang/include/clang/AST/Decl.h
--- a/clang/include/clang/AST/Decl.h
+++ b/clang/include/clang/AST/Decl.h
@@ -1720,6 +1720,7 @@
 
 /// Represents a parameter to a function.
 class ParmVarDecl : public VarDecl {
+
 public:
   enum { MaxFunctionScopeDepth = 255 };
   enum { MaxFunctionScopeIndex = 255 };
@@ -1807,6 +1808,18 @@
     ParmVarDeclBits.IsKNRPromoted = promoted;
   }
 
+  bool isExplicitObjectParameter() const {
+    return ExplicitObjectParameterIntroducerLoc.isValid();
+  }
+
+  void setExplicitObjectParameterLoc(SourceLocation Loc) {
+    ExplicitObjectParameterIntroducerLoc = Loc;
+  }
+
+  SourceLocation getExplicitObjectParamThisLoc() const {
+    return ExplicitObjectParameterIntroducerLoc;
+  }
+
   Expr *getDefaultArg();
   const Expr *getDefaultArg() const {
     return const_cast<ParmVarDecl *>(this)->getDefaultArg();
@@ -1873,7 +1886,10 @@
   static bool classofKind(Kind K) { return K == ParmVar; }
 
 private:
+  friend class ASTDeclReader;
+
   enum { ParameterIndexSentinel = (1 << NumParameterIndexBits) - 1 };
+  SourceLocation ExplicitObjectParameterIntroducerLoc;
 
   void setParameterIndex(unsigned parameterIndex) {
     if (parameterIndex >= ParameterIndexSentinel) {
@@ -2638,6 +2654,13 @@
   /// parameters have default arguments (in C++).
   unsigned getMinRequiredArguments() const;
 
+  /// Returns the minimum number of non-object arguments needed to call this
+  /// function. This produces the same value as getMinRequiredArguments except
+  /// it does not count the explicit object argument, if any.
+  unsigned getMinRequiredExplicitArguments() const;
+
+  bool hasCXXExplicitFunctionObjectParameter() const;
+
   /// Determine whether this function has a single parameter, or multiple
   /// parameters where all but the first have default arguments.
   ///
diff --git a/clang/include/clang/AST/DeclCXX.h b/clang/include/clang/AST/DeclCXX.h
--- a/clang/include/clang/AST/DeclCXX.h
+++ b/clang/include/clang/AST/DeclCXX.h
@@ -2060,6 +2060,14 @@
 
   bool isStatic() const;
   bool isInstance() const { return !isStatic(); }
+  bool isExplicitObjectMemberFunction() const;
+  bool isImplicitObjectMemberFunction() const;
+  const ParmVarDecl *getNonObjectParameter(unsigned I) const {
+    return getParamDecl(isExplicitObjectMemberFunction() ? I + 1 : I);
+  }
+  ParmVarDecl *getNonObjectParameter(unsigned I) {
+    return getParamDecl(isExplicitObjectMemberFunction() ? I + 1 : I);
+  }
 
   /// Returns true if the given operator is implicitly static in a record
   /// context.
@@ -2169,14 +2177,19 @@
   /// Return the type of the object pointed by \c this.
   ///
   /// See getThisType() for usage restriction.
-  QualType getThisObjectType() const;
+
+  QualType getFunctionObjectParameterReferenceType() const;
+  QualType getFunctionObjectParameterType() const {
+    return getFunctionObjectParameterReferenceType().getNonReferenceType();
+  }
+
+  unsigned getNumExplicitParams() const {
+    return getNumParams() - (isExplicitObjectMemberFunction() ? 1 : 0);
+  }
 
   static QualType getThisType(const FunctionProtoType *FPT,
                               const CXXRecordDecl *Decl);
 
-  static QualType getThisObjectType(const FunctionProtoType *FPT,
-                                    const CXXRecordDecl *Decl);
-
   Qualifiers getMethodQualifiers() const {
     return getType()->castAs<FunctionProtoType>()->getMethodQuals();
   }
diff --git a/clang/include/clang/AST/Expr.h b/clang/include/clang/AST/Expr.h
--- a/clang/include/clang/AST/Expr.h
+++ b/clang/include/clang/AST/Expr.h
@@ -1449,6 +1449,16 @@
     DeclRefExprBits.IsImmediateEscalating = Set;
   }
 
+  bool isCapturedByCopyInLambdaWithExplicitObjectParameter() const {
+    return DeclRefExprBits.CapturedByCopyInLambdaWithExplicitObjectParameter;
+  }
+
+  void setCapturedByCopyInLambdaWithExplicitObjectParameter(
+      bool Set, const ASTContext &Context) {
+    DeclRefExprBits.CapturedByCopyInLambdaWithExplicitObjectParameter = Set;
+    setDependence(computeDependence(this, Context));
+  }
+
   static bool classof(const Stmt *T) {
     return T->getStmtClass() == DeclRefExprClass;
   }
diff --git a/clang/include/clang/AST/Stmt.h b/clang/include/clang/AST/Stmt.h
--- a/clang/include/clang/AST/Stmt.h
+++ b/clang/include/clang/AST/Stmt.h
@@ -383,6 +383,7 @@
     unsigned HasFoundDecl : 1;
     unsigned HadMultipleCandidates : 1;
     unsigned RefersToEnclosingVariableOrCapture : 1;
+    unsigned CapturedByCopyInLambdaWithExplicitObjectParameter : 1;
     unsigned NonOdrUseReason : 2;
     unsigned IsImmediateEscalating : 1;
 
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -4640,12 +4640,14 @@
 // Note that we don't treat templates differently for this diagnostic.
 def note_ovl_candidate_arity : Note<"candidate "
     "%sub{select_ovl_candidate_kind}0,1,2 not viable: "
-    "requires%select{ at least| at most|}3 %4 argument%s4, but %5 "
+    "requires%select{ at least| at most|}3 %4 "
+    "%select{|non-object }6argument%s4, but %5 "
     "%plural{1:was|:were}5 provided">;
 
 def note_ovl_candidate_arity_one : Note<"candidate "
     "%sub{select_ovl_candidate_kind}0,1,2 not viable: "
     "%select{requires at least|allows at most single|requires single}3 "
+    "%select{|non-object }6"
     "argument %4, but %plural{0:no|:%5}5 arguments were provided">;
 
 def note_ovl_candidate_deleted : Note<
@@ -4828,7 +4830,7 @@
   "call to deleted function call operator in type %0">;
 def note_ovl_surrogate_cand : Note<"conversion candidate of type %0">;
 def err_member_call_without_object : Error<
-  "call to non-static member function without an object argument">;
+  "call to %select{non-static|explicit}0 member function without an object argument">;
 
 // C++ Address of Overloaded Function
 def err_addr_ovl_no_viable : Error<
@@ -7238,12 +7240,14 @@
   "add parentheses around left hand side expression to silence this warning">;
 
 def err_invalid_this_use : Error<
-  "invalid use of 'this' outside of a non-static member function">;
+  "invalid use of 'this' %select{outside of a non-static member function"
+  "|in a function with an explicit object parameter}0">;
 def err_this_static_member_func : Error<
   "'this' cannot be%select{| implicitly}0 used in a static member function "
   "declaration">;
-def err_invalid_member_use_in_static_method : Error<
-  "invalid use of member %0 in static member function">;
+def err_invalid_member_use_in_method : Error<
+  "invalid use of member %0 in %select{static|explicit object}1 member function">;
+
 def err_invalid_qualified_function_type : Error<
   "%select{non-member function|static member function|deduction guide}0 "
   "%select{of type %2 |}1cannot have '%3' qualifier">;
@@ -7253,6 +7257,27 @@
 def err_qualified_function_typeid : Error<
   "type operand %0 of 'typeid' cannot have '%1' qualifier">;
 
+def err_cxx20_deducing_this  : Error<
+  "explicit object parameters are incompatible with C++ standards before C++2b">;
+def err_explicit_object_default_arg: Error<
+  "an explicit object parameter cannot have a default argument">;
+def err_explicit_object_parameter_pack: Error<
+  "an explicit object parameter cannot be a function parameter pack">;
+def err_explicit_object_parameter_must_be_first: Error<
+  "an explicit object parameter can only appear as the first parameter "
+  "of the %select{function|lambda}0">;
+def err_explicit_object_parameter_nonmember: Error<
+  "an explicit object parameter cannot appear in a "
+  "%select{static|virtual|non-member}0 %select{function|lambda}1">;
+def err_explicit_object_parameter_constructor: Error<
+  "an explicit object parameter cannot appear in a %select{constructor|destructor}0">;
+def err_explicit_object_parameter_qualifiers: Error<
+  "a %select{function|lambda}0 with an explicit object parameter cannot "
+  "%select{be const|be mutable|be reference-qualified|have qualifiers}1">;
+def err_invalid_explicit_object_type_in_lambda: Error<
+  "invalid explicit object parameter type %0 in lambda with capture; "
+  "the type must be the same as, or derived from, the lambda">;
+
 def err_ref_qualifier_overload : Error<
   "cannot overload a member function %select{without a ref-qualifier|with "
   "ref-qualifier '&'|with ref-qualifier '&&'}0 with a member function %select{"
@@ -8474,53 +8499,65 @@
 def err_call_incomplete_argument : Error<
   "argument type %0 is incomplete">;
 def err_typecheck_call_too_few_args : Error<
-  "too few %select{|||execution configuration }0arguments to "
+  "too few %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
   "expected %1, have %2">;
 def err_typecheck_call_too_few_args_one : Error<
-  "too few %select{|||execution configuration }0arguments to "
+  "too few %select{|||execution configuration }0"
+  "%select{|non-object }2arguments to "
   "%select{function|block|method|kernel function}0 call, "
   "single argument %1 was not specified">;
 def err_typecheck_call_too_few_args_at_least : Error<
-  "too few %select{|||execution configuration }0arguments to "
+  "too few %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
   "expected at least %1, have %2">;
 def err_typecheck_call_too_few_args_at_least_one : Error<
-  "too few %select{|||execution configuration }0arguments to "
+  "too few %select{|||execution configuration }0"
+  "%select{|non-object }2arguments to "
   "%select{function|block|method|kernel function}0 call, "
   "at least argument %1 must be specified">;
 def err_typecheck_call_too_few_args_suggest : Error<
-  "too few %select{|||execution configuration }0arguments to "
+  "too few %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
-  "expected %1, have %2; did you mean %3?">;
+  "expected %1, have %2; did you mean %4?">;
 def err_typecheck_call_too_few_args_at_least_suggest : Error<
-  "too few %select{|||execution configuration }0arguments to "
+  "too few %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
-  "expected at least %1, have %2; did you mean %3?">;
+  "expected at least %1, have %2; did you mean %4?">;
 def err_typecheck_call_too_many_args : Error<
-  "too many %select{|||execution configuration }0arguments to "
+  "too many %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
   "expected %1, have %2">;
 def err_typecheck_call_too_many_args_one : Error<
-  "too many %select{|||execution configuration }0arguments to "
+  "too many %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
   "expected single argument %1, have %2 arguments">;
 def err_typecheck_call_too_many_args_at_most : Error<
-  "too many %select{|||execution configuration }0arguments to "
+  "too many %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
   "expected at most %1, have %2">;
 def err_typecheck_call_too_many_args_at_most_one : Error<
   "too many %select{|||execution configuration }0arguments to "
   "%select{function|block|method|kernel function}0 call, "
-  "expected at most single argument %1, have %2 arguments">;
+  "expected at most single %select{|non-object }3argument %1, "
+  "have %2%select{|non-object}3 arguments">;
 def err_typecheck_call_too_many_args_suggest : Error<
-  "too many %select{|||execution configuration }0arguments to "
+  "too many %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
-  "expected %1, have %2; did you mean %3?">;
+  "expected %1, have %2; did you mean %4?">;
 def err_typecheck_call_too_many_args_at_most_suggest : Error<
-  "too many %select{|||execution configuration }0arguments to "
+  "too many %select{|||execution configuration }0"
+  "%select{|non-object }3arguments to "
   "%select{function|block|method|kernel function}0 call, "
-  "expected at most %1, have %2; did you mean %3?">;
+  "expected at most %1, have %2; did you mean %4?">;
 
 def err_arc_typecheck_convert_incompatible_pointer : Error<
   "incompatible pointer types passing retainable parameter of type %0"
@@ -9338,10 +9375,10 @@
 
 // C++11 defaulted functions
 def err_defaulted_special_member_params : Error<
-  "an explicitly-defaulted %select{|copy |move }0constructor cannot "
+  "an explicitly-defaulted %sub{select_special_member_kind}0 cannot "
   "have default arguments">;
 def err_defaulted_special_member_variadic : Error<
-  "an explicitly-defaulted %select{|copy |move }0constructor cannot "
+  "an explicitly-defaulted %sub{select_special_member_kind}0 cannot "
   "be variadic">;
 def err_defaulted_special_member_return_type : Error<
   "explicitly-defaulted %select{copy|move}0 assignment operator must "
@@ -9404,7 +9441,7 @@
   "comparison operator template cannot be defaulted">;
 def err_defaulted_comparison_num_args : Error<
   "%select{non-member|member}0 %sub{select_defaulted_comparison_kind}1"
-  " comparison operator must have %select{2|1}0 parameters">;
+  " must have %select{2|1}0 parameters">;
 def err_defaulted_comparison_param : Error<
   "invalid parameter type for defaulted %sub{select_defaulted_comparison_kind}0"
   "; found %1, expected %2%select{| or %4}3">;
diff --git a/clang/include/clang/Sema/ScopeInfo.h b/clang/include/clang/Sema/ScopeInfo.h
--- a/clang/include/clang/Sema/ScopeInfo.h
+++ b/clang/include/clang/Sema/ScopeInfo.h
@@ -847,6 +847,8 @@
   /// is known.
   bool AfterParameterList = true;
 
+  ParmVarDecl *ExplicitObjectParameter = nullptr;
+
   /// Source range covering the lambda introducer [...].
   SourceRange IntroducerRange;
 
@@ -1042,6 +1044,8 @@
 
   void visitPotentialCaptures(
       llvm::function_ref<void(ValueDecl *, Expr *)> Callback) const;
+
+  bool lambdaCaptureShouldBeConst() const;
 };
 
 FunctionScopeInfo::WeakObjectProfileTy::WeakObjectProfileTy()
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -2175,20 +2175,17 @@
       const FunctionProtoType *Old, SourceLocation OldLoc,
       const FunctionProtoType *New, SourceLocation NewLoc);
   bool handlerCanCatch(QualType HandlerType, QualType ExceptionType);
-  bool CheckExceptionSpecSubset(const PartialDiagnostic &DiagID,
-                                const PartialDiagnostic &NestedDiagID,
-                                const PartialDiagnostic &NoteID,
-                                const PartialDiagnostic &NoThrowDiagID,
-                                const FunctionProtoType *Superset,
-                                SourceLocation SuperLoc,
-                                const FunctionProtoType *Subset,
-                                SourceLocation SubLoc);
-  bool CheckParamExceptionSpec(const PartialDiagnostic &NestedDiagID,
-                               const PartialDiagnostic &NoteID,
-                               const FunctionProtoType *Target,
-                               SourceLocation TargetLoc,
-                               const FunctionProtoType *Source,
-                               SourceLocation SourceLoc);
+  bool CheckExceptionSpecSubset(
+      const PartialDiagnostic &DiagID, const PartialDiagnostic &NestedDiagID,
+      const PartialDiagnostic &NoteID, const PartialDiagnostic &NoThrowDiagID,
+      const FunctionProtoType *Superset, bool SkipSupersetFirstParameter,
+      SourceLocation SuperLoc, const FunctionProtoType *Subset,
+      bool SkipSubsetFirstParameter, SourceLocation SubLoc);
+  bool CheckParamExceptionSpec(
+      const PartialDiagnostic &NestedDiagID, const PartialDiagnostic &NoteID,
+      const FunctionProtoType *Target, bool SkipTargetFirstParameter,
+      SourceLocation TargetLoc, const FunctionProtoType *Source,
+      bool SkipSourceFirstParameter, SourceLocation SourceLoc);
 
   TypeResult ActOnTypeName(Scope *S, Declarator &D);
 
@@ -3005,7 +3002,8 @@
   Attr *getImplicitCodeSegOrSectionAttrForFunction(const FunctionDecl *FD,
                                                    bool IsDefinition);
   void CheckFunctionOrTemplateParamDeclarator(Scope *S, Declarator &D);
-  Decl *ActOnParamDeclarator(Scope *S, Declarator &D);
+  Decl *ActOnParamDeclarator(Scope *S, Declarator &D,
+                             SourceLocation ExplicitThisLoc = {});
   ParmVarDecl *BuildParmVarDeclForTypedef(DeclContext *DC,
                                           SourceLocation Loc,
                                           QualType T);
@@ -3774,7 +3772,7 @@
                              bool UseMemberUsingDeclRules);
   bool IsOverload(FunctionDecl *New, FunctionDecl *Old,
                   bool UseMemberUsingDeclRules, bool ConsiderCudaAttrs = true,
-                  bool ConsiderRequiresClauses = true);
+                  bool UseOverrideRules = false);
 
   // Calculates whether the expression Constraint depends on an enclosing
   // template, for the purposes of [temp.friend] p9.
@@ -3832,6 +3830,12 @@
                                  QualType &ConvertedType);
   bool IsBlockPointerConversion(QualType FromType, QualType ToType,
                                 QualType& ConvertedType);
+
+  bool FunctionParamTypesAreEqual(ArrayRef<QualType> Old,
+                                  ArrayRef<QualType> New,
+                                  unsigned *ArgPos = nullptr,
+                                  bool Reversed = false);
+
   bool FunctionParamTypesAreEqual(const FunctionProtoType *OldType,
                                   const FunctionProtoType *NewType,
                                   unsigned *ArgPos = nullptr,
@@ -3872,10 +3876,11 @@
                                        ExprResult Init,
                                        bool TopLevelOfInitList = false,
                                        bool AllowExplicit = false);
-  ExprResult PerformObjectArgumentInitialization(Expr *From,
-                                                 NestedNameSpecifier *Qualifier,
-                                                 NamedDecl *FoundDecl,
-                                                 CXXMethodDecl *Method);
+  ExprResult InitializeExplicitObjectArgument(Sema &S, Expr *Obj,
+                                              FunctionDecl *Fun);
+  ExprResult PerformImplicitObjectArgumentInitialization(
+      Expr *From, NestedNameSpecifier *Qualifier, NamedDecl *FoundDecl,
+      CXXMethodDecl *Method);
 
   /// Check that the lifetime of the initializer (and its subobjects) is
   /// sufficient for initializing the entity, and perform lifetime extension
@@ -4197,9 +4202,8 @@
       QualType DestTypeForComplaining = QualType(),
       unsigned DiagIDForComplaining = 0);
 
-  Expr *FixOverloadedFunctionReference(Expr *E,
-                                       DeclAccessPair FoundDecl,
-                                       FunctionDecl *Fn);
+  ExprResult FixOverloadedFunctionReference(Expr *E, DeclAccessPair FoundDecl,
+                                            FunctionDecl *Fn);
   ExprResult FixOverloadedFunctionReference(ExprResult,
                                             DeclAccessPair FoundDecl,
                                             FunctionDecl *Fn);
@@ -5762,6 +5766,10 @@
                           Expr *Input, bool IsAfterAmp = false);
 
   bool isQualifiedMemberAccess(Expr *E);
+  bool DiagnoseMissingQualifiersInAddressOfOperand(SourceLocation OpLoc,
+                                                   const Expr *Op,
+                                                   const CXXMethodDecl *MD);
+
   QualType CheckAddressOfOperand(ExprResult &Operand, SourceLocation OpLoc);
 
   bool CheckTypeTraitArity(unsigned Arity, SourceLocation Loc, size_t N);
@@ -7186,6 +7194,8 @@
       StorageClass SC, ArrayRef<ParmVarDecl *> Params,
       bool HasExplicitResultType);
 
+  void DiagnoseInvalidExplicitObjectParameterInLambda(CXXMethodDecl *Method);
+
   /// Perform initialization analysis of the init-capture and perform
   /// any implicit conversions such as an lvalue-to-rvalue conversion if
   /// not being used to initialize a reference.
@@ -7856,6 +7866,13 @@
   void DefineDefaultedComparison(SourceLocation Loc, FunctionDecl *FD,
                                  DefaultedComparisonKind DCK);
 
+  void CheckExplicitObjectMemberFunction(Declarator &D, DeclarationName Name,
+                                         QualType R, bool IsLambda,
+                                         DeclContext *DC = nullptr);
+  void CheckExplicitObjectMemberFunction(DeclContext *DC, Declarator &D,
+                                         DeclarationName Name, QualType R);
+  void CheckExplicitObjectLambda(Declarator &D);
+
   //===--------------------------------------------------------------------===//
   // C++ Derived Classes
   //
@@ -7907,6 +7924,10 @@
   bool CheckOverridingFunctionReturnType(const CXXMethodDecl *New,
                                          const CXXMethodDecl *Old);
 
+  // Check that the overriding method has no explicit object parameter.
+  bool CheckOverridingExplicitObjectMembers(CXXMethodDecl *New,
+                                            const CXXMethodDecl *Old);
+
   /// CheckOverridingFunctionExceptionSpec - Checks whether the exception
   /// spec is a subset of base spec.
   bool CheckOverridingFunctionExceptionSpec(const CXXMethodDecl *New,
@@ -9148,6 +9169,7 @@
       TemplateArgumentListInfo *ExplicitTemplateArgs, ArrayRef<Expr *> Args,
       FunctionDecl *&Specialization, sema::TemplateDeductionInfo &Info,
       bool PartialOverloading, bool AggregateDeductionCandidate,
+      QualType ObjectType, Expr::Classification ObjectClassification,
       llvm::function_ref<bool(ArrayRef<QualType>)> CheckNonDependent);
 
   TemplateDeductionResult
@@ -9158,11 +9180,10 @@
                           sema::TemplateDeductionInfo &Info,
                           bool IsAddressOfFunction = false);
 
-  TemplateDeductionResult
-  DeduceTemplateArguments(FunctionTemplateDecl *FunctionTemplate,
-                          QualType ToType,
-                          CXXConversionDecl *&Specialization,
-                          sema::TemplateDeductionInfo &Info);
+  TemplateDeductionResult DeduceTemplateArguments(
+      FunctionTemplateDecl *FunctionTemplate, QualType ObjectType,
+      Expr::Classification ObjectClassification, QualType ToType,
+      CXXConversionDecl *&Specialization, sema::TemplateDeductionInfo &Info);
 
   TemplateDeductionResult
   DeduceTemplateArguments(FunctionTemplateDecl *FunctionTemplate,
diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -4413,6 +4413,8 @@
 Error ASTNodeImporter::ImportDefaultArgOfParmVarDecl(
     const ParmVarDecl *FromParam, ParmVarDecl *ToParam) {
   ToParam->setHasInheritedDefaultArg(FromParam->hasInheritedDefaultArg());
+  ToParam->setExplicitObjectParameterLoc(
+      FromParam->getExplicitObjectParamThisLoc());
   ToParam->setKNRPromoted(FromParam->isKNRPromoted());
 
   if (FromParam->hasUninstantiatedDefaultArg()) {
diff --git a/clang/lib/AST/ASTStructuralEquivalence.cpp b/clang/lib/AST/ASTStructuralEquivalence.cpp
--- a/clang/lib/AST/ASTStructuralEquivalence.cpp
+++ b/clang/lib/AST/ASTStructuralEquivalence.cpp
@@ -1362,6 +1362,8 @@
       Method1->getAccess() == Method2->getAccess() &&
       Method1->getOverloadedOperator() == Method2->getOverloadedOperator() &&
       Method1->isStatic() == Method2->isStatic() &&
+      Method1->isImplicitObjectMemberFunction() ==
+          Method2->isImplicitObjectMemberFunction() &&
       Method1->isConst() == Method2->isConst() &&
       Method1->isVolatile() == Method2->isVolatile() &&
       Method1->isVirtual() == Method2->isVirtual() &&
diff --git a/clang/lib/AST/ComputeDependence.cpp b/clang/lib/AST/ComputeDependence.cpp
--- a/clang/lib/AST/ComputeDependence.cpp
+++ b/clang/lib/AST/ComputeDependence.cpp
@@ -484,6 +484,10 @@
 
   //    - an identifier associated by name lookup with one or more declarations
   //      declared with a dependent type
+  //    - an identifier associated by name lookup with an entity captured by
+  //    copy ([expr.prim.lambda.capture])
+  //      in a lambda-expression that has an explicit object parameter whose
+  //      type is dependent ([dcl.fct]),
   //
   // [The "or more" case is not modeled as a DeclRefExpr. There are a bunch
   // more bullets here that we handle by treating the declaration as having a
@@ -493,6 +497,12 @@
   else if (Type->isInstantiationDependentType())
     Deps |= ExprDependence::Instantiation;
 
+  //    - an identifier associated by name lookup with an entity captured by
+  //    copy ([expr.prim.lambda.capture])
+  if (E->isCapturedByCopyInLambdaWithExplicitObjectParameter()) {
+    Deps |= ExprDependence::Type;
+  }
+
   //    - a conversion-function-id that specifies a dependent type
   if (Decl->getDeclName().getNameKind() ==
       DeclarationName::CXXConversionFunctionName) {
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -3616,6 +3616,15 @@
   return NumRequiredArgs;
 }
 
+bool FunctionDecl::hasCXXExplicitFunctionObjectParameter() const {
+  return getNumParams() != 0 && getParamDecl(0)->isExplicitObjectParameter();
+}
+
+unsigned FunctionDecl::getMinRequiredExplicitArguments() const {
+  return getMinRequiredArguments() -
+         (hasCXXExplicitFunctionObjectParameter() ? 1 : 0);
+}
+
 bool FunctionDecl::hasOneParamOrDefaultArgs() const {
   return getNumParams() == 1 ||
          (getNumParams() > 1 &&
diff --git a/clang/lib/AST/DeclCXX.cpp b/clang/lib/AST/DeclCXX.cpp
--- a/clang/lib/AST/DeclCXX.cpp
+++ b/clang/lib/AST/DeclCXX.cpp
@@ -838,7 +838,7 @@
       SMKind |= SMF_CopyAssignment;
 
       const auto *ParamTy =
-          Method->getParamDecl(0)->getType()->getAs<ReferenceType>();
+          Method->getNonObjectParameter(0)->getType()->getAs<ReferenceType>();
       if (!ParamTy || ParamTy->getPointeeType().isConstQualified())
         data().HasDeclaredCopyAssignmentWithConstParam = true;
     }
@@ -2409,6 +2409,17 @@
   return Result;
 }
 
+bool CXXMethodDecl::isExplicitObjectMemberFunction() const {
+  // C++2b [dcl.fct]p6:
+  // An explicit object member function is a non-static member
+  // function with an explicit object parameter
+  return !isStatic() && hasCXXExplicitFunctionObjectParameter();
+}
+
+bool CXXMethodDecl::isImplicitObjectMemberFunction() const {
+  return !isStatic() && !hasCXXExplicitFunctionObjectParameter();
+}
+
 bool CXXMethodDecl::isCopyAssignmentOperator() const {
   // C++0x [class.copy]p17:
   //  A user-declared copy assignment operator X::operator= is a non-static
@@ -2416,11 +2427,12 @@
   //  type X, X&, const X&, volatile X& or const volatile X&.
   if (/*operator=*/getOverloadedOperator() != OO_Equal ||
       /*non-static*/ isStatic() ||
-      /*non-template*/getPrimaryTemplate() || getDescribedFunctionTemplate() ||
-      getNumParams() != 1)
+
+      /*non-template*/ getPrimaryTemplate() || getDescribedFunctionTemplate() ||
+      getNumExplicitParams() != 1)
     return false;
 
-  QualType ParamType = getParamDecl(0)->getType();
+  QualType ParamType = getNonObjectParameter(0)->getType();
   if (const auto *Ref = ParamType->getAs<LValueReferenceType>())
     ParamType = Ref->getPointeeType();
 
@@ -2437,10 +2449,10 @@
   //  X&&, const X&&, volatile X&&, or const volatile X&&.
   if (getOverloadedOperator() != OO_Equal || isStatic() ||
       getPrimaryTemplate() || getDescribedFunctionTemplate() ||
-      getNumParams() != 1)
+      getNumExplicitParams() != 1)
     return false;
 
-  QualType ParamType = getParamDecl(0)->getType();
+  QualType ParamType = getNonObjectParameter(0)->getType();
   if (!ParamType->isRValueReferenceType())
     return false;
   ParamType = ParamType->getPointeeType();
@@ -2495,12 +2507,6 @@
   return C.getPointerType(ObjectTy);
 }
 
-QualType CXXMethodDecl::getThisObjectType(const FunctionProtoType *FPT,
-                                          const CXXRecordDecl *Decl) {
-  ASTContext &C = Decl->getASTContext();
-  return ::getThisObjectType(C, FPT, Decl);
-}
-
 QualType CXXMethodDecl::getThisType() const {
   // C++ 9.3.2p1: The type of this in a member function of a class X is X*.
   // If the member function is declared const, the type of this is const X*,
@@ -2512,11 +2518,17 @@
                                     getParent());
 }
 
-QualType CXXMethodDecl::getThisObjectType() const {
-  // Ditto getThisType.
-  assert(isInstance() && "No 'this' for static methods!");
-  return CXXMethodDecl::getThisObjectType(
-      getType()->castAs<FunctionProtoType>(), getParent());
+QualType CXXMethodDecl::getFunctionObjectParameterReferenceType() const {
+  if (isExplicitObjectMemberFunction())
+    return parameters()[0]->getType();
+
+  ASTContext &C = getParentASTContext();
+  const FunctionProtoType *FPT = getType()->castAs<FunctionProtoType>();
+  QualType Type = ::getThisObjectType(C, FPT, getParent());
+  RefQualifierKind RK = FPT->getRefQualifier();
+  if (RK == RefQualifierKind::RQ_RValue)
+    return C.getRValueReferenceType(Type);
+  return C.getLValueReferenceType(Type);
 }
 
 bool CXXMethodDecl::hasInlineBody() const {
diff --git a/clang/lib/AST/DeclPrinter.cpp b/clang/lib/AST/DeclPrinter.cpp
--- a/clang/lib/AST/DeclPrinter.cpp
+++ b/clang/lib/AST/DeclPrinter.cpp
@@ -865,6 +865,10 @@
 void DeclPrinter::VisitVarDecl(VarDecl *D) {
   prettyPrintPragmas(D);
 
+  if (const auto *Param = dyn_cast<ParmVarDecl>(D);
+      Param && Param->isExplicitObjectParameter())
+    Out << "this ";
+
   QualType T = D->getTypeSourceInfo()
     ? D->getTypeSourceInfo()->getType()
     : D->getASTContext().getUnqualifiedObjCPointerType(D->getType());
diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -491,6 +491,7 @@
   DeclRefExprBits.HadMultipleCandidates = false;
   DeclRefExprBits.RefersToEnclosingVariableOrCapture =
       RefersToEnclosingVariableOrCapture;
+  DeclRefExprBits.CapturedByCopyInLambdaWithExplicitObjectParameter = false;
   DeclRefExprBits.NonOdrUseReason = NOUR;
   DeclRefExprBits.IsImmediateEscalating = false;
   DeclRefExprBits.Loc = L;
@@ -518,6 +519,7 @@
     = (TemplateArgs || TemplateKWLoc.isValid()) ? 1 : 0;
   DeclRefExprBits.RefersToEnclosingVariableOrCapture =
       RefersToEnclosingVariableOrCapture;
+  DeclRefExprBits.CapturedByCopyInLambdaWithExplicitObjectParameter = false;
   DeclRefExprBits.NonOdrUseReason = NOUR;
   if (TemplateArgs) {
     auto Deps = TemplateArgumentDependence::None;
diff --git a/clang/lib/AST/ExprClassification.cpp b/clang/lib/AST/ExprClassification.cpp
--- a/clang/lib/AST/ExprClassification.cpp
+++ b/clang/lib/AST/ExprClassification.cpp
@@ -465,8 +465,13 @@
   // lvalue unless it's a reference type (C++ [temp.param]p6), so we need to
   // special-case this.
 
-  if (isa<CXXMethodDecl>(D) && cast<CXXMethodDecl>(D)->isInstance())
-    return Cl::CL_MemberFunction;
+  if (const auto *M = dyn_cast<CXXMethodDecl>(D)) {
+    if (M->isImplicitObjectMemberFunction())
+      return Cl::CL_MemberFunction;
+    if (M->isStatic())
+      return Cl::CL_LValue;
+    return Cl::CL_PRValue;
+  }
 
   bool islvalue;
   if (const auto *NTTParm = dyn_cast<NonTypeTemplateParmDecl>(D))
@@ -551,8 +556,13 @@
   //      -- If it refers to a static member function [...], then E1.E2 is an
   //         lvalue; [...]
   //      -- Otherwise [...] E1.E2 is a prvalue.
-  if (const auto *Method = dyn_cast<CXXMethodDecl>(Member))
-    return Method->isStatic() ? Cl::CL_LValue : Cl::CL_MemberFunction;
+  if (const auto *Method = dyn_cast<CXXMethodDecl>(Member)) {
+    if (Method->isStatic())
+      return Cl::CL_LValue;
+    if (Method->isImplicitObjectMemberFunction())
+      return Cl::CL_MemberFunction;
+    return Cl::CL_PRValue;
+  }
 
   //   -- If E2 is a member enumerator [...], the expression E1.E2 is a prvalue.
   // So is everything else we haven't handled yet.
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -1945,9 +1945,9 @@
 /// Produce a string describing the given constexpr call.
 void CallStackFrame::describe(raw_ostream &Out) const {
   unsigned ArgIndex = 0;
-  bool IsMemberCall = isa<CXXMethodDecl>(Callee) &&
-                      !isa<CXXConstructorDecl>(Callee) &&
-                      cast<CXXMethodDecl>(Callee)->isInstance();
+  bool IsMemberCall =
+      isa<CXXMethodDecl>(Callee) && !isa<CXXConstructorDecl>(Callee) &&
+      cast<CXXMethodDecl>(Callee)->isImplicitObjectMemberFunction();
 
   if (!IsMemberCall)
     Callee->getNameForDiagnostic(Out, Info.Ctx.getPrintingPolicy(),
@@ -4698,6 +4698,9 @@
   if (Object->getType()->isLiteralType(Info.Ctx))
     return EvaluateTemporary(Object, This, Info);
 
+  if (Object->getType()->isRecordType() && Object->isPRValue())
+    return EvaluateTemporary(Object, This, Info);
+
   Info.FFDiag(Object, diag::note_constexpr_nonliteral) << Object->getType();
   return false;
 }
@@ -7762,15 +7765,18 @@
       if (OCE && OCE->isAssignmentOp()) {
         assert(Args.size() == 2 && "wrong number of arguments in assignment");
         Call = Info.CurrentCall->createCall(FD);
-        if (!EvaluateArgs(isa<CXXMethodDecl>(FD) ? Args.slice(1) : Args, Call,
-                          Info, FD, /*RightToLeft=*/true))
+        bool HasThis = false;
+        if (const auto *MD = dyn_cast<CXXMethodDecl>(FD))
+          HasThis = MD->isImplicitObjectMemberFunction();
+        if (!EvaluateArgs(HasThis ? Args.slice(1) : Args, Call, Info, FD,
+                          /*RightToLeft=*/true))
           return false;
       }
 
       // Overloaded operator calls to member functions are represented as normal
       // calls with '*this' as the first argument.
       const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(FD);
-      if (MD && !MD->isStatic()) {
+      if (MD && MD->isImplicitObjectMemberFunction()) {
         // FIXME: When selecting an implicit conversion for an overloaded
         // operator delete, we sometimes try to evaluate calls to conversion
         // operators without a 'this' parameter!
@@ -7854,7 +7860,7 @@
                                    CovariantAdjustmentPath);
         if (!FD)
           return false;
-      } else {
+      } else if (NamedMember && NamedMember->isImplicitObjectMemberFunction()) {
         // Check that the 'this' pointer points to an object of the right type.
         // FIXME: If this is an assignment operator call, we may need to change
         // the active union member before we check this.
@@ -16227,7 +16233,8 @@
 #ifndef NDEBUG
     auto *MD = dyn_cast<CXXMethodDecl>(Callee);
     assert(MD && "Don't provide `this` for non-methods.");
-    assert(!MD->isStatic() && "Don't provide `this` for static methods.");
+    assert(MD->isImplicitObjectMemberFunction() &&
+           "Don't provide `this` for methods without an implicit object.");
 #endif
     if (!This->isValueDependent() &&
         EvaluateObjectArgument(Info, This, ThisVal) &&
@@ -16322,9 +16329,10 @@
     HandleConstructorCall(&VIE, This, Args, CD, Info, Scratch);
   } else {
     SourceLocation Loc = FD->getLocation();
-    HandleFunctionCall(Loc, FD, (MD && MD->isInstance()) ? &This : nullptr,
-                       &VIE, Args, CallRef(), FD->getBody(), Info, Scratch,
-                       /*ResultSlot=*/nullptr);
+    HandleFunctionCall(
+        Loc, FD, (MD && MD->isImplicitObjectMemberFunction()) ? &This : nullptr,
+        &VIE, Args, CallRef(), FD->getBody(), Info, Scratch,
+        /*ResultSlot=*/nullptr);
   }
 
   return Diags.empty();
diff --git a/clang/lib/AST/Interp/ByteCodeEmitter.cpp b/clang/lib/AST/Interp/ByteCodeEmitter.cpp
--- a/clang/lib/AST/Interp/ByteCodeEmitter.cpp
+++ b/clang/lib/AST/Interp/ByteCodeEmitter.cpp
@@ -46,7 +46,7 @@
   // InterpStack when calling the function.
   bool HasThisPointer = false;
   if (const auto *MD = dyn_cast<CXXMethodDecl>(FuncDecl)) {
-    if (MD->isInstance()) {
+    if (MD->isImplicitObjectMemberFunction()) {
       HasThisPointer = true;
       ParamTypes.push_back(PT_Ptr);
       ParamOffsets.push_back(ParamOffset);
diff --git a/clang/lib/AST/ItaniumMangle.cpp b/clang/lib/AST/ItaniumMangle.cpp
--- a/clang/lib/AST/ItaniumMangle.cpp
+++ b/clang/lib/AST/ItaniumMangle.cpp
@@ -1666,7 +1666,7 @@
 
       // If we have a member function, we need to include the 'this' pointer.
       if (const auto *MD = dyn_cast<CXXMethodDecl>(ND))
-        if (!MD->isStatic())
+        if (MD->isImplicitObjectMemberFunction())
           Arity++;
     }
     [[fallthrough]];
@@ -1726,6 +1726,8 @@
     Qualifiers MethodQuals = Method->getMethodQualifiers();
     // We do not consider restrict a distinguishing attribute for overloading
     // purposes so we must not mangle it.
+    if (Method->isExplicitObjectMemberFunction())
+      Out << 'H';
     MethodQuals.removeRestrict();
     mangleQualifiers(MethodQuals);
     mangleRefQualifier(Method->getRefQualifier());
diff --git a/clang/lib/AST/JSONNodeDumper.cpp b/clang/lib/AST/JSONNodeDumper.cpp
--- a/clang/lib/AST/JSONNodeDumper.cpp
+++ b/clang/lib/AST/JSONNodeDumper.cpp
@@ -843,6 +843,9 @@
 void JSONNodeDumper::VisitVarDecl(const VarDecl *VD) {
   VisitNamedDecl(VD);
   JOS.attribute("type", createQualType(VD->getType()));
+  if (const auto *P = dyn_cast<ParmVarDecl>(VD))
+    attributeOnlyIfTrue("explicitObjectParameter",
+                        P->isExplicitObjectParameter());
 
   StorageClass SC = VD->getStorageClass();
   if (SC != SC_None)
diff --git a/clang/lib/AST/Mangle.cpp b/clang/lib/AST/Mangle.cpp
--- a/clang/lib/AST/Mangle.cpp
+++ b/clang/lib/AST/Mangle.cpp
@@ -225,7 +225,7 @@
   assert(!Proto->isVariadic());
   unsigned ArgWords = 0;
   if (const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(FD))
-    if (!MD->isStatic())
+    if (MD->isImplicitObjectMemberFunction())
       ++ArgWords;
   uint64_t DefaultPtrWidth = TI.getPointerWidth(LangAS::Default);
   for (const auto &AT : Proto->param_types()) {
diff --git a/clang/lib/AST/MicrosoftMangle.cpp b/clang/lib/AST/MicrosoftMangle.cpp
--- a/clang/lib/AST/MicrosoftMangle.cpp
+++ b/clang/lib/AST/MicrosoftMangle.cpp
@@ -2636,7 +2636,7 @@
   if (const CXXMethodDecl *MD = dyn_cast_or_null<CXXMethodDecl>(D)) {
     if (MD->getParent()->isLambda())
       IsInLambda = true;
-    if (MD->isInstance())
+    if (MD->isImplicitObjectMemberFunction())
       HasThisQuals = true;
     if (isa<CXXDestructorDecl>(MD)) {
       IsStructor = true;
@@ -2745,6 +2745,10 @@
   } else {
     // Happens for function pointer type arguments for example.
     for (unsigned I = 0, E = Proto->getNumParams(); I != E; ++I) {
+      // Explicit object parameters are prefixed by "_V".
+      if (I == 0 && D && D->getParamDecl(I)->isExplicitObjectParameter())
+        Out << "_V";
+
       mangleFunctionArgumentType(Proto->getParamType(I), Range);
       // Mangle each pass_object_size parameter as if it's a parameter of enum
       // type passed directly after the parameter with the pass_object_size
@@ -2810,7 +2814,7 @@
       case AS_none:
         llvm_unreachable("Unsupported access specifier");
       case AS_private:
-        if (MD->isStatic())
+        if (!MD->isImplicitObjectMemberFunction())
           Out << 'C';
         else if (IsVirtual)
           Out << 'E';
@@ -2818,7 +2822,7 @@
           Out << 'A';
         break;
       case AS_protected:
-        if (MD->isStatic())
+        if (!MD->isImplicitObjectMemberFunction())
           Out << 'K';
         else if (IsVirtual)
           Out << 'M';
@@ -2826,7 +2830,7 @@
           Out << 'I';
         break;
       case AS_public:
-        if (MD->isStatic())
+        if (!MD->isImplicitObjectMemberFunction())
           Out << 'S';
         else if (IsVirtual)
           Out << 'U';
diff --git a/clang/lib/AST/TextNodeDumper.cpp b/clang/lib/AST/TextNodeDumper.cpp
--- a/clang/lib/AST/TextNodeDumper.cpp
+++ b/clang/lib/AST/TextNodeDumper.cpp
@@ -1782,6 +1782,10 @@
 
 void TextNodeDumper::VisitVarDecl(const VarDecl *D) {
   dumpName(D);
+  if (const auto *P = dyn_cast<ParmVarDecl>(D);
+      P && P->isExplicitObjectParameter())
+    OS << " this";
+
   dumpType(D->getType());
   StorageClass SC = D->getStorageClass();
   if (SC != SC_None)
diff --git a/clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp b/clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp
--- a/clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp
+++ b/clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp
@@ -317,8 +317,8 @@
       MethodDecl = dyn_cast<CXXMethodDecl>(Parent->getDeclContext());
 
     // FIXME: Initialize the ThisPointeeLoc of lambdas too.
-    if (MethodDecl && !MethodDecl->isStatic()) {
-      QualType ThisPointeeType = MethodDecl->getThisObjectType();
+    if (MethodDecl && MethodDecl->isImplicitObjectMemberFunction()) {
+      QualType ThisPointeeType = MethodDecl->getFunctionObjectParameterType();
       ThisPointeeLoc =
           &cast<RecordValue>(createValue(ThisPointeeType))->getLoc();
     }
diff --git a/clang/lib/Analysis/ThreadSafetyCommon.cpp b/clang/lib/Analysis/ThreadSafetyCommon.cpp
--- a/clang/lib/Analysis/ThreadSafetyCommon.cpp
+++ b/clang/lib/Analysis/ThreadSafetyCommon.cpp
@@ -149,7 +149,9 @@
     // If the attribute has no arguments, then assume the argument is "this".
     if (!AttrExp)
       return CapabilityExpr(
-          Self, ClassifyDiagnostic(cast<CXXMethodDecl>(D)->getThisObjectType()),
+          Self,
+          ClassifyDiagnostic(
+              cast<CXXMethodDecl>(D)->getFunctionObjectParameterType()),
           false);
     else  // For most attributes.
       return translateAttrExpr(AttrExp, &Ctx);
diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -297,7 +297,7 @@
   setCUDAKernelCallingConvention(FT, CGM, MD);
   auto prototype = FT.getAs<FunctionProtoType>();
 
-  if (MD->isInstance()) {
+  if (MD->isImplicitObjectMemberFunction()) {
     // The abstract case is perfectly fine.
     const CXXRecordDecl *ThisType = TheCXXABI.getThisArgumentTypeForMethod(MD);
     return arrangeCXXMethodType(ThisType, prototype.getTypePtr(), MD);
@@ -447,7 +447,7 @@
 const CGFunctionInfo &
 CodeGenTypes::arrangeFunctionDeclaration(const FunctionDecl *FD) {
   if (const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(FD))
-    if (MD->isInstance())
+    if (MD->isImplicitObjectMemberFunction())
       return arrangeCXXMethodDeclaration(MD);
 
   CanQualType FTy = FD->getType()->getCanonicalTypeUnqualified();
diff --git a/clang/lib/CodeGen/CGClass.cpp b/clang/lib/CodeGen/CGClass.cpp
--- a/clang/lib/CodeGen/CGClass.cpp
+++ b/clang/lib/CodeGen/CGClass.cpp
@@ -516,7 +516,7 @@
       const CXXDestructorDecl *D = BaseClass->getDestructor();
       // We are already inside a destructor, so presumably the object being
       // destroyed should have the expected type.
-      QualType ThisTy = D->getThisObjectType();
+      QualType ThisTy = D->getFunctionObjectParameterType();
       Address Addr =
         CGF.GetAddressOfDirectBaseInCompleteClass(CGF.LoadCXXThisAddress(),
                                                   DerivedClass, BaseClass,
@@ -1462,7 +1462,7 @@
     RunCleanupsScope DtorEpilogue(*this);
     EnterDtorCleanups(Dtor, Dtor_Deleting);
     if (HaveInsertPoint()) {
-      QualType ThisTy = Dtor->getThisObjectType();
+      QualType ThisTy = Dtor->getFunctionObjectParameterType();
       EmitCXXDestructorCall(Dtor, Dtor_Complete, /*ForVirtualBase=*/false,
                             /*Delegating=*/false, LoadCXXThisAddress(), ThisTy);
     }
@@ -1496,7 +1496,7 @@
     EnterDtorCleanups(Dtor, Dtor_Complete);
 
     if (!isTryBody) {
-      QualType ThisTy = Dtor->getThisObjectType();
+      QualType ThisTy = Dtor->getFunctionObjectParameterType();
       EmitCXXDestructorCall(Dtor, Dtor_Base, /*ForVirtualBase=*/false,
                             /*Delegating=*/false, LoadCXXThisAddress(), ThisTy);
       break;
@@ -2463,7 +2463,7 @@
     void Emit(CodeGenFunction &CGF, Flags flags) override {
       // We are calling the destructor from within the constructor.
       // Therefore, "this" should have the expected type.
-      QualType ThisTy = Dtor->getThisObjectType();
+      QualType ThisTy = Dtor->getFunctionObjectParameterType();
       CGF.EmitCXXDestructorCall(Dtor, Type, /*ForVirtualBase=*/false,
                                 /*Delegating=*/true, Addr, ThisTy);
     }
diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -2127,14 +2127,14 @@
       // attribute, i.e. that value is not available at the host side.
       if (!CGM.getLangOpts().CUDA || CGM.getLangOpts().CUDAIsDevice ||
           !D->hasAttr<CUDADeviceAttr>()) {
-        const CXXMethodDecl *MD;
         // Variable pointer template parameters have a value that is the address
         // of the variable.
         if (const auto *VD = dyn_cast<VarDecl>(D))
           V = CGM.GetAddrOfGlobalVar(VD);
         // Member function pointers have special support for building them,
         // though this is currently unsupported in LLVM CodeGen.
-        else if ((MD = dyn_cast<CXXMethodDecl>(D)) && MD->isInstance())
+        else if (const auto *MD = dyn_cast<CXXMethodDecl>(D);
+                 MD && MD->isImplicitObjectMemberFunction())
           V = CGM.getCXXABI().EmitMemberFunctionPointer(MD);
         else if (const auto *FD = dyn_cast<FunctionDecl>(D))
           V = CGM.GetAddrOfFunction(FD);
diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -2654,9 +2654,8 @@
 
 static LValue EmitCapturedFieldLValue(CodeGenFunction &CGF, const FieldDecl *FD,
                                       llvm::Value *ThisValue) {
-  QualType TagType = CGF.getContext().getTagDeclType(FD->getParent());
-  LValue LV = CGF.MakeNaturalAlignAddrLValue(ThisValue, TagType);
-  return CGF.EmitLValueForField(LV, FD);
+
+  return CGF.EmitLValueForLambdaField(FD, ThisValue);
 }
 
 /// Named Registers are named metadata pointing to the register name
@@ -4269,17 +4268,38 @@
 
 /// Given that we are currently emitting a lambda, emit an l-value for
 /// one of its members.
-LValue CodeGenFunction::EmitLValueForLambdaField(const FieldDecl *Field) {
-  if (CurCodeDecl) {
-    assert(cast<CXXMethodDecl>(CurCodeDecl)->getParent()->isLambda());
-    assert(cast<CXXMethodDecl>(CurCodeDecl)->getParent() == Field->getParent());
+///
+LValue CodeGenFunction::EmitLValueForLambdaField(const FieldDecl *Field,
+                                                 llvm::Value *ThisValue) {
+  bool HasExplicitObjectParameter = false;
+  if (const auto *MD = dyn_cast_if_present<CXXMethodDecl>(CurCodeDecl)) {
+    HasExplicitObjectParameter = MD->isExplicitObjectMemberFunction();
+    assert(MD->getParent()->isLambda());
+    assert(MD->getParent() == Field->getParent());
+  }
+  LValue LambdaLV;
+  if (HasExplicitObjectParameter) {
+    const VarDecl *D = cast<CXXMethodDecl>(CurCodeDecl)->getParamDecl(0);
+    auto It = LocalDeclMap.find(D);
+    assert(It != LocalDeclMap.end() && "explicit parameter not loaded?");
+    Address AddrOfExplicitObject = It->getSecond();
+    if (D->getType()->isReferenceType())
+      LambdaLV = EmitLoadOfReferenceLValue(AddrOfExplicitObject, D->getType(),
+                                           AlignmentSource::Decl);
+    else
+      LambdaLV = MakeNaturalAlignAddrLValue(AddrOfExplicitObject.getPointer(),
+                                            D->getType().getNonReferenceType());
+  } else {
+    QualType LambdaTagType = getContext().getTagDeclType(Field->getParent());
+    LambdaLV = MakeNaturalAlignAddrLValue(ThisValue, LambdaTagType);
   }
-  QualType LambdaTagType =
-    getContext().getTagDeclType(Field->getParent());
-  LValue LambdaLV = MakeNaturalAlignAddrLValue(CXXABIThisValue, LambdaTagType);
   return EmitLValueForField(LambdaLV, Field);
 }
 
+LValue CodeGenFunction::EmitLValueForLambdaField(const FieldDecl *Field) {
+  return EmitLValueForLambdaField(Field, CXXABIThisValue);
+}
+
 /// Get the field index in the debug info. The debug info structure/union
 /// will ignore the unnamed bitfields.
 unsigned CodeGenFunction::getDebugInfoFIndex(const RecordDecl *Rec,
@@ -4991,9 +5011,12 @@
   if (const auto *CE = dyn_cast<CUDAKernelCallExpr>(E))
     return EmitCUDAKernelCallExpr(CE, ReturnValue);
 
+  // A CXXOperatorCallExpr is created even for explicit object methods, but
+  // these should be treated like static function call.
   if (const auto *CE = dyn_cast<CXXOperatorCallExpr>(E))
-    if (const CXXMethodDecl *MD =
-          dyn_cast_or_null<CXXMethodDecl>(CE->getCalleeDecl()))
+    if (const auto *MD =
+            dyn_cast_if_present<CXXMethodDecl>(CE->getCalleeDecl());
+        MD && MD->isImplicitObjectMemberFunction())
       return EmitCXXOperatorMemberCallExpr(CE, MD, ReturnValue);
 
   CGCallee callee = EmitCallee(E->getCallee());
diff --git a/clang/lib/CodeGen/CGExprCXX.cpp b/clang/lib/CodeGen/CGExprCXX.cpp
--- a/clang/lib/CodeGen/CGExprCXX.cpp
+++ b/clang/lib/CodeGen/CGExprCXX.cpp
@@ -41,7 +41,7 @@
 
   assert(CE == nullptr || isa<CXXMemberCallExpr>(CE) ||
          isa<CXXOperatorCallExpr>(CE));
-  assert(MD->isInstance() &&
+  assert(MD->isImplicitObjectMemberFunction() &&
          "Trying to emit a member or operator call expr on a static method!");
 
   // Push the this ptr.
@@ -66,7 +66,11 @@
     Args.addFrom(*RtlArgs);
   } else if (CE) {
     // Special case: skip first argument of CXXOperatorCall (it is "this").
-    unsigned ArgsToSkip = isa<CXXOperatorCallExpr>(CE) ? 1 : 0;
+    unsigned ArgsToSkip = 0;
+    if (const auto *Op = dyn_cast<CXXOperatorCallExpr>(CE)) {
+      if (const auto *M = dyn_cast<CXXMethodDecl>(Op->getCalleeDecl()))
+        ArgsToSkip = (unsigned)!M->isExplicitObjectMemberFunction();
+    }
     CGF.EmitCallArgs(Args, FPT, drop_begin(CE->arguments(), ArgsToSkip),
                      CE->getDirectCallee());
   } else {
@@ -484,7 +488,7 @@
 CodeGenFunction::EmitCXXOperatorMemberCallExpr(const CXXOperatorCallExpr *E,
                                                const CXXMethodDecl *MD,
                                                ReturnValueSlot ReturnValue) {
-  assert(MD->isInstance() &&
+  assert(MD->isImplicitObjectMemberFunction() &&
          "Trying to emit a member call expr on a static method!");
   return EmitCXXMemberOrOperatorMemberCallExpr(
       E, MD, ReturnValue, /*HasQualifier=*/false, /*Qualifier=*/nullptr,
diff --git a/clang/lib/CodeGen/CodeGenFunction.h b/clang/lib/CodeGen/CodeGenFunction.h
--- a/clang/lib/CodeGen/CodeGenFunction.h
+++ b/clang/lib/CodeGen/CodeGenFunction.h
@@ -4017,6 +4017,8 @@
                                            const ObjCIvarDecl *Ivar);
   LValue EmitLValueForField(LValue Base, const FieldDecl* Field);
   LValue EmitLValueForLambdaField(const FieldDecl *Field);
+  LValue EmitLValueForLambdaField(const FieldDecl *Field,
+                                  llvm::Value *ThisValue);
 
   /// EmitLValueForFieldInitialization - Like EmitLValueForField, except that
   /// if the Field is a reference, this will return the address of the reference
diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -1167,12 +1167,13 @@
 
   EmitFunctionProlog(*CurFnInfo, CurFn, Args);
 
-  if (isa_and_nonnull<CXXMethodDecl>(D) &&
-      cast<CXXMethodDecl>(D)->isInstance()) {
-    CGM.getCXXABI().EmitInstanceFunctionProlog(*this);
-    const CXXMethodDecl *MD = cast<CXXMethodDecl>(D);
-    if (MD->getParent()->isLambda() &&
-        MD->getOverloadedOperator() == OO_Call) {
+  if (const CXXMethodDecl *MD = dyn_cast_if_present<CXXMethodDecl>(D);
+      MD && !MD->isStatic()) {
+    bool IsInLambda =
+        MD->getParent()->isLambda() && MD->getOverloadedOperator() == OO_Call;
+    if (MD->isImplicitObjectMemberFunction())
+      CGM.getCXXABI().EmitInstanceFunctionProlog(*this);
+    if (IsInLambda) {
       // We're in a lambda; figure out the captures.
       MD->getParent()->getCaptureFields(LambdaCaptureFields,
                                         LambdaThisCaptureField);
@@ -1202,7 +1203,7 @@
           VLASizeMap[VAT->getSizeExpr()] = ExprArg;
         }
       }
-    } else {
+    } else if (MD->isImplicitObjectMemberFunction()) {
       // Not in a lambda; just use 'this' from the method.
       // FIXME: Should we generate a new load for each use of 'this'?  The
       // fast register allocator would be happier...
@@ -1313,7 +1314,7 @@
   QualType ResTy = FD->getReturnType();
 
   const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(FD);
-  if (MD && MD->isInstance()) {
+  if (MD && MD->isImplicitObjectMemberFunction()) {
     if (CGM.getCXXABI().HasThisReturn(GD))
       ResTy = MD->getThisType();
     else if (CGM.getCXXABI().hasMostDerivedReturn(GD))
diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -2237,8 +2237,8 @@
 
   // Only functions whose address can be taken with a member function pointer
   // need this sort of type metadata.
-  return !MD->isStatic() && !MD->isVirtual() && !isa<CXXConstructorDecl>(MD) &&
-         !isa<CXXDestructorDecl>(MD);
+  return MD->isImplicitObjectMemberFunction() && !MD->isVirtual() &&
+         !isa<CXXConstructorDecl, CXXDestructorDecl>(MD);
 }
 
 SmallVector<const CXXRecordDecl *, 0>
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -5753,8 +5753,7 @@
 bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide,
                                      DeclSpec::FriendSpecified IsFriend,
                                      const ParsedTemplateInfo *TemplateInfo) {
-  TentativeParsingAction TPA(*this);
-
+  RevertingTentativeParsingAction TPA(*this);
   // Parse the C++ scope specifier.
   CXXScopeSpec SS;
   if (TemplateInfo && TemplateInfo->TemplateParams)
@@ -5763,7 +5762,6 @@
   if (ParseOptionalCXXScopeSpecifier(SS, /*ObjectType=*/nullptr,
                                      /*ObjectHasErrors=*/false,
                                      /*EnteringContext=*/true)) {
-    TPA.Revert();
     return false;
   }
 
@@ -5775,7 +5773,6 @@
   } else if (Tok.is(tok::annot_template_id)) {
     ConsumeAnnotationToken();
   } else {
-    TPA.Revert();
     return false;
   }
 
@@ -5785,7 +5782,6 @@
 
   // Current class name must be followed by a left parenthesis.
   if (Tok.isNot(tok::l_paren)) {
-    TPA.Revert();
     return false;
   }
   ConsumeParen();
@@ -5794,7 +5790,6 @@
   // that we have a constructor.
   if (Tok.is(tok::r_paren) ||
       (Tok.is(tok::ellipsis) && NextToken().is(tok::r_paren))) {
-    TPA.Revert();
     return true;
   }
 
@@ -5803,7 +5798,6 @@
   if (getLangOpts().CPlusPlus11 &&
       isCXX11AttributeSpecifier(/*Disambiguate*/ false,
                                 /*OuterMightBeMessageSend*/ true)) {
-    TPA.Revert();
     return true;
   }
 
@@ -5824,9 +5818,17 @@
   // If we parsed a scope specifier as well as friend,
   // we might be parsing a friend constructor.
   bool IsConstructor = false;
-  if (isDeclarationSpecifier(IsFriend && !SS.isSet()
-                                 ? ImplicitTypenameContext::No
-                                 : ImplicitTypenameContext::Yes))
+  ImplicitTypenameContext ITC = IsFriend && !SS.isSet()
+                                    ? ImplicitTypenameContext::No
+                                    : ImplicitTypenameContext::Yes;
+  // Constructors cannot have this parameters, but we support that scenario here
+  // to improve diagnostic.
+  if (Tok.is(tok::kw_this)) {
+    ConsumeToken();
+    return isDeclarationSpecifier(ITC);
+  }
+
+  if (isDeclarationSpecifier(ITC))
     IsConstructor = true;
   else if (Tok.is(tok::identifier) ||
            (Tok.is(tok::annot_cxxscope) && NextToken().is(tok::identifier))) {
@@ -5895,8 +5897,6 @@
       break;
     }
   }
-
-  TPA.Revert();
   return IsConstructor;
 }
 
@@ -7274,6 +7274,7 @@
 ///           '=' assignment-expression
 /// [GNU]   declaration-specifiers abstract-declarator[opt] attributes
 /// [C++11] attribute-specifier-seq parameter-declaration
+/// [C++2b] attribute-specifier-seq 'this' parameter-declaration
 ///
 void Parser::ParseParameterDeclarationClause(
     DeclaratorContext DeclaratorCtx, ParsedAttributes &FirstArgAttrs,
@@ -7338,9 +7339,16 @@
 
     SourceLocation DSStart = Tok.getLocation();
 
+    // Parse a C++23 Explicit Object Parameter
+    // We do that in all language modes to produce a better diagnostic.
+    SourceLocation ThisLoc;
+    if (getLangOpts().CPlusPlus && Tok.is(tok::kw_this))
+      ThisLoc = ConsumeToken();
+
     ParseDeclarationSpecifiers(DS, /*TemplateInfo=*/ParsedTemplateInfo(),
                                AS_none, DeclSpecContext::DSC_normal,
                                /*LateAttrs=*/nullptr, AllowImplicitTypename);
+
     DS.takeAttributesFrom(ArgDeclSpecAttrs);
 
     // Parse the declarator.  This is "PrototypeContext" or
@@ -7354,6 +7362,9 @@
                                   : DeclaratorContext::Prototype);
     ParseDeclarator(ParmDeclarator);
 
+    if (ThisLoc.isValid())
+      ParmDeclarator.SetRangeBegin(ThisLoc);
+
     // Parse GNU attributes, if present.
     MaybeParseGNUAttributes(ParmDeclarator);
     if (getLangOpts().HLSL)
@@ -7423,7 +7434,8 @@
       }
       // Inform the actions module about the parameter declarator, so it gets
       // added to the current scope.
-      Decl *Param = Actions.ActOnParamDeclarator(getCurScope(), ParmDeclarator);
+      Decl *Param =
+          Actions.ActOnParamDeclarator(getCurScope(), ParmDeclarator, ThisLoc);
       // Parse the default argument, if any. We parse the default
       // arguments in all dialects; the semantic analysis in
       // ActOnParamDefaultArgument will reject the default argument in
diff --git a/clang/lib/Parse/ParseTentative.cpp b/clang/lib/Parse/ParseTentative.cpp
--- a/clang/lib/Parse/ParseTentative.cpp
+++ b/clang/lib/Parse/ParseTentative.cpp
@@ -1559,6 +1559,17 @@
   case tok::kw___vector:
     return TPResult::True;
 
+  case tok::kw_this: {
+    // Try to parse a C++23 Explicit Object Parameter
+    // We do that in all language modes to produce a better diagnostic.
+    if (getLangOpts().CPlusPlus) {
+      RevertingTentativeParsingAction PA(*this);
+      ConsumeToken();
+      return isCXXDeclarationSpecifier(AllowImplicitTypename, BracedCastResult,
+                                       InvalidAsDeclSpec);
+    }
+    return TPResult::False;
+  }
   case tok::annot_template_id: {
     TemplateIdAnnotation *TemplateId = takeTemplateIdAnnotation(Tok);
     // If lookup for the template-name found nothing, don't assume we have a
diff --git a/clang/lib/Sema/ScopeInfo.cpp b/clang/lib/Sema/ScopeInfo.cpp
--- a/clang/lib/Sema/ScopeInfo.cpp
+++ b/clang/lib/Sema/ScopeInfo.cpp
@@ -248,6 +248,14 @@
   }
 }
 
+bool LambdaScopeInfo::lambdaCaptureShouldBeConst() const {
+  if (ExplicitObjectParameter)
+    return ExplicitObjectParameter->getType()
+        .getNonReferenceType()
+        .isConstQualified();
+  return !Mutable;
+}
+
 FunctionScopeInfo::~FunctionScopeInfo() { }
 BlockScopeInfo::~BlockScopeInfo() { }
 CapturedRegionScopeInfo::~CapturedRegionScopeInfo() { }
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -125,7 +125,7 @@
 
   return S.Diag(Call->getEndLoc(), diag::err_typecheck_call_too_few_args)
          << 0 /*function call*/ << MinArgCount << ArgCount
-         << Call->getSourceRange();
+         << /*is non object*/ 0 << Call->getSourceRange();
 }
 
 /// Checks that a call expression's argument count is at most the desired
@@ -138,7 +138,7 @@
   return S.Diag(Call->getEndLoc(),
                 diag::err_typecheck_call_too_many_args_at_most)
          << 0 /*function call*/ << MaxArgCount << ArgCount
-         << Call->getSourceRange();
+         << /*is non object*/ 0 << Call->getSourceRange();
 }
 
 /// Checks that a call expression's argument count is in the desired range. This
@@ -167,7 +167,7 @@
 
   return S.Diag(Range.getBegin(), diag::err_typecheck_call_too_many_args)
          << 0 /*function call*/ << DesiredArgCount << ArgCount
-         << Call->getArg(1)->getSourceRange();
+         << /*is non object*/ 0 << Call->getArg(1)->getSourceRange();
 }
 
 static bool convertArgumentToType(Sema &S, Expr *&Value, QualType Ty) {
@@ -216,7 +216,7 @@
   // We need at least one argument.
   if (TheCall->getNumArgs() < 1) {
     S.Diag(TheCall->getEndLoc(), diag::err_typecheck_call_too_few_args_at_least)
-        << 0 << 1 << TheCall->getNumArgs()
+        << 0 << 1 << TheCall->getNumArgs() << /*is non object*/ 0
         << TheCall->getCallee()->getSourceRange();
     return true;
   }
@@ -1530,7 +1530,7 @@
   if (NumArgs < 4) {
     S.Diag(TheCall->getBeginLoc(),
            diag::err_typecheck_call_too_few_args_at_least)
-        << 0 << 4 << NumArgs;
+        << 0 << 4 << NumArgs << /*is non object*/ 0;
     return true;
   }
 
@@ -6781,8 +6781,9 @@
       Proto->isVariadic() ? VariadicConstructor : VariadicDoesNotApply;
 
   auto *Ctor = cast<CXXConstructorDecl>(FDecl);
-  CheckArgAlignment(Loc, FDecl, "'this'", Context.getPointerType(ThisType),
-                    Context.getPointerType(Ctor->getThisObjectType()));
+  CheckArgAlignment(
+      Loc, FDecl, "'this'", Context.getPointerType(ThisType),
+      Context.getPointerType(Ctor->getFunctionObjectParameterType()));
 
   checkCall(FDecl, Proto, /*ThisArg=*/nullptr, Args, /*IsMemberFunction=*/true,
             Loc, SourceRange(), CallType);
@@ -6802,14 +6803,16 @@
   unsigned NumArgs = TheCall->getNumArgs();
 
   Expr *ImplicitThis = nullptr;
-  if (IsMemberOperatorCall && !FDecl->isStatic()) {
+  if (IsMemberOperatorCall && !FDecl->isStatic() &&
+      !FDecl->hasCXXExplicitFunctionObjectParameter()) {
     // If this is a call to a non-static member operator, hide the first
     // argument from checkCall.
     // FIXME: Our choice of AST representation here is less than ideal.
     ImplicitThis = Args[0];
     ++Args;
     --NumArgs;
-  } else if (IsMemberFunction && !FDecl->isStatic())
+  } else if (IsMemberFunction && !FDecl->isStatic() &&
+             !FDecl->hasCXXExplicitFunctionObjectParameter())
     ImplicitThis =
         cast<CXXMemberCallExpr>(TheCall)->getImplicitObjectArgument();
 
@@ -6822,8 +6825,8 @@
       ThisType = Context.getPointerType(ThisType);
     }
 
-    QualType ThisTypeFromDecl =
-        Context.getPointerType(cast<CXXMethodDecl>(FDecl)->getThisObjectType());
+    QualType ThisTypeFromDecl = Context.getPointerType(
+        cast<CXXMethodDecl>(FDecl)->getFunctionObjectParameterType());
 
     CheckArgAlignment(TheCall->getRParenLoc(), FDecl, "'this'", ThisType,
                       ThisTypeFromDecl);
@@ -7150,13 +7153,13 @@
   if (Args.size() < AdjustedNumArgs) {
     Diag(CallRange.getEnd(), diag::err_typecheck_call_too_few_args)
         << 0 << AdjustedNumArgs << static_cast<unsigned>(Args.size())
-        << ExprRange;
+        << /*is non object*/ 0 << ExprRange;
     return ExprError();
   } else if (Args.size() > AdjustedNumArgs) {
     Diag(Args[AdjustedNumArgs]->getBeginLoc(),
          diag::err_typecheck_call_too_many_args)
         << 0 << AdjustedNumArgs << static_cast<unsigned>(Args.size())
-        << ExprRange;
+        << /*is non object*/ 0 << ExprRange;
     return ExprError();
   }
 
@@ -7519,7 +7522,8 @@
 bool Sema::BuiltinWasmRefNullFunc(CallExpr *TheCall) {
   if (TheCall->getNumArgs() != 0) {
     Diag(TheCall->getBeginLoc(), diag::err_typecheck_call_too_many_args)
-         << 0 /*function call*/ << 0 << TheCall->getNumArgs();
+        << 0 /*function call*/ << /*expected*/ 0 << TheCall->getNumArgs()
+        << /*is non object*/ 0;
     return true;
   }
 
@@ -7552,7 +7556,8 @@
   // Ensure that we have at least one argument to do type inference from.
   if (TheCall->getNumArgs() < 1) {
     Diag(TheCall->getEndLoc(), diag::err_typecheck_call_too_few_args_at_least)
-        << 0 << 1 << TheCall->getNumArgs() << Callee->getSourceRange();
+        << 0 << 1 << TheCall->getNumArgs() << /*is non object*/ 0
+        << Callee->getSourceRange();
     return ExprError();
   }
 
@@ -7828,7 +7833,7 @@
   // have at least that many.
   if (TheCall->getNumArgs() < 1+NumFixed) {
     Diag(TheCall->getEndLoc(), diag::err_typecheck_call_too_few_args_at_least)
-        << 0 << 1 + NumFixed << TheCall->getNumArgs()
+        << 0 << 1 + NumFixed << TheCall->getNumArgs() << /*is non object*/ 0
         << Callee->getSourceRange();
     return ExprError();
   }
@@ -8218,7 +8223,8 @@
   if (Call->getNumArgs() < 3)
     return Diag(Call->getEndLoc(),
                 diag::err_typecheck_call_too_few_args_at_least)
-           << 0 /*function call*/ << 3 << Call->getNumArgs();
+           << 0 /*function call*/ << 3 << Call->getNumArgs()
+           << /*is non object*/ 0;
 
   // Type-check the first argument normally.
   if (checkBuiltinArgument(*this, Call, 0))
@@ -8459,7 +8465,7 @@
     return ExprError(Diag(TheCall->getEndLoc(),
                           diag::err_typecheck_call_too_few_args_at_least)
                      << 0 /*function call*/ << 2 << TheCall->getNumArgs()
-                     << TheCall->getSourceRange());
+                     << /*is non object*/ 0 << TheCall->getSourceRange());
 
   // Determine which of the following types of shufflevector we're checking:
   // 1) unary, vector mask: (lhs, mask)
@@ -8579,7 +8585,8 @@
   if (NumArgs > 3)
     return Diag(TheCall->getEndLoc(),
                 diag::err_typecheck_call_too_many_args_at_most)
-           << 0 /*function call*/ << 3 << NumArgs << TheCall->getSourceRange();
+           << 0 /*function call*/ << 3 << NumArgs << /*is non object*/ 0
+           << TheCall->getSourceRange();
 
   // Argument 0 is checked for us and the remaining arguments must be
   // constant integers.
@@ -8718,13 +8725,13 @@
   if (NumArgs < NumRequiredArgs) {
     return Diag(TheCall->getEndLoc(), diag::err_typecheck_call_too_few_args)
            << 0 /* function call */ << NumRequiredArgs << NumArgs
-           << TheCall->getSourceRange();
+           << /*is non object*/ 0 << TheCall->getSourceRange();
   }
   if (NumArgs >= NumRequiredArgs + 0x100) {
     return Diag(TheCall->getEndLoc(),
                 diag::err_typecheck_call_too_many_args_at_most)
            << 0 /* function call */ << (NumRequiredArgs + 0xff) << NumArgs
-           << TheCall->getSourceRange();
+           << /*is non object*/ 0 << TheCall->getSourceRange();
   }
   unsigned i = 0;
 
diff --git a/clang/lib/Sema/SemaCoroutine.cpp b/clang/lib/Sema/SemaCoroutine.cpp
--- a/clang/lib/Sema/SemaCoroutine.cpp
+++ b/clang/lib/Sema/SemaCoroutine.cpp
@@ -70,7 +70,7 @@
   // If the function is a non-static member function, add the type
   // of the implicit object parameter before the formal parameters.
   if (auto *MD = dyn_cast<CXXMethodDecl>(FD)) {
-    if (MD->isInstance()) {
+    if (MD->isImplicitObjectMemberFunction()) {
       // [over.match.funcs]4
       // For non-static member functions, the type of the implicit object
       // parameter is
@@ -480,10 +480,10 @@
   assert(isa<FunctionDecl>(CurContext) && "not in a function scope");
   auto *FD = cast<FunctionDecl>(CurContext);
   bool IsThisDependentType = [&] {
-    if (auto *MD = dyn_cast_or_null<CXXMethodDecl>(FD))
-      return MD->isInstance() && MD->getThisType()->isDependentType();
-    else
-      return false;
+    if (const auto *MD = dyn_cast_if_present<CXXMethodDecl>(FD))
+      return MD->isImplicitObjectMemberFunction() &&
+             MD->getThisType()->isDependentType();
+    return false;
   }();
 
   QualType T = FD->getType()->isDependentType() || IsThisDependentType
@@ -508,7 +508,7 @@
 
   // Add implicit object parameter.
   if (auto *MD = dyn_cast<CXXMethodDecl>(FD)) {
-    if (MD->isInstance() && !isLambdaCallOperator(MD)) {
+    if (MD->isImplicitObjectMemberFunction() && !isLambdaCallOperator(MD)) {
       ExprResult ThisExpr = ActOnCXXThis(Loc);
       if (ThisExpr.isInvalid())
         return nullptr;
@@ -1283,7 +1283,7 @@
 static bool collectPlacementArgs(Sema &S, FunctionDecl &FD, SourceLocation Loc,
                                  SmallVectorImpl<Expr *> &PlacementArgs) {
   if (auto *MD = dyn_cast<CXXMethodDecl>(&FD)) {
-    if (MD->isInstance() && !isLambdaCallOperator(MD)) {
+    if (MD->isImplicitObjectMemberFunction() && !isLambdaCallOperator(MD)) {
       ExprResult ThisExpr = S.ActOnCXXThis(Loc);
       if (ThisExpr.isInvalid())
         return false;
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -8847,9 +8847,10 @@
                      /*ConsiderCudaAttrs=*/true,
                      // C++2a [class.virtual]p2 does not consider requires
                      // clauses when overriding.
-                     /*ConsiderRequiresClauses=*/false))
+                     /*UseOverrideRules=*/true))
+        continue;
+      if (!CheckOverridingExplicitObjectMembers(MD, BaseMD))
         continue;
-
       if (Overridden.insert(BaseMD).second) {
         MD->addOverriddenMethod(BaseMD);
         CheckOverridingFunctionReturnType(MD, BaseMD);
@@ -9183,6 +9184,8 @@
   }
   Expr *TrailingRequiresClause = D.getTrailingRequiresClause();
 
+  SemaRef.CheckExplicitObjectMemberFunction(DC, D, Name, R);
+
   if (Name.getNameKind() == DeclarationName::CXXConstructorName) {
     // This is a C++ constructor declaration.
     assert(DC->isRecord() &&
@@ -14718,9 +14721,32 @@
   }
 }
 
+static void CheckExplicitObjectParameter(Sema &S, ParmVarDecl *P,
+                                         SourceLocation ExplicitThisLoc) {
+  if (!ExplicitThisLoc.isValid())
+    return;
+  assert(S.getLangOpts().CPlusPlus &&
+         "explicit parameter in non-cplusplus mode");
+  if (!S.getLangOpts().CPlusPlus23)
+    S.Diag(ExplicitThisLoc, diag::err_cxx20_deducing_this)
+        << P->getSourceRange();
+
+  // C++2b [dcl.fct/7] An explicit object parameter shall not be a function
+  // parameter pack.
+  if (P->isParameterPack()) {
+    S.Diag(P->getBeginLoc(), diag::err_explicit_object_parameter_pack)
+        << P->getSourceRange();
+    return;
+  }
+  P->setExplicitObjectParameterLoc(ExplicitThisLoc);
+  if (LambdaScopeInfo *LSI = S.getCurLambda())
+    LSI->ExplicitObjectParameter = P;
+}
+
 /// ActOnParamDeclarator - Called from Parser::ParseFunctionDeclarator()
 /// to introduce parameters into function prototype scope.
-Decl *Sema::ActOnParamDeclarator(Scope *S, Declarator &D) {
+Decl *Sema::ActOnParamDeclarator(Scope *S, Declarator &D,
+                                 SourceLocation ExplicitThisLoc) {
   const DeclSpec &DS = D.getDeclSpec();
 
   // Verify C99 6.7.5.3p2: The only SCS allowed is 'register'.
@@ -14798,6 +14824,8 @@
   if (D.isInvalidType())
     New->setInvalidDecl();
 
+  CheckExplicitObjectParameter(*this, New, ExplicitThisLoc);
+
   assert(S->isFunctionPrototypeScope());
   assert(S->getFunctionPrototypeDepth() >= 1);
   New->setScopeInfo(S->getFunctionPrototypeDepth() - 1,
@@ -15191,6 +15219,8 @@
 
   LSI->IntroducerRange = DNI.getCXXOperatorNameRange();
   LSI->Mutable = !CallOperator->isConst();
+  if (CallOperator->isExplicitObjectMemberFunction())
+    LSI->ExplicitObjectParameter = CallOperator->getParamDecl(0);
 
   // Add the captures to the LSI so they can be noted as already
   // captured within tryCaptureVar.
@@ -18578,10 +18608,12 @@
   if (CSM == Sema::CXXDefaultConstructor)
     return bool(M1->getDescribedFunctionTemplate()) ==
            bool(M2->getDescribedFunctionTemplate());
-  if (!Context.hasSameType(M1->getParamDecl(0)->getType(),
-                           M2->getParamDecl(0)->getType()))
+  // https://github.com/cplusplus/CWG/issues/390
+  if (!Context.hasSameType(M1->getNonObjectParameter(0)->getType(),
+                           M2->getNonObjectParameter(0)->getType()))
     return false;
-  if (!Context.hasSameType(M1->getThisType(), M2->getThisType()))
+  if (!Context.hasSameType(M1->getFunctionObjectParameterReferenceType(),
+                           M2->getFunctionObjectParameterReferenceType()))
     return false;
 
   return true;
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -7611,7 +7611,7 @@
   unsigned ExpectedParams = 1;
   if (CSM == CXXDefaultConstructor || CSM == CXXDestructor)
     ExpectedParams = 0;
-  if (MD->getNumParams() != ExpectedParams) {
+  if (MD->getNumExplicitParams() != ExpectedParams) {
     // This checks for default arguments: a copy or move constructor with a
     // default argument is classified as a default constructor, and assignment
     // operations and destructors can't have default arguments.
@@ -7640,10 +7640,12 @@
   if (CSM == CXXCopyAssignment || CSM == CXXMoveAssignment) {
     // Check for return type matching.
     ReturnType = Type->getReturnType();
+    QualType ThisType = MD->getFunctionObjectParameterType();
 
     QualType DeclType = Context.getTypeDeclType(RD);
     DeclType = Context.getElaboratedType(ETK_None, nullptr, DeclType, nullptr);
-    DeclType = Context.getAddrSpaceQualType(DeclType, MD->getMethodQualifiers().getAddressSpace());
+    DeclType = Context.getAddrSpaceQualType(
+        DeclType, ThisType.getQualifiers().getAddressSpace());
     QualType ExpectedReturnType = Context.getLValueReferenceType(DeclType);
 
     if (!Context.hasSameType(ReturnType, ExpectedReturnType)) {
@@ -7653,7 +7655,7 @@
     }
 
     // A defaulted special member cannot have cv-qualifiers.
-    if (Type->getMethodQuals().hasConst() || Type->getMethodQuals().hasVolatile()) {
+    if (ThisType.isConstQualified() || ThisType.isVolatileQualified()) {
       if (DeleteOnTypeMismatch)
         ShouldDeleteForTypeMismatch = true;
       else {
@@ -7665,7 +7667,10 @@
   }
 
   // Check for parameter type matching.
-  QualType ArgType = ExpectedParams ? Type->getParamType(0) : QualType();
+  QualType ArgType =
+      ExpectedParams
+          ? Type->getParamType(MD->isExplicitObjectMemberFunction() ? 1 : 0)
+          : QualType();
   bool HasConstParam = false;
   if (ExpectedParams && ArgType->isReferenceType()) {
     // Argument must be reference to possibly-const T.
@@ -7759,8 +7764,8 @@
       FunctionProtoType::ExtProtoInfo EPI = Type->getExtProtoInfo();
       EPI.ExceptionSpec.Type = EST_Unevaluated;
       EPI.ExceptionSpec.SourceDecl = MD;
-      MD->setType(Context.getFunctionType(
-          ReturnType, llvm::ArrayRef(&ArgType, ExpectedParams), EPI));
+      MD->setType(
+          Context.getFunctionType(ReturnType, Type->getParamTypes(), EPI));
     }
   }
 
@@ -8411,7 +8416,8 @@
   ExprPair getCompleteObject() {
     unsigned Param = 0;
     ExprResult LHS;
-    if (isa<CXXMethodDecl>(FD)) {
+    if (const auto *MD = dyn_cast<CXXMethodDecl>(FD);
+        MD && MD->isImplicitObjectMemberFunction()) {
       // LHS is '*this'.
       LHS = S.ActOnCXXThis(Loc);
       if (!LHS.isInvalid())
@@ -8717,15 +8723,22 @@
     // If we're out-of-class, this is the class we're comparing.
     if (!RD)
       RD = MD->getParent();
-
-    if (!MD->isConst()) {
-      SourceLocation InsertLoc;
-      if (FunctionTypeLoc Loc = MD->getFunctionTypeLoc())
-        InsertLoc = getLocForEndOfToken(Loc.getRParenLoc());
+    QualType T = MD->getFunctionObjectParameterType();
+    if (!T.isConstQualified()) {
+      SourceLocation Loc, InsertLoc;
+      if (MD->isExplicitObjectMemberFunction()) {
+        Loc = MD->getParamDecl(0)->getBeginLoc();
+        InsertLoc = getLocForEndOfToken(
+            MD->getParamDecl(0)->getExplicitObjectParamThisLoc());
+      } else {
+        Loc = MD->getLocation();
+        if (FunctionTypeLoc Loc = MD->getFunctionTypeLoc())
+          InsertLoc = Loc.getRParenLoc();
+      }
       // Don't diagnose an implicit 'operator=='; we will have diagnosed the
       // corresponding defaulted 'operator<=>' already.
       if (!MD->isImplicit()) {
-        Diag(MD->getLocation(), diag::err_defaulted_comparison_non_const)
+        Diag(Loc, diag::err_defaulted_comparison_non_const)
             << (int)DCK << FixItHint::CreateInsertion(InsertLoc, " const");
       }
 
@@ -8749,7 +8762,9 @@
     }
   }
 
-  if (FD->getNumParams() != (IsMethod ? 1 : 2)) {
+  if ((FD->getNumParams() -
+       (unsigned)FD->hasCXXExplicitFunctionObjectParameter()) !=
+      (IsMethod ? 1 : 2)) {
     // Let's not worry about using a variadic template pack here -- who would do
     // such a thing?
     Diag(FD->getLocation(), diag::err_defaulted_comparison_num_args)
@@ -8759,6 +8774,8 @@
 
   const ParmVarDecl *KnownParm = nullptr;
   for (const ParmVarDecl *Param : FD->parameters()) {
+    if (Param->isExplicitObjectParameter())
+      continue;
     QualType ParmTy = Param->getType();
 
     if (!KnownParm) {
@@ -9142,9 +9159,9 @@
       llvm_unreachable("invalid special member kind");
     }
 
-    if (MD->getNumParams()) {
+    if (MD->getNumExplicitParams()) {
       if (const ReferenceType *RT =
-              MD->getParamDecl(0)->getType()->getAs<ReferenceType>())
+              MD->getNonObjectParameter(0)->getType()->getAs<ReferenceType>())
         ConstArg = RT->getPointeeType().isConstQualified();
     }
   }
@@ -10024,7 +10041,7 @@
 
   case CXXCopyConstructor:
   case CXXCopyAssignment: {
-    const ParmVarDecl *Param0 = MD->getParamDecl(0);
+    const ParmVarDecl *Param0 = MD->getNonObjectParameter(0);
     const ReferenceType *RT = Param0->getType()->getAs<ReferenceType>();
 
     // When ClangABICompat14 is true, CXX copy constructors will only be trivial
@@ -10055,7 +10072,7 @@
   case CXXMoveConstructor:
   case CXXMoveAssignment: {
     // Trivial move operations always have non-cv-qualified parameters.
-    const ParmVarDecl *Param0 = MD->getParamDecl(0);
+    const ParmVarDecl *Param0 = MD->getNonObjectParameter(0);
     const RValueReferenceType *RT =
       Param0->getType()->getAs<RValueReferenceType>();
     if (!RT || RT->getPointeeType().getCVRQualifiers()) {
@@ -11021,15 +11038,25 @@
         << SourceRange(D.getIdentifierLoc()) << 0;
     D.setInvalidType();
   }
-
   const auto *Proto = R->castAs<FunctionProtoType>();
-
   // Make sure we don't have any parameters.
-  if (Proto->getNumParams() > 0) {
-    Diag(D.getIdentifierLoc(), diag::err_conv_function_with_params);
+  DeclaratorChunk::FunctionTypeInfo &FTI = D.getFunctionTypeInfo();
+  unsigned NumParam = Proto->getNumParams();
 
+  // [C++2b]
+  // A conversion function shall have no non-object parameters.
+  if (NumParam == 1) {
+    DeclaratorChunk::FunctionTypeInfo &FTI = D.getFunctionTypeInfo();
+    if (const auto *First =
+            dyn_cast_if_present<ParmVarDecl>(FTI.Params[0].Param);
+        First && First->isExplicitObjectParameter())
+      NumParam--;
+  }
+
+  if (NumParam != 0) {
+    Diag(D.getIdentifierLoc(), diag::err_conv_function_with_params);
     // Delete the parameters.
-    D.getFunctionTypeInfo().freeParams();
+    FTI.freeParams();
     D.setInvalidType();
   } else if (Proto->isVariadic()) {
     Diag(D.getIdentifierLoc(), diag::err_conv_function_variadic);
@@ -11187,6 +11214,103 @@
   return Conversion;
 }
 
+void Sema::CheckExplicitObjectMemberFunction(DeclContext *DC, Declarator &D,
+                                             DeclarationName Name, QualType R) {
+  CheckExplicitObjectMemberFunction(D, Name, R, false, DC);
+}
+
+void Sema::CheckExplicitObjectLambda(Declarator &D) {
+  CheckExplicitObjectMemberFunction(D, {}, {}, true);
+}
+
+void Sema::CheckExplicitObjectMemberFunction(Declarator &D,
+                                             DeclarationName Name, QualType R,
+                                             bool IsLambda, DeclContext *DC) {
+  if (!D.isFunctionDeclarator())
+    return;
+
+  DeclaratorChunk::FunctionTypeInfo &FTI = D.getFunctionTypeInfo();
+  if (FTI.NumParams == 0)
+    return;
+  ParmVarDecl *ExplicitObjectParam = nullptr;
+  for (unsigned Idx = 0; Idx < FTI.NumParams; Idx++) {
+    const auto &ParamInfo = FTI.Params[Idx];
+    if (!ParamInfo.Param)
+      continue;
+    ParmVarDecl *Param = cast<ParmVarDecl>(ParamInfo.Param);
+    if (!Param->isExplicitObjectParameter())
+      continue;
+    if (Idx == 0) {
+      ExplicitObjectParam = Param;
+      continue;
+    } else {
+      Diag(Param->getLocation(),
+           diag::err_explicit_object_parameter_must_be_first)
+          << IsLambda << Param->getSourceRange();
+    }
+  }
+  if (!ExplicitObjectParam)
+    return;
+
+  if (ExplicitObjectParam->hasDefaultArg()) {
+    Diag(ExplicitObjectParam->getLocation(),
+         diag::err_explicit_object_default_arg)
+        << ExplicitObjectParam->getSourceRange();
+  }
+
+  if (D.getDeclSpec().getStorageClassSpec() == DeclSpec::SCS_static) {
+    Diag(ExplicitObjectParam->getBeginLoc(),
+         diag::err_explicit_object_parameter_nonmember)
+        << D.getSourceRange() << /*static=*/0 << IsLambda;
+    return;
+  }
+
+  if (D.getDeclSpec().isVirtualSpecified()) {
+    Diag(ExplicitObjectParam->getBeginLoc(),
+         diag::err_explicit_object_parameter_nonmember)
+        << D.getSourceRange() << /*virtual=*/1 << IsLambda;
+  }
+
+  if (FTI.hasMutableQualifier()) {
+    Diag(ExplicitObjectParam->getBeginLoc(),
+         diag::err_explicit_object_parameter_qualifiers)
+        << D.getSourceRange() << IsLambda << /*mutable=*/1;
+  }
+
+  if (IsLambda)
+    return;
+
+  if (!DC || !DC->isRecord()) {
+    Diag(ExplicitObjectParam->getLocation(),
+         diag::err_explicit_object_parameter_nonmember)
+        << D.getSourceRange() << /*non-member=*/2 << IsLambda;
+    return;
+  }
+
+  if (FTI.hasMethodTypeQualifiers()) {
+    if (FTI.getConstQualifierLoc().isValid())
+      Diag(FTI.getConstQualifierLoc(),
+           diag::err_explicit_object_parameter_qualifiers)
+          << /*function=*/0 << /*const=*/0 << D.getSourceRange();
+    else
+      // Generic message for the less common qualifiers
+      Diag(D.getBeginLoc(), diag::err_explicit_object_parameter_qualifiers)
+          << /*function=*/0 << /*qualifiers=*/3 << D.getSourceRange();
+  }
+  if (FTI.hasRefQualifier())
+    Diag(FTI.getRefQualifierLoc(),
+         diag::err_explicit_object_parameter_qualifiers)
+        << /*function=*/0 << /*reference=*/2 << D.getSourceRange();
+
+  // CWG2674: constructors and destructors cannot have explicit parameters.
+  if (Name.getNameKind() == DeclarationName::CXXConstructorName ||
+      Name.getNameKind() == DeclarationName::CXXDestructorName)
+    Diag(ExplicitObjectParam->getBeginLoc(),
+         diag::err_explicit_object_parameter_constructor)
+        << (Name.getNameKind() == DeclarationName::CXXDestructorName)
+        << D.getSourceRange();
+}
+
 namespace {
 /// Utility class to accumulate and print a diagnostic listing the invalid
 /// specifier(s) on a declaration.
@@ -14781,12 +14905,11 @@
   SmallVector<Stmt*, 8> Statements;
 
   // The parameter for the "other" object, which we are copying from.
-  ParmVarDecl *Other = CopyAssignOperator->getParamDecl(0);
+  ParmVarDecl *Other = CopyAssignOperator->getNonObjectParameter(0);
   Qualifiers OtherQuals = Other->getType().getQualifiers();
   QualType OtherRefType = Other->getType();
-  if (const LValueReferenceType *OtherRef
-                                = OtherRefType->getAs<LValueReferenceType>()) {
-    OtherRefType = OtherRef->getPointeeType();
+  if (OtherRefType->isLValueReferenceType()) {
+    OtherRefType = OtherRefType->getPointeeType();
     OtherQuals = OtherRefType.getQualifiers();
   }
 
@@ -14798,8 +14921,25 @@
   // Builds a DeclRefExpr for the "other" object.
   RefBuilder OtherRef(Other, OtherRefType);
 
-  // Builds the "this" pointer.
-  ThisBuilder This;
+  // Builds the function object parameter.
+  std::optional<ThisBuilder> This;
+  std::optional<DerefBuilder> DerefThis;
+  std::optional<RefBuilder> ExplicitObject;
+  bool IsArrow = false;
+  QualType ObjectType;
+  if (CopyAssignOperator->isExplicitObjectMemberFunction()) {
+    ObjectType = CopyAssignOperator->getParamDecl(0)->getType();
+    if (ObjectType->isReferenceType())
+      ObjectType = ObjectType->getPointeeType();
+    ExplicitObject.emplace(CopyAssignOperator->getParamDecl(0), ObjectType);
+  } else {
+    ObjectType = getCurrentThisType();
+    This.emplace();
+    DerefThis.emplace(*This);
+    IsArrow = !LangOpts.HLSL;
+  }
+  ExprBuilder &ObjectParameter =
+      ExplicitObject ? (ExprBuilder &)*ExplicitObject : (ExprBuilder &)*This;
 
   // Assign base classes.
   bool Invalid = false;
@@ -14821,11 +14961,11 @@
                      VK_LValue, BasePath);
 
     // Dereference "this".
-    DerefBuilder DerefThis(This);
-    CastBuilder To(DerefThis,
-                   Context.getQualifiedType(
-                       BaseType, CopyAssignOperator->getMethodQualifiers()),
-                   VK_LValue, BasePath);
+    CastBuilder To(
+        ExplicitObject ? (ExprBuilder &)*ExplicitObject
+                       : (ExprBuilder &)*DerefThis,
+        Context.getQualifiedType(BaseType, ObjectType.getQualifiers()),
+        VK_LValue, BasePath);
 
     // Build the copy.
     StmtResult Copy = buildSingleCopyAssign(*this, Loc, BaseType,
@@ -14891,10 +15031,7 @@
     MemberLookup.resolveKind();
 
     MemberBuilder From(OtherRef, OtherRefType, /*IsArrow=*/false, MemberLookup);
-
-    MemberBuilder To(This, getCurrentThisType(), /*IsArrow=*/!LangOpts.HLSL,
-                     MemberLookup);
-
+    MemberBuilder To(ObjectParameter, ObjectType, IsArrow, MemberLookup);
     // Build the copy of this field.
     StmtResult Copy = buildSingleCopyAssign(*this, Loc, FieldType,
                                             To, From,
@@ -14911,15 +15048,10 @@
 
   if (!Invalid) {
     // Add a "return *this;"
-    Expr *ThisExpr = nullptr;
-    if (!LangOpts.HLSL) {
-      ExprResult ThisObj =
-          CreateBuiltinUnaryOp(Loc, UO_Deref, This.build(*this, Loc));
-      ThisExpr = ThisObj.get();
-    } else {
-      ThisExpr = This.build(*this, Loc);
-    }
-
+    Expr *ThisExpr = (ExplicitObject  ? (ExprBuilder &)*ExplicitObject
+                      : LangOpts.HLSL ? (ExprBuilder &)*This
+                                      : (ExprBuilder &)*DerefThis)
+                         .build(*this, Loc);
     StmtResult Return = BuildReturnStmt(Loc, ThisExpr);
     if (Return.isInvalid())
       Invalid = true;
@@ -15150,7 +15282,7 @@
   SmallVector<Stmt*, 8> Statements;
 
   // The parameter for the "other" object, which we are move from.
-  ParmVarDecl *Other = MoveAssignOperator->getParamDecl(0);
+  ParmVarDecl *Other = MoveAssignOperator->getNonObjectParameter(0);
   QualType OtherRefType =
       Other->getType()->castAs<RValueReferenceType>()->getPointeeType();
 
@@ -15164,8 +15296,23 @@
   // Cast to rvalue.
   MoveCastBuilder MoveOther(OtherRef);
 
-  // Builds the "this" pointer.
-  ThisBuilder This;
+  // Builds the function object parameter.
+  std::optional<ThisBuilder> This;
+  std::optional<DerefBuilder> DerefThis;
+  std::optional<RefBuilder> ExplicitObject;
+  QualType ObjectType;
+  if (MoveAssignOperator->isExplicitObjectMemberFunction()) {
+    ObjectType = MoveAssignOperator->getParamDecl(0)->getType();
+    if (ObjectType->isReferenceType())
+      ObjectType = ObjectType->getPointeeType();
+    ExplicitObject.emplace(MoveAssignOperator->getParamDecl(0), ObjectType);
+  } else {
+    ObjectType = getCurrentThisType();
+    This.emplace();
+    DerefThis.emplace(*This);
+  }
+  ExprBuilder &ObjectParameter =
+      ExplicitObject ? (ExprBuilder &)*ExplicitObject : (ExprBuilder &)*This;
 
   // Assign base classes.
   bool Invalid = false;
@@ -15193,14 +15340,13 @@
     // appropriately-qualified base type.
     CastBuilder From(OtherRef, BaseType, VK_XValue, BasePath);
 
-    // Dereference "this".
-    DerefBuilder DerefThis(This);
-
     // Implicitly cast "this" to the appropriately-qualified base type.
-    CastBuilder To(DerefThis,
-                   Context.getQualifiedType(
-                       BaseType, MoveAssignOperator->getMethodQualifiers()),
-                   VK_LValue, BasePath);
+    // Dereference "this".
+    CastBuilder To(
+        ExplicitObject ? (ExprBuilder &)*ExplicitObject
+                       : (ExprBuilder &)*DerefThis,
+        Context.getQualifiedType(BaseType, ObjectType.getQualifiers()),
+        VK_LValue, BasePath);
 
     // Build the move.
     StmtResult Move = buildSingleCopyAssign(*this, Loc, BaseType,
@@ -15265,8 +15411,8 @@
     MemberLookup.resolveKind();
     MemberBuilder From(MoveOther, OtherRefType,
                        /*IsArrow=*/false, MemberLookup);
-    MemberBuilder To(This, getCurrentThisType(),
-                     /*IsArrow=*/true, MemberLookup);
+    MemberBuilder To(ObjectParameter, ObjectType, /*IsArrow=*/!ExplicitObject,
+                     MemberLookup);
 
     assert(!From.build(*this, Loc)->isLValue() && // could be xvalue or prvalue
         "Member reference with rvalue base must be rvalue except for reference "
@@ -15288,10 +15434,11 @@
 
   if (!Invalid) {
     // Add a "return *this;"
-    ExprResult ThisObj =
-        CreateBuiltinUnaryOp(Loc, UO_Deref, This.build(*this, Loc));
+    Expr *ThisExpr = (ExplicitObject ? (ExprBuilder &)*ExplicitObject
+                                     : (ExprBuilder &)*DerefThis)
+                         .build(*this, Loc);
 
-    StmtResult Return = BuildReturnStmt(Loc, ThisObj.get());
+    StmtResult Return = BuildReturnStmt(Loc, ThisExpr);
     if (Return.isInvalid())
       Invalid = true;
     else
@@ -15610,7 +15757,9 @@
   CXXRecordDecl *Lambda = Conv->getParent();
   FunctionDecl *CallOp = Lambda->getLambdaCallOperator();
   FunctionDecl *Invoker =
-      CallOp->isStatic() ? CallOp : Lambda->getLambdaStaticInvoker(CC);
+      CallOp->hasCXXExplicitFunctionObjectParameter() || CallOp->isStatic()
+          ? CallOp
+          : Lambda->getLambdaStaticInvoker(CC);
 
   if (auto *TemplateArgs = Conv->getTemplateSpecializationArgs()) {
     CallOp = InstantiateFunctionDeclaration(
@@ -16216,8 +16365,11 @@
   //   [...] Operator functions cannot have more or fewer parameters
   //   than the number required for the corresponding operator, as
   //   described in the rest of this subclause.
-  unsigned NumParams = FnDecl->getNumParams()
-                     + (isa<CXXMethodDecl>(FnDecl)? 1 : 0);
+  unsigned NumParams = FnDecl->getNumParams() +
+                       (isa<CXXMethodDecl>(FnDecl) &&
+                                !FnDecl->hasCXXExplicitFunctionObjectParameter()
+                            ? 1
+                            : 0);
   if (Op != OO_Call && Op != OO_Subscript &&
       ((NumParams == 1 && !CanBeUnaryOperator) ||
        (NumParams == 2 && !CanBeBinaryOperator) || (NumParams < 1) ||
@@ -18015,6 +18167,20 @@
   return true;
 }
 
+bool Sema::CheckOverridingExplicitObjectMembers(CXXMethodDecl *New,
+                                                const CXXMethodDecl *Old) {
+  // CWG2553
+  // A virtual function shall not be an explicit object member function.
+  if (!New->isExplicitObjectMemberFunction())
+    return true;
+  Diag(New->getParamDecl(0)->getBeginLoc(),
+       diag::err_explicit_object_parameter_nonmember)
+      << New->getSourceRange() << /*virtual*/ 1 << /*IsLambda*/ false;
+  Diag(Old->getLocation(), diag::note_overridden_virtual_function);
+  New->setInvalidDecl();
+  return false;
+}
+
 bool Sema::CheckOverridingFunctionReturnType(const CXXMethodDecl *New,
                                              const CXXMethodDecl *Old) {
   QualType NewTy = New->getType()->castAs<FunctionType>()->getReturnType();
diff --git a/clang/lib/Sema/SemaExceptionSpec.cpp b/clang/lib/Sema/SemaExceptionSpec.cpp
--- a/clang/lib/Sema/SemaExceptionSpec.cpp
+++ b/clang/lib/Sema/SemaExceptionSpec.cpp
@@ -769,14 +769,12 @@
 /// CheckExceptionSpecSubset - Check whether the second function type's
 /// exception specification is a subset (or equivalent) of the first function
 /// type. This is used by override and pointer assignment checks.
-bool Sema::CheckExceptionSpecSubset(const PartialDiagnostic &DiagID,
-                                    const PartialDiagnostic &NestedDiagID,
-                                    const PartialDiagnostic &NoteID,
-                                    const PartialDiagnostic &NoThrowDiagID,
-                                    const FunctionProtoType *Superset,
-                                    SourceLocation SuperLoc,
-                                    const FunctionProtoType *Subset,
-                                    SourceLocation SubLoc) {
+bool Sema::CheckExceptionSpecSubset(
+    const PartialDiagnostic &DiagID, const PartialDiagnostic &NestedDiagID,
+    const PartialDiagnostic &NoteID, const PartialDiagnostic &NoThrowDiagID,
+    const FunctionProtoType *Superset, bool SkipSupersetFirstParameter,
+    SourceLocation SuperLoc, const FunctionProtoType *Subset,
+    bool SkipSubsetFirstParameter, SourceLocation SubLoc) {
 
   // Just auto-succeed under -fno-exceptions.
   if (!getLangOpts().CXXExceptions)
@@ -816,8 +814,9 @@
   // done.
   if ((SuperCanThrow == CT_Can && SuperEST != EST_Dynamic) ||
       SubCanThrow == CT_Cannot)
-    return CheckParamExceptionSpec(NestedDiagID, NoteID, Superset, SuperLoc,
-                                   Subset, SubLoc);
+    return CheckParamExceptionSpec(NestedDiagID, NoteID, Superset,
+                                   SkipSupersetFirstParameter, SuperLoc, Subset,
+                                   SkipSubsetFirstParameter, SubLoc);
 
   // Allow __declspec(nothrow) to be missing on redeclaration as an extension in
   // some cases.
@@ -869,8 +868,9 @@
     }
   }
   // We've run half the gauntlet.
-  return CheckParamExceptionSpec(NestedDiagID, NoteID, Superset, SuperLoc,
-                                 Subset, SubLoc);
+  return CheckParamExceptionSpec(NestedDiagID, NoteID, Superset,
+                                 SkipSupersetFirstParameter, SuperLoc, Subset,
+                                 SkipSupersetFirstParameter, SubLoc);
 }
 
 static bool
@@ -894,12 +894,11 @@
 /// assignment and override compatibility check. We do not check the parameters
 /// of parameter function pointers recursively, as no sane programmer would
 /// even be able to write such a function type.
-bool Sema::CheckParamExceptionSpec(const PartialDiagnostic &DiagID,
-                                   const PartialDiagnostic &NoteID,
-                                   const FunctionProtoType *Target,
-                                   SourceLocation TargetLoc,
-                                   const FunctionProtoType *Source,
-                                   SourceLocation SourceLoc) {
+bool Sema::CheckParamExceptionSpec(
+    const PartialDiagnostic &DiagID, const PartialDiagnostic &NoteID,
+    const FunctionProtoType *Target, bool SkipTargetFirstParameter,
+    SourceLocation TargetLoc, const FunctionProtoType *Source,
+    bool SkipSourceFirstParameter, SourceLocation SourceLoc) {
   auto RetDiag = DiagID;
   RetDiag << 0;
   if (CheckSpecForTypesEquivalent(
@@ -910,14 +909,16 @@
 
   // We shouldn't even be testing this unless the arguments are otherwise
   // compatible.
-  assert(Target->getNumParams() == Source->getNumParams() &&
+  assert((Target->getNumParams() - (unsigned)SkipTargetFirstParameter) ==
+             (Source->getNumParams() - (unsigned)SkipSourceFirstParameter) &&
          "Functions have different argument counts.");
   for (unsigned i = 0, E = Target->getNumParams(); i != E; ++i) {
     auto ParamDiag = DiagID;
     ParamDiag << 1;
     if (CheckSpecForTypesEquivalent(
             *this, ParamDiag, PDiag(),
-            Target->getParamType(i), TargetLoc, Source->getParamType(i),
+            Target->getParamType(i + (SkipTargetFirstParameter ? 1 : 0)),
+            TargetLoc, Source->getParamType(SkipSourceFirstParameter ? 1 : 0),
             SourceLoc))
       return true;
   }
@@ -958,9 +959,10 @@
   //     void (*q)(void (*) throw(int)) = p;
   //   }
   // ... because it might be instantiated with T=int.
-  return CheckExceptionSpecSubset(
-             PDiag(DiagID), PDiag(NestedDiagID), PDiag(), PDiag(), ToFunc,
-             From->getSourceRange().getBegin(), FromFunc, SourceLocation()) &&
+  return CheckExceptionSpecSubset(PDiag(DiagID), PDiag(NestedDiagID), PDiag(),
+                                  PDiag(), ToFunc, 0,
+                                  From->getSourceRange().getBegin(), FromFunc,
+                                  0, SourceLocation()) &&
          !getLangOpts().CPlusPlus17;
 }
 
@@ -989,14 +991,14 @@
   unsigned DiagID = diag::err_override_exception_spec;
   if (getLangOpts().MSVCCompat)
     DiagID = diag::ext_override_exception_spec;
-  return CheckExceptionSpecSubset(PDiag(DiagID),
-                                  PDiag(diag::err_deep_exception_specs_differ),
-                                  PDiag(diag::note_overridden_virtual_function),
-                                  PDiag(diag::ext_override_exception_spec),
-                                  Old->getType()->castAs<FunctionProtoType>(),
-                                  Old->getLocation(),
-                                  New->getType()->castAs<FunctionProtoType>(),
-                                  New->getLocation());
+  return CheckExceptionSpecSubset(
+      PDiag(DiagID), PDiag(diag::err_deep_exception_specs_differ),
+      PDiag(diag::note_overridden_virtual_function),
+      PDiag(diag::ext_override_exception_spec),
+      Old->getType()->castAs<FunctionProtoType>(),
+      Old->hasCXXExplicitFunctionObjectParameter(), Old->getLocation(),
+      New->getType()->castAs<FunctionProtoType>(),
+      New->hasCXXExplicitFunctionObjectParameter(), New->getLocation());
 }
 
 static CanThrowResult canSubStmtsThrow(Sema &Self, const Stmt *S) {
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -2355,7 +2355,7 @@
   // FIXME: Is this special case necessary? We could allow the caller to
   // diagnose this.
   if (isDefaultArgument && ((*R.begin())->isCXXInstanceMember())) {
-    Diag(R.getNameLoc(), diag::err_member_call_without_object);
+    Diag(R.getNameLoc(), diag::err_member_call_without_object) << 0;
     return true;
   }
 
@@ -3128,7 +3128,7 @@
       FromRecordType = FromType;
     }
   } else if (const auto *Method = dyn_cast<CXXMethodDecl>(Member)) {
-    if (Method->isStatic())
+    if (!Method->isImplicitObjectMemberFunction())
       return From;
 
     DestType = Method->getThisType();
@@ -6437,6 +6437,9 @@
 
   // C99 6.5.2.2p7 - the arguments are implicitly converted, as if by
   // assignment, to the types of the corresponding parameter, ...
+  bool HasExplicitObjectParameter =
+      FDecl && FDecl->hasCXXExplicitFunctionObjectParameter();
+  unsigned ExplicitObjectParameterOffset = HasExplicitObjectParameter ? 1 : 0;
   unsigned NumParams = Proto->getNumParams();
   bool Invalid = false;
   unsigned MinArgs = FDecl ? FDecl->getMinRequiredArguments() : NumParams;
@@ -6455,21 +6458,29 @@
             MinArgs == NumParams && !Proto->isVariadic()
                 ? diag::err_typecheck_call_too_few_args_suggest
                 : diag::err_typecheck_call_too_few_args_at_least_suggest;
-        diagnoseTypo(TC, PDiag(diag_id) << FnKind << MinArgs
-                                        << static_cast<unsigned>(Args.size())
-                                        << TC.getCorrectionRange());
-      } else if (MinArgs == 1 && FDecl && FDecl->getParamDecl(0)->getDeclName())
+        diagnoseTypo(
+            TC, PDiag(diag_id)
+                    << FnKind << MinArgs - ExplicitObjectParameterOffset
+                    << static_cast<unsigned>(Args.size()) -
+                           ExplicitObjectParameterOffset
+                    << HasExplicitObjectParameter << TC.getCorrectionRange());
+      } else if (MinArgs - ExplicitObjectParameterOffset == 1 && FDecl &&
+                 FDecl->getParamDecl(ExplicitObjectParameterOffset)
+                     ->getDeclName())
         Diag(RParenLoc,
              MinArgs == NumParams && !Proto->isVariadic()
                  ? diag::err_typecheck_call_too_few_args_one
                  : diag::err_typecheck_call_too_few_args_at_least_one)
-            << FnKind << FDecl->getParamDecl(0) << Fn->getSourceRange();
+            << FnKind << FDecl->getParamDecl(ExplicitObjectParameterOffset)
+            << HasExplicitObjectParameter << Fn->getSourceRange();
       else
         Diag(RParenLoc, MinArgs == NumParams && !Proto->isVariadic()
                             ? diag::err_typecheck_call_too_few_args
                             : diag::err_typecheck_call_too_few_args_at_least)
-            << FnKind << MinArgs << static_cast<unsigned>(Args.size())
-            << Fn->getSourceRange();
+            << FnKind << MinArgs - ExplicitObjectParameterOffset
+            << static_cast<unsigned>(Args.size()) -
+                   ExplicitObjectParameterOffset
+            << HasExplicitObjectParameter << Fn->getSourceRange();
 
       // Emit the location of the prototype.
       if (!TC && FDecl && !FDecl->getBuiltinID() && !IsExecConfig)
@@ -6494,17 +6505,23 @@
             MinArgs == NumParams && !Proto->isVariadic()
                 ? diag::err_typecheck_call_too_many_args_suggest
                 : diag::err_typecheck_call_too_many_args_at_most_suggest;
-        diagnoseTypo(TC, PDiag(diag_id) << FnKind << NumParams
-                                        << static_cast<unsigned>(Args.size())
-                                        << TC.getCorrectionRange());
-      } else if (NumParams == 1 && FDecl &&
-                 FDecl->getParamDecl(0)->getDeclName())
+        diagnoseTypo(
+            TC, PDiag(diag_id)
+                    << FnKind << NumParams - ExplicitObjectParameterOffset
+                    << static_cast<unsigned>(Args.size()) -
+                           ExplicitObjectParameterOffset
+                    << HasExplicitObjectParameter << TC.getCorrectionRange());
+      } else if (NumParams - ExplicitObjectParameterOffset == 1 && FDecl &&
+                 FDecl->getParamDecl(ExplicitObjectParameterOffset)
+                     ->getDeclName())
         Diag(Args[NumParams]->getBeginLoc(),
              MinArgs == NumParams
                  ? diag::err_typecheck_call_too_many_args_one
                  : diag::err_typecheck_call_too_many_args_at_most_one)
-            << FnKind << FDecl->getParamDecl(0)
-            << static_cast<unsigned>(Args.size()) << Fn->getSourceRange()
+            << FnKind << FDecl->getParamDecl(ExplicitObjectParameterOffset)
+            << static_cast<unsigned>(Args.size()) -
+                   ExplicitObjectParameterOffset
+            << HasExplicitObjectParameter << Fn->getSourceRange()
             << SourceRange(Args[NumParams]->getBeginLoc(),
                            Args.back()->getEndLoc());
       else
@@ -6512,8 +6529,10 @@
              MinArgs == NumParams
                  ? diag::err_typecheck_call_too_many_args
                  : diag::err_typecheck_call_too_many_args_at_most)
-            << FnKind << NumParams << static_cast<unsigned>(Args.size())
-            << Fn->getSourceRange()
+            << FnKind << NumParams - ExplicitObjectParameterOffset
+            << static_cast<unsigned>(Args.size()) -
+                   ExplicitObjectParameterOffset
+            << HasExplicitObjectParameter << Fn->getSourceRange()
             << SourceRange(Args[NumParams]->getBeginLoc(),
                            Args.back()->getEndLoc());
 
@@ -7589,9 +7608,9 @@
   }
 
   if (CXXMethodDecl *Method = dyn_cast_or_null<CXXMethodDecl>(FDecl))
-    if (!Method->isStatic())
+    if (Method->isImplicitObjectMemberFunction())
       return ExprError(Diag(LParenLoc, diag::err_member_call_without_object)
-        << Fn->getSourceRange());
+                       << Fn->getSourceRange() << 0);
 
   // Check for sentinels
   if (NDecl)
@@ -14865,6 +14884,33 @@
   S.Diag(Loc, diag::err_typecheck_address_of) << Type << E->getSourceRange();
 }
 
+bool Sema::DiagnoseMissingQualifiersInAddressOfOperand(
+    SourceLocation OpLoc, const Expr *Op, const CXXMethodDecl *MD) {
+  const auto *DRE = cast<DeclRefExpr>(Op->IgnoreParens());
+
+  if (Op != DRE)
+    return Diag(OpLoc, diag::err_parens_pointer_member_function)
+           << Op->getSourceRange();
+
+  // Taking the address of a dtor is illegal per C++ [class.dtor]p2.
+  if (isa<CXXDestructorDecl>(MD))
+    return Diag(OpLoc, diag::err_typecheck_addrof_dtor)
+           << DRE->getSourceRange();
+
+  if (DRE->getQualifier())
+    return false;
+
+  if (MD->getParent()->getName().empty())
+    return Diag(OpLoc, diag::err_unqualified_pointer_member_function)
+           << DRE->getSourceRange();
+
+  SmallString<32> Str;
+  StringRef Qual = (MD->getParent()->getName() + "::").toStringRef(Str);
+  return Diag(OpLoc, diag::err_unqualified_pointer_member_function)
+         << DRE->getSourceRange()
+         << FixItHint::CreateInsertion(DRE->getSourceRange().getBegin(), Qual);
+}
+
 /// CheckAddressOfOperand - The operand of & must be either a function
 /// designator or an lvalue designating an object. If it is an lvalue, the
 /// object cannot be declared with storage class register or be a bit field.
@@ -14974,28 +15020,7 @@
     DeclRefExpr *DRE = cast<DeclRefExpr>(op);
     CXXMethodDecl *MD = cast<CXXMethodDecl>(DRE->getDecl());
 
-    // The id-expression was parenthesized.
-    if (OrigOp.get() != DRE) {
-      Diag(OpLoc, diag::err_parens_pointer_member_function)
-        << OrigOp.get()->getSourceRange();
-
-    // The method was named without a qualifier.
-    } else if (!DRE->getQualifier()) {
-      if (MD->getParent()->getName().empty())
-        Diag(OpLoc, diag::err_unqualified_pointer_member_function)
-          << op->getSourceRange();
-      else {
-        SmallString<32> Str;
-        StringRef Qual = (MD->getParent()->getName() + "::").toStringRef(Str);
-        Diag(OpLoc, diag::err_unqualified_pointer_member_function)
-          << op->getSourceRange()
-          << FixItHint::CreateInsertion(op->getSourceRange().getBegin(), Qual);
-      }
-    }
-
-    // Taking the address of a dtor is illegal per C++ [class.dtor]p2.
-    if (isa<CXXDestructorDecl>(MD))
-      Diag(OpLoc, diag::err_typecheck_addrof_dtor) << op->getSourceRange();
+    DiagnoseMissingQualifiersInAddressOfOperand(OpLoc, OrigOp.get(), MD);
 
     QualType MPTy = Context.getMemberPointerType(
         op->getType(), Context.getTypeDeclType(MD->getParent()).getTypePtr());
@@ -15015,7 +15040,11 @@
           << op->getType() << op->getSourceRange();
         return QualType();
       }
+    } else if (const auto *DRE = dyn_cast<DeclRefExpr>(op)) {
+      if (const auto *MD = dyn_cast_or_null<CXXMethodDecl>(DRE->getDecl()))
+        DiagnoseMissingQualifiersInAddressOfOperand(OpLoc, OrigOp.get(), MD);
     }
+
   } else if (op->getObjectKind() == OK_BitField) { // C99 6.5.3.2p1
     // The operand cannot be a bit-field
     AddressOfError = AO_Bit_Field;
@@ -16406,7 +16435,7 @@
     if (isa<FieldDecl>(VD) || isa<IndirectFieldDecl>(VD))
       return true;
     if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(VD))
-      return Method->isInstance();
+      return Method->isImplicitObjectMemberFunction();
 
     return false;
   }
@@ -16417,7 +16446,7 @@
 
     for (NamedDecl *D : ULE->decls()) {
       if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(D)) {
-        if (Method->isInstance())
+        if (Method->isImplicitObjectMemberFunction())
           return true;
       } else {
         // Overload set does not contain methods.
@@ -19126,7 +19155,8 @@
     // private instances of the captured declarations.
     const Capture &Cap = CSI->getCapture(Var);
     if (Cap.isCopyCapture() &&
-        !(isa<LambdaScopeInfo>(CSI) && cast<LambdaScopeInfo>(CSI)->Mutable) &&
+        !(isa<LambdaScopeInfo>(CSI) &&
+          !cast<LambdaScopeInfo>(CSI)->lambdaCaptureShouldBeConst()) &&
         !(isa<CapturedRegionScopeInfo>(CSI) &&
           cast<CapturedRegionScopeInfo>(CSI)->CapRegionKind == CR_OpenMP))
       DeclRefType.addConst();
@@ -19443,7 +19473,8 @@
     //   declared const (9.3.1) if and only if the lambda-expression's
     //   parameter-declaration-clause is not followed by mutable.
     DeclRefType = CaptureType.getNonReferenceType();
-    if (!LSI->Mutable && !CaptureType->isReferenceType())
+    bool Const = LSI->lambdaCaptureShouldBeConst();
+    if (Const && !CaptureType->isReferenceType())
       DeclRefType.addConst();
   }
 
@@ -20510,6 +20541,34 @@
   DoMarkVarDeclReferenced(*this, Loc, Var, nullptr, RefsMinusAssignments);
 }
 
+// C++ [temp.dep.expr]p3:
+//   An id-expression is type-dependent if it contains:
+//     - an identifier associated by name lookup with an entity captured by copy
+//       in a lambda-expression that has an explicit object parameter whose type
+//       is dependent ([dcl.fct]),
+static void FixDependencyOfIdExpressionsInLambdaWithDependentObjectParameter(
+    Sema &SemaRef, ValueDecl *D, Expr *E) {
+  DeclRefExpr *ID = dyn_cast<DeclRefExpr>(E);
+  if (!ID || ID->isTypeDependent())
+    return;
+
+  auto IsDependent = [&]() {
+    const LambdaScopeInfo *LSI = SemaRef.getCurLambda();
+    if (!LSI)
+      return false;
+    if (!LSI->ExplicitObjectParameter ||
+        !LSI->ExplicitObjectParameter->getType()->isDependentType())
+      return false;
+    if (!LSI->CaptureMap.count(D))
+      return false;
+    const Capture &Cap = LSI->getCapture(D);
+    return !Cap.isCopyCapture();
+  }();
+
+  ID->setCapturedByCopyInLambdaWithExplicitObjectParameter(
+      IsDependent, SemaRef.getASTContext());
+}
+
 static void
 MarkExprReferenced(Sema &SemaRef, SourceLocation Loc, Decl *D, Expr *E,
                    bool MightBeOdrUse,
@@ -20519,14 +20578,19 @@
 
   if (VarDecl *Var = dyn_cast<VarDecl>(D)) {
     DoMarkVarDeclReferenced(SemaRef, Loc, Var, E, RefsMinusAssignments);
+    if (SemaRef.getLangOpts().CPlusPlus)
+      FixDependencyOfIdExpressionsInLambdaWithDependentObjectParameter(SemaRef,
+                                                                       Var, E);
     return;
   }
 
   if (BindingDecl *Decl = dyn_cast<BindingDecl>(D)) {
     DoMarkBindingDeclReferenced(SemaRef, Loc, Decl, E);
+    if (SemaRef.getLangOpts().CPlusPlus)
+      FixDependencyOfIdExpressionsInLambdaWithDependentObjectParameter(SemaRef,
+                                                                       Decl, E);
     return;
   }
-
   SemaRef.MarkAnyDeclReferenced(Loc, D, MightBeOdrUse);
 
   // If this is a call to a method via a cast, also mark the method in the
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -1153,7 +1153,7 @@
     auto C = CurLSI->getCXXThisCapture();
 
     if (C.isCopyCapture()) {
-      if (!CurLSI->Mutable)
+      if (CurLSI->lambdaCaptureShouldBeConst())
         ClassType.addConst();
       return ASTCtx.getPointerType(ClassType);
     }
@@ -1209,11 +1209,11 @@
   QualType ThisTy = CXXThisTypeOverride;
 
   if (CXXMethodDecl *method = dyn_cast<CXXMethodDecl>(DC)) {
-    if (method && method->isInstance())
+    if (method && method->isImplicitObjectMemberFunction())
       ThisTy = method->getThisType();
   }
 
-  if (ThisTy.isNull() && isLambdaCallOperator(CurContext) &&
+  if (ThisTy.isNull() && isLambdaCallWithImplicitObjectParameter(CurContext) &&
       inTemplateInstantiation() && isa<CXXRecordDecl>(DC)) {
 
     // This is a lambda call operator that is being instantiated as a default
@@ -1390,10 +1390,22 @@
   /// C++ 9.3.2: In the body of a non-static member function, the keyword this
   /// is a non-lvalue expression whose value is the address of the object for
   /// which the function is called.
-
   QualType ThisTy = getCurrentThisType();
-  if (ThisTy.isNull())
-    return Diag(Loc, diag::err_invalid_this_use);
+
+  if (ThisTy.isNull()) {
+    DeclContext *DC = getFunctionLevelDeclContext();
+
+    if (const auto *Method = dyn_cast<CXXMethodDecl>(DC);
+        Method && Method->isExplicitObjectMemberFunction()) {
+      return Diag(Loc, diag::err_invalid_this_use) << 1;
+    }
+
+    if (isLambdaCallWithExplicitObjectParameter(CurContext))
+      return Diag(Loc, diag::err_invalid_this_use) << 1;
+
+    return Diag(Loc, diag::err_invalid_this_use) << 0;
+  }
+
   return BuildCXXThisExpr(Loc, ThisTy, /*IsImplicit=*/false);
 }
 
@@ -4318,15 +4330,17 @@
     if (DiagnoseUseOfDecl(Fn, From->getBeginLoc()))
       return ExprError();
 
-    From = FixOverloadedFunctionReference(From, Found, Fn);
+    ExprResult Res = FixOverloadedFunctionReference(From, Found, Fn);
+    if (Res.isInvalid())
+      return ExprError();
 
     // We might get back another placeholder expression if we resolved to a
     // builtin.
-    ExprResult Checked = CheckPlaceholderExpr(From);
-    if (Checked.isInvalid())
+    Res = CheckPlaceholderExpr(Res.get());
+    if (Res.isInvalid())
       return ExprError();
 
-    From = Checked.get();
+    From = Res.get();
     FromType = From->getType();
   }
 
@@ -8038,68 +8052,6 @@
                                    Destructed);
 }
 
-ExprResult Sema::BuildCXXMemberCallExpr(Expr *E, NamedDecl *FoundDecl,
-                                        CXXConversionDecl *Method,
-                                        bool HadMultipleCandidates) {
-  // Convert the expression to match the conversion function's implicit object
-  // parameter.
-  ExprResult Exp = PerformObjectArgumentInitialization(E, /*Qualifier=*/nullptr,
-                                          FoundDecl, Method);
-  if (Exp.isInvalid())
-    return true;
-
-  if (Method->getParent()->isLambda() &&
-      Method->getConversionType()->isBlockPointerType()) {
-    // This is a lambda conversion to block pointer; check if the argument
-    // was a LambdaExpr.
-    Expr *SubE = E;
-    CastExpr *CE = dyn_cast<CastExpr>(SubE);
-    if (CE && CE->getCastKind() == CK_NoOp)
-      SubE = CE->getSubExpr();
-    SubE = SubE->IgnoreParens();
-    if (CXXBindTemporaryExpr *BE = dyn_cast<CXXBindTemporaryExpr>(SubE))
-      SubE = BE->getSubExpr();
-    if (isa<LambdaExpr>(SubE)) {
-      // For the conversion to block pointer on a lambda expression, we
-      // construct a special BlockLiteral instead; this doesn't really make
-      // a difference in ARC, but outside of ARC the resulting block literal
-      // follows the normal lifetime rules for block literals instead of being
-      // autoreleased.
-      PushExpressionEvaluationContext(
-          ExpressionEvaluationContext::PotentiallyEvaluated);
-      ExprResult BlockExp = BuildBlockForLambdaConversion(
-          Exp.get()->getExprLoc(), Exp.get()->getExprLoc(), Method, Exp.get());
-      PopExpressionEvaluationContext();
-
-      // FIXME: This note should be produced by a CodeSynthesisContext.
-      if (BlockExp.isInvalid())
-        Diag(Exp.get()->getExprLoc(), diag::note_lambda_to_block_conv);
-      return BlockExp;
-    }
-  }
-
-  MemberExpr *ME =
-      BuildMemberExpr(Exp.get(), /*IsArrow=*/false, SourceLocation(),
-                      NestedNameSpecifierLoc(), SourceLocation(), Method,
-                      DeclAccessPair::make(FoundDecl, FoundDecl->getAccess()),
-                      HadMultipleCandidates, DeclarationNameInfo(),
-                      Context.BoundMemberTy, VK_PRValue, OK_Ordinary);
-
-  QualType ResultType = Method->getReturnType();
-  ExprValueKind VK = Expr::getValueKindForType(ResultType);
-  ResultType = ResultType.getNonLValueExprType(Context);
-
-  CXXMemberCallExpr *CE = CXXMemberCallExpr::Create(
-      Context, ME, /*Args=*/{}, ResultType, VK, Exp.get()->getEndLoc(),
-      CurFPFeatureOverrides());
-
-  if (CheckFunctionCall(Method, CE,
-                        Method->getType()->castAs<FunctionProtoType>()))
-    return ExprError();
-
-  return CheckForImmediateInvocation(CE, CE->getMethodDecl());
-}
-
 ExprResult Sema::BuildCXXNoexceptExpr(SourceLocation KeyLoc, Expr *Operand,
                                       SourceLocation RParen) {
   // If the operand is an unresolved lookup expression, the expression is ill-
diff --git a/clang/lib/Sema/SemaExprMember.cpp b/clang/lib/Sema/SemaExprMember.cpp
--- a/clang/lib/Sema/SemaExprMember.cpp
+++ b/clang/lib/Sema/SemaExprMember.cpp
@@ -46,7 +46,7 @@
 
   /// The reference may be to an instance member, but it might be invalid if
   /// so, because the context is not an instance method.
-  IMA_Mixed_StaticContext,
+  IMA_Mixed_StaticOrExplicitContext,
 
   /// The reference may be to an instance member, but it is invalid if
   /// so, because the context is from an unrelated class.
@@ -63,7 +63,7 @@
 
   /// The reference may be to an unresolved using declaration and the
   /// context is not an instance method.
-  IMA_Unresolved_StaticContext,
+  IMA_Unresolved_StaticOrExplicitContext,
 
   // The reference refers to a field which is not a member of the containing
   // class, which is allowed because we're in C++11 mode and the context is
@@ -72,7 +72,7 @@
 
   /// All possible referrents are instance members and the current
   /// context is not an instance method.
-  IMA_Error_StaticContext,
+  IMA_Error_StaticOrExplicitContext,
 
   /// All possible referrents are instance members of an unrelated
   /// class.
@@ -91,11 +91,14 @@
 
   DeclContext *DC = SemaRef.getFunctionLevelDeclContext();
 
-  bool isStaticContext = SemaRef.CXXThisTypeOverride.isNull() &&
-    (!isa<CXXMethodDecl>(DC) || cast<CXXMethodDecl>(DC)->isStatic());
+  bool isStaticOrExplicitContext =
+      SemaRef.CXXThisTypeOverride.isNull() &&
+      (!isa<CXXMethodDecl>(DC) || cast<CXXMethodDecl>(DC)->isStatic() ||
+       cast<CXXMethodDecl>(DC)->isExplicitObjectMemberFunction());
 
   if (R.isUnresolvableResult())
-    return isStaticContext ? IMA_Unresolved_StaticContext : IMA_Unresolved;
+    return isStaticOrExplicitContext ? IMA_Unresolved_StaticOrExplicitContext
+                                     : IMA_Unresolved;
 
   // Collect all the declaring classes of instance members we find.
   bool hasNonInstance = false;
@@ -152,12 +155,12 @@
 
   // If the current context is not an instance method, it can't be
   // an implicit member reference.
-  if (isStaticContext) {
+  if (isStaticOrExplicitContext) {
     if (hasNonInstance)
-      return IMA_Mixed_StaticContext;
+      return IMA_Mixed_StaticOrExplicitContext;
 
     return AbstractInstanceResult ? AbstractInstanceResult
-                                  : IMA_Error_StaticContext;
+                                  : IMA_Error_StaticOrExplicitContext;
   }
 
   CXXRecordDecl *contextClass;
@@ -167,7 +170,7 @@
     contextClass = RD;
   else
     return AbstractInstanceResult ? AbstractInstanceResult
-                                  : IMA_Error_StaticContext;
+                                  : IMA_Error_StaticOrExplicitContext;
 
   // [class.mfct.non-static]p3:
   // ...is used in the body of a non-static member function of class X,
@@ -214,14 +217,31 @@
   CXXRecordDecl *RepClass = dyn_cast<CXXRecordDecl>(Rep->getDeclContext());
 
   bool InStaticMethod = Method && Method->isStatic();
+  bool InExplicitObjectMethod =
+      Method && Method->isExplicitObjectMemberFunction();
   bool IsField = isa<FieldDecl>(Rep) || isa<IndirectFieldDecl>(Rep);
 
+  std::string Replacement;
+  if (InExplicitObjectMethod) {
+    DeclarationName N = Method->getParamDecl(0)->getDeclName();
+    if (!N.isEmpty()) {
+      Replacement.append(N.getAsString());
+      Replacement.append(".");
+    }
+  }
   if (IsField && InStaticMethod)
     // "invalid use of member 'x' in static member function"
-    SemaRef.Diag(Loc, diag::err_invalid_member_use_in_static_method)
-        << Range << nameInfo.getName();
-  else if (ContextClass && RepClass && SS.isEmpty() && !InStaticMethod &&
-           !RepClass->Equals(ContextClass) && RepClass->Encloses(ContextClass))
+    SemaRef.Diag(Loc, diag::err_invalid_member_use_in_method)
+        << Range << nameInfo.getName() << /*static*/ 0;
+  else if (IsField && InExplicitObjectMethod) {
+    auto Diag = SemaRef.Diag(Loc, diag::err_invalid_member_use_in_method)
+                << Range << nameInfo.getName() << /*explicit*/ 1;
+    if (!Replacement.empty())
+      Diag << FixItHint::CreateInsertion(Loc, Replacement);
+  } else if (ContextClass && RepClass && SS.isEmpty() &&
+             !InExplicitObjectMethod && !InStaticMethod &&
+             !RepClass->Equals(ContextClass) &&
+             RepClass->Encloses(ContextClass))
     // Unqualified lookup in a non-static member function found a member of an
     // enclosing class.
     SemaRef.Diag(Loc, diag::err_nested_non_static_member_use)
@@ -229,9 +249,16 @@
   else if (IsField)
     SemaRef.Diag(Loc, diag::err_invalid_non_static_member_use)
       << nameInfo.getName() << Range;
-  else
+  else if (!InExplicitObjectMethod)
     SemaRef.Diag(Loc, diag::err_member_call_without_object)
-      << Range;
+        << Range << /*static*/ 0;
+  else {
+    const auto *Callee = dyn_cast<CXXMethodDecl>(Rep);
+    auto Diag = SemaRef.Diag(Loc, diag::err_member_call_without_object)
+                << Range << Callee->isExplicitObjectMemberFunction();
+    if (!Replacement.empty())
+      Diag << FixItHint::CreateInsertion(Loc, Replacement);
+  }
 }
 
 /// Builds an expression which might be an implicit member expression.
@@ -255,13 +282,13 @@
     [[fallthrough]];
   case IMA_Static:
   case IMA_Abstract:
-  case IMA_Mixed_StaticContext:
-  case IMA_Unresolved_StaticContext:
+  case IMA_Mixed_StaticOrExplicitContext:
+  case IMA_Unresolved_StaticOrExplicitContext:
     if (TemplateArgs || TemplateKWLoc.isValid())
       return BuildTemplateIdExpr(SS, TemplateKWLoc, R, false, TemplateArgs);
     return AsULE ? AsULE : BuildDeclarationNameExpr(SS, R, false);
 
-  case IMA_Error_StaticContext:
+  case IMA_Error_StaticOrExplicitContext:
   case IMA_Error_Unrelated:
     diagnoseInstanceReference(*this, SS, R.getRepresentativeDecl(),
                               R.getLookupNameInfo());
diff --git a/clang/lib/Sema/SemaExprObjC.cpp b/clang/lib/Sema/SemaExprObjC.cpp
--- a/clang/lib/Sema/SemaExprObjC.cpp
+++ b/clang/lib/Sema/SemaExprObjC.cpp
@@ -1800,7 +1800,8 @@
   // FIXME. This need be cleaned up.
   if (Args.size() < NumNamedArgs) {
     Diag(SelLoc, diag::err_typecheck_call_too_few_args)
-      << 2 << NumNamedArgs << static_cast<unsigned>(Args.size());
+        << 2 << NumNamedArgs << static_cast<unsigned>(Args.size())
+        << /*is non object*/ 0;
     return false;
   }
 
@@ -1898,7 +1899,7 @@
       Diag(Args[NumNamedArgs]->getBeginLoc(),
            diag::err_typecheck_call_too_many_args)
           << 2 /*method*/ << NumNamedArgs << static_cast<unsigned>(Args.size())
-          << Method->getSourceRange()
+          << Method->getSourceRange() << /*is non object*/ 0
           << SourceRange(Args[NumNamedArgs]->getBeginLoc(),
                          Args.back()->getEndLoc());
     }
diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -7395,8 +7395,9 @@
       return true;
   if (!isInStlNamespace(Callee->getParent()))
     return false;
-  if (!isRecordWithAttr<PointerAttr>(Callee->getThisObjectType()) &&
-      !isRecordWithAttr<OwnerAttr>(Callee->getThisObjectType()))
+  if (!isRecordWithAttr<PointerAttr>(
+          Callee->getFunctionObjectParameterType()) &&
+      !isRecordWithAttr<OwnerAttr>(Callee->getFunctionObjectParameterType()))
     return false;
   if (Callee->getReturnType()->isPointerType() ||
       isRecordWithAttr<PointerAttr>(Callee->getReturnType())) {
@@ -7531,7 +7532,7 @@
       QualType LHST;
       auto *MD = dyn_cast<CXXMethodDecl>(FD);
       if (MD && MD->isCXXInstanceMember())
-        LHST = Ctx.getLValueReferenceType(MD->getThisObjectType());
+        LHST = Ctx.getLValueReferenceType(MD->getFunctionObjectParameterType());
       else
         LHST = MD->getParamDecl(0)->getType();
       if (Ctx.hasSameType(RetT, LHST))
diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
--- a/clang/lib/Sema/SemaLambda.cpp
+++ b/clang/lib/Sema/SemaLambda.cpp
@@ -378,6 +378,38 @@
   return MethodType;
 }
 
+// [C++2b] [expr.prim.lambda.closure] p4
+//  Given a lambda with a lambda-capture, the type of the explicit object
+//  parameter, if any, of the lambda's function call operator (possibly
+//  instantiated from a function call operator template) shall be either:
+//  - the closure type,
+//  - class type derived from the closure type, or
+//  - a reference to a possibly cv-qualified such type.
+void Sema::DiagnoseInvalidExplicitObjectParameterInLambda(
+    CXXMethodDecl *Method) {
+  if (!isLambdaCallWithExplicitObjectParameter(Method))
+    return;
+  CXXRecordDecl *RD = Method->getParent();
+  if (Method->isDependentContext())
+    return;
+  if (RD->getLambdaCaptureDefault() == LambdaCaptureDefault::LCD_None &&
+      RD->capture_size() == 0)
+    return;
+  QualType ExplicitObjectParameterType = Method->getParamDecl(0)
+                                             ->getType()
+                                             .getNonReferenceType()
+                                             .getUnqualifiedType()
+                                             .getDesugaredType(getASTContext());
+  QualType LambdaType = getASTContext().getRecordType(RD);
+  if (LambdaType == ExplicitObjectParameterType)
+    return;
+  if (IsDerivedFrom(RD->getLocation(), ExplicitObjectParameterType, LambdaType))
+    return;
+  Diag(Method->getParamDecl(0)->getLocation(),
+       diag::err_invalid_explicit_object_type_in_lambda)
+      << ExplicitObjectParameterType;
+}
+
 void Sema::handleLambdaNumbering(
     CXXRecordDecl *Class, CXXMethodDecl *Method,
     std::optional<CXXRecordDecl::LambdaNumbering> NumberingOverride) {
@@ -859,9 +891,18 @@
   if (ParamInfo.getNumTypeObjects() == 0) {
     MethodTyInfo = getDummyLambdaType(S, Loc);
   } else {
+    // Check explicit parameters
+    S.CheckExplicitObjectLambda(ParamInfo);
+
     DeclaratorChunk::FunctionTypeInfo &FTI = ParamInfo.getFunctionTypeInfo();
+
+    bool HasExplicitObjectParameter =
+        FTI.NumParams > 0 &&
+        cast<ParmVarDecl>(FTI.Params[0].Param)->isExplicitObjectParameter();
+
     ExplicitResultType = FTI.hasTrailingReturnType();
-    if (!FTI.hasMutableQualifier() && !IsLambdaStatic)
+    if (!FTI.hasMutableQualifier() && !IsLambdaStatic &&
+        !HasExplicitObjectParameter)
       FTI.getOrCreateMethodQualifiers().SetTypeQual(DeclSpec::TQ_const, Loc);
 
     if (ExplicitResultType && S.getLangOpts().HLSL) {
@@ -1659,7 +1700,7 @@
   // function that will be the result of the conversion with a
   // certain unique ID.
   // When it is static we just return the static call operator instead.
-  if (CallOperator->isInstance()) {
+  if (CallOperator->isImplicitObjectMemberFunction()) {
     DeclarationName InvokerName =
         &S.Context.Idents.get(getLambdaStaticInvokerName());
     // FIXME: Instead of passing in the CallOperator->getTypeSourceInfo()
diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -1237,7 +1237,7 @@
 
 bool Sema::IsOverload(FunctionDecl *New, FunctionDecl *Old,
                       bool UseMemberUsingDeclRules, bool ConsiderCudaAttrs,
-                      bool ConsiderRequiresClauses) {
+                      bool UseOverrideRules) {
   // C++ [basic.start.main]p2: This function shall not be overloaded.
   if (New->isMain())
     return false;
@@ -1275,10 +1275,7 @@
   // The signature of a function includes the types of its
   // parameters (C++ 1.3.10), which includes the presence or absence
   // of the ellipsis; see C++ DR 357).
-  if (OldQType != NewQType &&
-      (OldType->getNumParams() != NewType->getNumParams() ||
-       OldType->isVariadic() != NewType->isVariadic() ||
-       !FunctionParamTypesAreEqual(OldType, NewType)))
+  if (OldQType != NewQType && OldType->isVariadic() != NewType->isVariadic())
     return true;
 
   // For member-like friends, the enclosing class is part of the signature.
@@ -1286,6 +1283,124 @@
        Old->isMemberLikeConstrainedFriend()) &&
       !New->getLexicalDeclContext()->Equals(Old->getLexicalDeclContext()))
     return true;
+  const auto *OldMethod = dyn_cast<CXXMethodDecl>(Old);
+  const auto *NewMethod = dyn_cast<CXXMethodDecl>(New);
+
+  int OldParamsOffset = 0;
+  int NewParamsOffset = 0;
+
+  // When determining if a method is an overload from a base class, act as if
+  // the implicit object parameter are of the same type.
+
+  auto NormalizeQualifiers = [&](const CXXMethodDecl *M, Qualifiers Q) {
+    if (M->isExplicitObjectMemberFunction())
+      return Q;
+
+    // We do not allow overloading based off of '__restrict'.
+    Q.removeRestrict();
+
+    // We may not have applied the implicit const for a constexpr member
+    // function yet (because we haven't yet resolved whether this is a static
+    // or non-static member function). Add it now, on the assumption that this
+    // is a redeclaration of OldMethod.
+    if (!getLangOpts().CPlusPlus14 && (M->isConstexpr() || M->isConsteval()) &&
+        !isa<CXXConstructorDecl>(NewMethod))
+      Q.addConst();
+    return Q;
+  };
+
+  auto CompareType = [&](QualType Base, QualType D) {
+    auto BS = Base.getNonReferenceType().getCanonicalType().split();
+    BS.Quals = NormalizeQualifiers(OldMethod, BS.Quals);
+
+    auto DS = D.getNonReferenceType().getCanonicalType().split();
+    DS.Quals = NormalizeQualifiers(NewMethod, DS.Quals);
+
+    if (BS.Quals != DS.Quals)
+      return false;
+
+    if (OldMethod->isImplicitObjectMemberFunction() &&
+        OldMethod->getParent() != NewMethod->getParent()) {
+      QualType ParentType =
+          Context.getTypeDeclType(OldMethod->getParent()).getCanonicalType();
+      if (ParentType.getTypePtr() != BS.Ty)
+        return false;
+      BS.Ty = DS.Ty;
+    }
+
+    if (BS.Ty != DS.Ty)
+      return false;
+
+    if (Base->isLValueReferenceType())
+      return D->isLValueReferenceType();
+    return Base->isRValueReferenceType() == D->isRValueReferenceType();
+  };
+
+  // If the function is a class member, its signature includes the
+  // cv-qualifiers (if any) and ref-qualifier (if any) on the function itself.
+  auto DiagnoseInconsistentRefQualifiers = [&]() {
+    if (LangOpts.CPlusPlus23)
+      return false;
+    if (OldMethod->getRefQualifier() == NewMethod->getRefQualifier())
+      return false;
+    if (OldMethod->isExplicitObjectMemberFunction() ||
+        NewMethod->isExplicitObjectMemberFunction())
+      return false;
+    if (!UseMemberUsingDeclRules && (OldMethod->getRefQualifier() == RQ_None ||
+                                     NewMethod->getRefQualifier() == RQ_None)) {
+      Diag(NewMethod->getLocation(), diag::err_ref_qualifier_overload)
+          << NewMethod->getRefQualifier() << OldMethod->getRefQualifier();
+      Diag(OldMethod->getLocation(), diag::note_previous_declaration);
+      return true;
+    }
+    return false;
+  };
+
+  if (OldMethod && OldMethod->isExplicitObjectMemberFunction())
+    OldParamsOffset++;
+  if (NewMethod && NewMethod->isExplicitObjectMemberFunction())
+    NewParamsOffset++;
+
+  if (OldType->getNumParams() - OldParamsOffset !=
+          NewType->getNumParams() - NewParamsOffset ||
+      !FunctionParamTypesAreEqual(
+          {OldType->param_type_begin() + OldParamsOffset,
+           OldType->param_type_end()},
+          {NewType->param_type_begin() + NewParamsOffset,
+           NewType->param_type_end()},
+          nullptr)) {
+    return true;
+  }
+
+  if (OldMethod && NewMethod && !OldMethod->isStatic() &&
+      !OldMethod->isStatic()) {
+    bool HaveCorrespondingObjectParameters = [&](const CXXMethodDecl *Old,
+                                                 const CXXMethodDecl *New) {
+      auto NewObjectType = New->getFunctionObjectParameterReferenceType();
+      auto OldObjectType = Old->getFunctionObjectParameterReferenceType();
+
+      auto IsImplicitWithNoRefQual = [](const CXXMethodDecl *F) {
+        return F->getRefQualifier() == RQ_None &&
+               !F->isExplicitObjectMemberFunction();
+      };
+
+      if (IsImplicitWithNoRefQual(Old) != IsImplicitWithNoRefQual(New) &&
+          CompareType(OldObjectType.getNonReferenceType(),
+                      NewObjectType.getNonReferenceType()))
+        return true;
+      return CompareType(OldObjectType, NewObjectType);
+    }(OldMethod, NewMethod);
+
+    if (!HaveCorrespondingObjectParameters) {
+      DiagnoseInconsistentRefQualifiers();
+      // CWG2554
+      // and, if at least one is an explicit object member function, ignoring
+      // object parameters
+      if (!UseOverrideRules || (!NewMethod->isExplicitObjectMemberFunction() &&
+                                !OldMethod->isExplicitObjectMemberFunction()))
+        return true;
+    }
+  }
 
   if (NewTemplate) {
     // C++ [temp.over.link]p4:
@@ -1324,7 +1439,7 @@
       return true;
   }
 
-  if (ConsiderRequiresClauses) {
+  if (!UseOverrideRules) {
     Expr *NewRC = New->getTrailingRequiresClause(),
          *OldRC = Old->getTrailingRequiresClause();
     if ((NewRC != nullptr) != (OldRC != nullptr))
@@ -1334,49 +1449,9 @@
         return true;
   }
 
-  // If the function is a class member, its signature includes the
-  // cv-qualifiers (if any) and ref-qualifier (if any) on the function itself.
-  //
-  // As part of this, also check whether one of the member functions
-  // is static, in which case they are not overloads (C++
-  // 13.1p2). While not part of the definition of the signature,
-  // this check is important to determine whether these functions
-  // can be overloaded.
-  CXXMethodDecl *OldMethod = dyn_cast<CXXMethodDecl>(Old);
-  CXXMethodDecl *NewMethod = dyn_cast<CXXMethodDecl>(New);
-  if (OldMethod && NewMethod &&
-      !OldMethod->isStatic() && !NewMethod->isStatic()) {
-    if (OldMethod->getRefQualifier() != NewMethod->getRefQualifier()) {
-      if (!UseMemberUsingDeclRules &&
-          (OldMethod->getRefQualifier() == RQ_None ||
-           NewMethod->getRefQualifier() == RQ_None)) {
-        // C++20 [over.load]p2:
-        //   - Member function declarations with the same name, the same
-        //     parameter-type-list, and the same trailing requires-clause (if
-        //     any), as well as member function template declarations with the
-        //     same name, the same parameter-type-list, the same trailing
-        //     requires-clause (if any), and the same template-head, cannot be
-        //     overloaded if any of them, but not all, have a ref-qualifier.
-        Diag(NewMethod->getLocation(), diag::err_ref_qualifier_overload)
-            << NewMethod->getRefQualifier() << OldMethod->getRefQualifier();
-        Diag(OldMethod->getLocation(), diag::note_previous_declaration);
-      }
-      return true;
-    }
-
-    // We may not have applied the implicit const for a constexpr member
-    // function yet (because we haven't yet resolved whether this is a static
-    // or non-static member function). Add it now, on the assumption that this
-    // is a redeclaration of OldMethod.
-    auto OldQuals = OldMethod->getMethodQualifiers();
-    auto NewQuals = NewMethod->getMethodQualifiers();
-    if (!getLangOpts().CPlusPlus14 && NewMethod->isConstexpr() &&
-        !isa<CXXConstructorDecl>(NewMethod))
-      NewQuals.addConst();
-    // We do not allow overloading based off of '__restrict'.
-    OldQuals.removeRestrict();
-    NewQuals.removeRestrict();
-    if (OldQuals != NewQuals)
+  if (NewMethod && OldMethod && OldMethod->isImplicitObjectMemberFunction() &&
+      NewMethod->isImplicitObjectMemberFunction()) {
+    if (DiagnoseInconsistentRefQualifiers())
       return true;
   }
 
@@ -1864,7 +1939,8 @@
       // fact that non-static member functions *must* have such an address-of
       // expression.
       CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Fn);
-      if (Method && !Method->isStatic()) {
+      if (Method && !Method->isStatic() &&
+          !Method->isExplicitObjectMemberFunction()) {
         assert(isa<UnaryOperator>(From->IgnoreParens()) &&
                "Non-unary operator on non-static member address");
         assert(cast<UnaryOperator>(From->IgnoreParens())->getOpcode()
@@ -3084,30 +3160,40 @@
 /// If `Reversed` is true, the parameters of `NewType` will be compared in
 /// reverse order. That's useful if one of the functions is being used as a C++20
 /// synthesized operator overload with a reversed parameter order.
-bool Sema::FunctionParamTypesAreEqual(const FunctionProtoType *OldType,
-                                      const FunctionProtoType *NewType,
-                                      unsigned *ArgPos, bool Reversed) {
-  assert(OldType->getNumParams() == NewType->getNumParams() &&
+bool Sema::FunctionParamTypesAreEqual(ArrayRef<QualType> Old,
+                                      ArrayRef<QualType> New, unsigned *ArgPos,
+                                      bool Reversed) {
+  assert(llvm::size(Old) == llvm::size(New) &&
          "Can't compare parameters of functions with different number of "
          "parameters!");
-  for (size_t I = 0; I < OldType->getNumParams(); I++) {
+
+  for (auto &&[Idx, Type] : llvm::enumerate(Old)) {
     // Reverse iterate over the parameters of `OldType` if `Reversed` is true.
-    size_t J = Reversed ? (OldType->getNumParams() - I - 1) : I;
+    size_t J = Reversed ? (llvm::size(New) - Idx - 1) : Idx;
 
     // Ignore address spaces in pointee type. This is to disallow overloading
     // on __ptr32/__ptr64 address spaces.
-    QualType Old = Context.removePtrSizeAddrSpace(OldType->getParamType(I).getUnqualifiedType());
-    QualType New = Context.removePtrSizeAddrSpace(NewType->getParamType(J).getUnqualifiedType());
+    QualType OldType =
+        Context.removePtrSizeAddrSpace(Type.getUnqualifiedType());
+    QualType NewType =
+        Context.removePtrSizeAddrSpace((New.begin() + J)->getUnqualifiedType());
 
-    if (!Context.hasSameType(Old, New)) {
+    if (!Context.hasSameType(OldType, NewType)) {
       if (ArgPos)
-        *ArgPos = I;
+        *ArgPos = Idx;
       return false;
     }
   }
   return true;
 }
 
+bool Sema::FunctionParamTypesAreEqual(const FunctionProtoType *OldType,
+                                      const FunctionProtoType *NewType,
+                                      unsigned *ArgPos, bool Reversed) {
+  return FunctionParamTypesAreEqual(OldType->param_types(),
+                                    NewType->param_types(), ArgPos, Reversed);
+}
+
 /// CheckPointerConversion - Check the pointer conversion from the
 /// expression From to the type ToType. This routine checks for
 /// ambiguous or inaccessible derived-to-base pointer
@@ -5486,11 +5572,45 @@
 /// TryObjectArgumentInitialization - Try to initialize the object
 /// parameter of the given member function (@c Method) from the
 /// expression @p From.
-static ImplicitConversionSequence
-TryObjectArgumentInitialization(Sema &S, SourceLocation Loc, QualType FromType,
-                                Expr::Classification FromClassification,
-                                CXXMethodDecl *Method,
-                                CXXRecordDecl *ActingContext) {
+static ImplicitConversionSequence TryObjectArgumentInitialization(
+    Sema &S, SourceLocation Loc, QualType FromType,
+    Expr::Classification FromClassification, CXXMethodDecl *Method,
+    CXXRecordDecl *ActingContext, bool InOverloadResolution = false,
+    QualType ExplicitParameterType = QualType(),
+    bool SuppressUserConversion = false) {
+
+  // We need to have an object of class type.
+  if (const PointerType *PT = FromType->getAs<PointerType>()) {
+    FromType = PT->getPointeeType();
+
+    // When we had a pointer, it's implicitly dereferenced, so we
+    // better have an lvalue.
+    assert(FromClassification.isLValue());
+  }
+
+  auto ValueKindFromClassification = [](Expr::Classification C) {
+    if (C.isPRValue())
+      return clang::VK_PRValue;
+    if (C.isXValue())
+      return VK_XValue;
+    return clang::VK_LValue;
+  };
+
+  if (Method->isExplicitObjectMemberFunction()) {
+    if (ExplicitParameterType.isNull())
+      ExplicitParameterType = Method->getFunctionObjectParameterReferenceType();
+    OpaqueValueExpr TmpExpr(Loc, FromType.getNonReferenceType(),
+                            ValueKindFromClassification(FromClassification));
+    ImplicitConversionSequence ICS = TryCopyInitialization(
+        S, &TmpExpr, ExplicitParameterType, SuppressUserConversion,
+        /*InOverloadResolution=*/true, false);
+    if (ICS.isBad())
+      ICS.Bad.FromExpr = nullptr;
+    return ICS;
+  }
+
+  assert(FromType->isRecordType());
+
   QualType ClassType = S.Context.getTypeDeclType(ActingContext);
   // [class.dtor]p2: A destructor can be invoked for a const, volatile or
   //                 const volatile object.
@@ -5506,17 +5626,6 @@
   // to exit early.
   ImplicitConversionSequence ICS;
 
-  // We need to have an object of class type.
-  if (const PointerType *PT = FromType->getAs<PointerType>()) {
-    FromType = PT->getPointeeType();
-
-    // When we had a pointer, it's implicitly dereferenced, so we
-    // better have an lvalue.
-    assert(FromClassification.isLValue());
-  }
-
-  assert(FromType->isRecordType());
-
   // C++0x [over.match.funcs]p4:
   //   For non-static member functions, the type of the implicit object
   //   parameter is
@@ -5563,7 +5672,7 @@
     SecondKind = ICK_Identity;
   } else if (S.IsDerivedFrom(Loc, FromType, ClassType))
     SecondKind = ICK_Derived_To_Base;
-  else {
+  else if (!Method->isExplicitObjectMemberFunction()) {
     ICS.setBad(BadConversionSequence::unrelated_class,
                FromType, ImplicitParamType);
     return ICS;
@@ -5613,11 +5722,9 @@
 /// PerformObjectArgumentInitialization - Perform initialization of
 /// the implicit object parameter for the given Method with the given
 /// expression.
-ExprResult
-Sema::PerformObjectArgumentInitialization(Expr *From,
-                                          NestedNameSpecifier *Qualifier,
-                                          NamedDecl *FoundDecl,
-                                          CXXMethodDecl *Method) {
+ExprResult Sema::PerformImplicitObjectArgumentInitialization(
+    Expr *From, NestedNameSpecifier *Qualifier, NamedDecl *FoundDecl,
+    CXXMethodDecl *Method) {
   QualType FromRecordType, DestType;
   QualType ImplicitParamRecordType  =
     Method->getThisType()->castAs<PointerType>()->getPointeeType();
@@ -6114,6 +6221,65 @@
   return ExprResult();
 }
 
+static QualType GetExplicitObjectType(Sema &S, Expr *MemExprE) {
+  Expr *Base = nullptr;
+  assert((isa<UnresolvedMemberExpr, MemberExpr>(MemExprE)) &&
+         "expected a member expression");
+
+  if (auto M = dyn_cast<UnresolvedMemberExpr>(MemExprE);
+      M && !M->isImplicitAccess())
+    Base = M->getBase();
+  else if (auto M = dyn_cast<MemberExpr>(MemExprE); M && !M->isImplicitAccess())
+    Base = M->getBase();
+  QualType T;
+  if (!Base)
+    T = S.getCurrentThisType();
+  else
+    T = Base->getType();
+
+  if (T->isPointerType())
+    T = T->getPointeeType();
+
+  return T;
+}
+
+static Expr *GetExplicitObjectExpr(Sema &S, Expr *Obj, FunctionDecl *Fun) {
+  QualType ObjType = Obj->getType();
+  if (ObjType->isPointerType()) {
+    ObjType = ObjType->getPointeeType();
+    Obj = UnaryOperator::Create(S.getASTContext(), Obj, UO_Deref, ObjType,
+                                VK_LValue, OK_Ordinary, SourceLocation(),
+                                /*CanOverflow=*/false, FPOptionsOverride());
+  }
+  if (Obj->Classify(S.getASTContext()).isPRValue()) {
+    Obj = S.CreateMaterializeTemporaryExpr(
+        ObjType, Obj,
+        !Fun->getParamDecl(0)->getType()->isRValueReferenceType());
+  }
+  return Obj;
+}
+
+ExprResult Sema::InitializeExplicitObjectArgument(Sema &S, Expr *Obj,
+                                                  FunctionDecl *Fun) {
+  Obj = GetExplicitObjectExpr(S, Obj, Fun);
+  return S.PerformCopyInitialization(
+      InitializedEntity::InitializeParameter(S.Context, Fun->getParamDecl(0)),
+      Obj->getExprLoc(), Obj);
+}
+
+static void PrepareExplicitObjectArgument(Sema &S, CXXMethodDecl *Method,
+                                          Expr *Object, MultiExprArg &Args,
+                                          SmallVectorImpl<Expr *> &NewArgs) {
+  assert(Method->isExplicitObjectMemberFunction() &&
+         "Method is not an explicit member function");
+  assert(NewArgs.empty() && "NewArgs should be empty");
+  NewArgs.reserve(Args.size() + 1);
+  Expr *This = GetExplicitObjectExpr(S, Object, Method);
+  NewArgs.push_back(This);
+  NewArgs.append(Args.begin(), Args.end());
+  Args = NewArgs;
+}
+
 /// Determine whether the provided type is an integral type, or an enumeration
 /// type of a permitted flavor.
 bool Sema::ICEConvertDiagnoser::match(QualType T) {
@@ -6197,7 +6363,6 @@
     Converter.diagnoseConversion(SemaRef, Loc, T, ToType)
         << From->getSourceRange();
   }
-
   ExprResult Result = SemaRef.BuildCXXMemberCallExpr(From, Found, Conversion,
                                                      HadMultipleCandidates);
   if (Result.isInvalid())
@@ -6348,7 +6513,6 @@
 
     QualType CurToType = Conversion->getConversionType().getNonReferenceType();
     if (Converter.match(CurToType) || ConvTemplate) {
-
       if (Conversion->isExplicit()) {
         // FIXME: For C++1y, do we need this restriction?
         // cf. diagnoseNoViableConversion()
@@ -6844,7 +7008,7 @@
     assert(!isa<CXXConstructorDecl>(Method) &&
            "Shouldn't have `this` for ctors!");
     assert(!Method->isStatic() && "Shouldn't have `this` for static methods!");
-    ExprResult R = S.PerformObjectArgumentInitialization(
+    ExprResult R = S.PerformImplicitObjectArgumentInitialization(
         ThisArg, /*Qualifier=*/nullptr, Method, Method);
     if (R.isInvalid())
       return false;
@@ -7144,7 +7308,8 @@
   Candidate.IgnoreObjectArgument = false;
   Candidate.ExplicitCallArguments = Args.size();
 
-  unsigned NumParams = Proto->getNumParams();
+  unsigned NumParams = Method->getNumExplicitParams();
+  unsigned ExplicitOffset = Method->isExplicitObjectMemberFunction() ? 1 : 0;
 
   // (C++ 13.3.2p2): A candidate function having fewer than m
   // parameters is viable only if it has an ellipsis in its parameter
@@ -7162,7 +7327,7 @@
   // (8.3.6). For the purposes of overload resolution, the
   // parameter list is truncated on the right, so that there are
   // exactly m parameters.
-  unsigned MinRequiredArgs = Method->getMinRequiredArguments();
+  unsigned MinRequiredArgs = Method->getMinRequiredExplicitArguments();
   if (Args.size() < MinRequiredArgs && !PartialOverloading) {
     // Not enough arguments.
     Candidate.Viable = false;
@@ -7191,7 +7356,7 @@
     // parameter.
     Candidate.Conversions[FirstConvIdx] = TryObjectArgumentInitialization(
         *this, CandidateSet.getLocation(), ObjectType, ObjectClassification,
-        Method, ActingContext);
+        Method, ActingContext, /*InOverloadResolution=*/true);
     if (Candidate.Conversions[FirstConvIdx].isBad()) {
       Candidate.Viable = false;
       Candidate.FailureKind = ovl_fail_bad_conversion;
@@ -7232,7 +7397,7 @@
       // exist for each argument an implicit conversion sequence
       // (13.3.3.1) that converts that argument to the corresponding
       // parameter of F.
-      QualType ParamType = Proto->getParamType(ArgIdx);
+      QualType ParamType = Proto->getParamType(ArgIdx + ExplicitOffset);
       Candidate.Conversions[ConvIdx]
         = TryCopyInitialization(*this, Args[ArgIdx], ParamType,
                                 SuppressUserConversions,
@@ -7297,8 +7462,8 @@
   ConversionSequenceList Conversions;
   if (TemplateDeductionResult Result = DeduceTemplateArguments(
           MethodTmpl, ExplicitTemplateArgs, Args, Specialization, Info,
-          PartialOverloading, /*AggregateDeductionCandidate=*/false,
-          [&](ArrayRef<QualType> ParamTypes) {
+          PartialOverloading, /*AggregateDeductionCandidate=*/false, ObjectType,
+          ObjectClassification, [&](ArrayRef<QualType> ParamTypes) {
             return CheckNonDependentConversions(
                 MethodTmpl, ParamTypes, Args, CandidateSet, Conversions,
                 SuppressUserConversions, ActingContext, ObjectType,
@@ -7382,6 +7547,8 @@
   if (TemplateDeductionResult Result = DeduceTemplateArguments(
           FunctionTemplate, ExplicitTemplateArgs, Args, Specialization, Info,
           PartialOverloading, AggregateCandidateDeduction,
+          /*ObjectType=*/QualType(),
+          /*ObjectClassification=*/Expr::Classification(),
           [&](ArrayRef<QualType> ParamTypes) {
             return CheckNonDependentConversions(
                 FunctionTemplate, ParamTypes, Args, CandidateSet, Conversions,
@@ -7454,16 +7621,24 @@
   if (HasThisConversion && !cast<CXXMethodDecl>(FD)->isStatic() &&
       !ObjectType.isNull()) {
     unsigned ConvIdx = PO == OverloadCandidateParamOrder::Reversed ? 1 : 0;
-    Conversions[ConvIdx] = TryObjectArgumentInitialization(
-        *this, CandidateSet.getLocation(), ObjectType, ObjectClassification,
-        Method, ActingContext);
-    if (Conversions[ConvIdx].isBad())
-      return true;
+    if (!FD->hasCXXExplicitFunctionObjectParameter() ||
+        !ParamTypes[0]->isDependentType()) {
+      Conversions[ConvIdx] = TryObjectArgumentInitialization(
+          *this, CandidateSet.getLocation(), ObjectType, ObjectClassification,
+          Method, ActingContext, /*InOverloadResolution=*/true,
+          FD->hasCXXExplicitFunctionObjectParameter() ? ParamTypes[0]
+                                                      : QualType());
+      if (Conversions[ConvIdx].isBad())
+        return true;
+    }
   }
 
+  unsigned Offset =
+      Method && Method->hasCXXExplicitFunctionObjectParameter() ? 1 : 0;
+
   for (unsigned I = 0, N = std::min(ParamTypes.size(), Args.size()); I != N;
        ++I) {
-    QualType ParamType = ParamTypes[I];
+    QualType ParamType = ParamTypes[I + Offset];
     if (!ParamType->isDependentType()) {
       unsigned ConvIdx = PO == OverloadCandidateParamOrder::Reversed
                              ? 0
@@ -7596,15 +7771,21 @@
   //
   // Determine the implicit conversion sequence for the implicit
   // object parameter.
-  QualType ImplicitParamType = From->getType();
-  if (const PointerType *FromPtrType = ImplicitParamType->getAs<PointerType>())
-    ImplicitParamType = FromPtrType->getPointeeType();
-  CXXRecordDecl *ConversionContext
-    = cast<CXXRecordDecl>(ImplicitParamType->castAs<RecordType>()->getDecl());
-
+  QualType ObjectType = From->getType();
+  if (const PointerType *FromPtrType = ObjectType->getAs<PointerType>())
+    ObjectType = FromPtrType->getPointeeType();
+  CXXRecordDecl *ConversionContext =
+      cast<CXXRecordDecl>(ObjectType->castAs<RecordType>()->getDecl());
+
+  // C++23 [over.best.ics.general]
+  // However, if the target is [...]
+  // - the object parameter of a user-defined conversion function
+  // [...] user-defined conversion sequences are not considered.
   Candidate.Conversions[0] = TryObjectArgumentInitialization(
       *this, CandidateSet.getLocation(), From->getType(),
-      From->Classify(Context), Conversion, ConversionContext);
+      From->Classify(Context), Conversion, ConversionContext,
+      /*InOverloadResolution*/ false, /*ExplicitParameterType=*/QualType(),
+      /*SuppressUserConversion*/ true);
 
   if (Candidate.Conversions[0].isBad()) {
     Candidate.Viable = false;
@@ -7758,11 +7939,14 @@
     return;
   }
 
+  QualType ObjectType = From->getType();
+  Expr::Classification ObjectClassification = From->Classify(getASTContext());
+
   TemplateDeductionInfo Info(CandidateSet.getLocation());
   CXXConversionDecl *Specialization = nullptr;
-  if (TemplateDeductionResult Result
-        = DeduceTemplateArguments(FunctionTemplate, ToType,
-                                  Specialization, Info)) {
+  if (TemplateDeductionResult Result = DeduceTemplateArguments(
+          FunctionTemplate, ObjectType, ObjectClassification, ToType,
+          Specialization, Info)) {
     OverloadCandidate &Candidate = CandidateSet.addCandidate();
     Candidate.FoundDecl = FoundDecl;
     Candidate.Function = FunctionTemplate->getTemplatedDecl();
@@ -7814,9 +7998,18 @@
 
   // Determine the implicit conversion sequence for the implicit
   // object parameter.
-  ImplicitConversionSequence ObjectInit = TryObjectArgumentInitialization(
-      *this, CandidateSet.getLocation(), Object->getType(),
-      Object->Classify(Context), Conversion, ActingContext);
+  ImplicitConversionSequence ObjectInit;
+  if (Conversion->hasCXXExplicitFunctionObjectParameter()) {
+    ObjectInit = TryCopyInitialization(*this, Object,
+                                       Conversion->getParamDecl(0)->getType(),
+                                       /*SuppressUserConversions=*/false,
+                                       /*InOverloadResolution=*/true, false);
+  } else {
+    ObjectInit = TryObjectArgumentInitialization(
+        *this, CandidateSet.getLocation(), Object->getType(),
+        Object->Classify(Context), Conversion, ActingContext);
+  }
+
   if (ObjectInit.isBad()) {
     Candidate.Viable = false;
     Candidate.FailureKind = ovl_fail_bad_conversion;
@@ -9842,11 +10035,7 @@
   // Static member functions' object parameters match all types.
   if (M->isStatic())
     return QualType();
-
-  QualType T = M->getThisObjectType();
-  if (M->getRefQualifier() == RQ_RValue)
-    return Context.getRValueReferenceType(T);
-  return Context.getLValueReferenceType(T);
+  return M->getFunctionObjectParameterReferenceType();
 }
 
 static bool haveSameParameterTypes(ASTContext &Context, const FunctionDecl *F1,
@@ -11044,39 +11233,43 @@
 
   // TODO: treat calls to a missing default constructor as a special case
   const auto *FnTy = Fn->getType()->castAs<FunctionProtoType>();
-  unsigned MinParams = Fn->getMinRequiredArguments();
+  unsigned MinParams = Fn->getMinRequiredExplicitArguments();
 
   // at least / at most / exactly
+  bool hasExplicitObjectParam = Fn->hasCXXExplicitFunctionObjectParameter();
+  unsigned ParamCount = FnTy->getNumParams() - (hasExplicitObjectParam ? 1 : 0);
   unsigned mode, modeCount;
   if (NumFormalArgs < MinParams) {
-    if (MinParams != FnTy->getNumParams() || FnTy->isVariadic() ||
+    if (MinParams != ParamCount || FnTy->isVariadic() ||
         FnTy->isTemplateVariadic())
       mode = 0; // "at least"
     else
       mode = 2; // "exactly"
     modeCount = MinParams;
   } else {
-    if (MinParams != FnTy->getNumParams())
+    if (MinParams != ParamCount)
       mode = 1; // "at most"
     else
       mode = 2; // "exactly"
-    modeCount = FnTy->getNumParams();
+    modeCount = ParamCount;
   }
 
   std::string Description;
   std::pair<OverloadCandidateKind, OverloadCandidateSelect> FnKindPair =
       ClassifyOverloadCandidate(S, Found, Fn, CRK_None, Description);
 
-  if (modeCount == 1 && Fn->getParamDecl(0)->getDeclName())
+  if (modeCount == 1 &&
+      Fn->getParamDecl(hasExplicitObjectParam ? 1 : 0)->getDeclName())
     S.Diag(Fn->getLocation(), diag::note_ovl_candidate_arity_one)
         << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second
-        << Description << mode << Fn->getParamDecl(0) << NumFormalArgs
-        << Fn->getParametersSourceRange();
+        << Description << mode
+        << Fn->getParamDecl(hasExplicitObjectParam ? 1 : 0) << NumFormalArgs
+        << hasExplicitObjectParam << Fn->getParametersSourceRange();
   else
     S.Diag(Fn->getLocation(), diag::note_ovl_candidate_arity)
         << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second
         << Description << mode << modeCount << NumFormalArgs
-        << Fn->getParametersSourceRange();
+        << hasExplicitObjectParam << Fn->getParametersSourceRange();
 
   MaybeEmitInheritedConstructorNote(S, Found);
 }
@@ -12420,7 +12613,9 @@
               = dyn_cast<CXXMethodDecl>(FunctionTemplate->getTemplatedDecl())) {
       // Skip non-static function templates when converting to pointer, and
       // static when converting to member pointer.
-      if (Method->isStatic() == TargetTypeIsNonStaticMemberFunction)
+      bool CanConvertToFunctionPointer =
+          Method->isStatic() || Method->isExplicitObjectMemberFunction();
+      if (CanConvertToFunctionPointer == TargetTypeIsNonStaticMemberFunction)
         return false;
     }
     else if (TargetTypeIsNonStaticMemberFunction)
@@ -12465,7 +12660,9 @@
     if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Fn)) {
       // Skip non-static functions when converting to pointer, and static
       // when converting to member pointer.
-      if (Method->isStatic() == TargetTypeIsNonStaticMemberFunction)
+      bool CanConvertToFunctionPointer =
+          Method->isStatic() || Method->isExplicitObjectMemberFunction();
+      if (CanConvertToFunctionPointer == TargetTypeIsNonStaticMemberFunction)
         return false;
     }
     else if (TargetTypeIsNonStaticMemberFunction)
@@ -12832,7 +13029,10 @@
   // for both.
   DiagnoseUseOfDecl(Found, E->getExprLoc());
   CheckAddressOfMemberAccess(E, DAP);
-  Expr *Fixed = FixOverloadedFunctionReference(E, DAP, Found);
+  ExprResult Res = FixOverloadedFunctionReference(E, DAP, Found);
+  if (Res.isInvalid())
+    return false;
+  Expr *Fixed = Res.get();
   if (DoFunctionPointerConversion && Fixed->getType()->isFunctionType())
     SrcExpr = DefaultFunctionArrayConversion(Fixed, /*Diagnose=*/false);
   else
@@ -13494,10 +13694,13 @@
     SemaRef.CheckUnresolvedLookupAccess(ULE, (*Best)->FoundDecl);
     if (SemaRef.DiagnoseUseOfDecl(FDecl, ULE->getNameLoc()))
       return ExprError();
-    Fn = SemaRef.FixOverloadedFunctionReference(Fn, (*Best)->FoundDecl, FDecl);
-    return SemaRef.BuildResolvedCallExpr(Fn, FDecl, LParenLoc, Args, RParenLoc,
-                                         ExecConfig, /*IsExecConfig=*/false,
-                                         (*Best)->IsADLCandidate);
+    ExprResult Res =
+        SemaRef.FixOverloadedFunctionReference(Fn, (*Best)->FoundDecl, FDecl);
+    if (Res.isInvalid())
+      return ExprError();
+    return SemaRef.BuildResolvedCallExpr(
+        Res.get(), FDecl, LParenLoc, Args, RParenLoc, ExecConfig,
+        /*IsExecConfig=*/false, (*Best)->IsADLCandidate);
   }
 
   case OR_No_Viable_Function: {
@@ -13552,10 +13755,13 @@
     // We emitted an error for the unavailable/deleted function call but keep
     // the call in the AST.
     FunctionDecl *FDecl = (*Best)->Function;
-    Fn = SemaRef.FixOverloadedFunctionReference(Fn, (*Best)->FoundDecl, FDecl);
-    return SemaRef.BuildResolvedCallExpr(Fn, FDecl, LParenLoc, Args, RParenLoc,
-                                         ExecConfig, /*IsExecConfig=*/false,
-                                         (*Best)->IsADLCandidate);
+    ExprResult Res =
+        SemaRef.FixOverloadedFunctionReference(Fn, (*Best)->FoundDecl, FDecl);
+    if (Res.isInvalid())
+      return ExprError();
+    return SemaRef.BuildResolvedCallExpr(
+        Res.get(), FDecl, LParenLoc, Args, RParenLoc, ExecConfig,
+        /*IsExecConfig=*/false, (*Best)->IsADLCandidate);
   }
   }
 
@@ -13629,6 +13835,83 @@
                                       Fns.begin(), Fns.end());
 }
 
+ExprResult Sema::BuildCXXMemberCallExpr(Expr *E, NamedDecl *FoundDecl,
+                                        CXXConversionDecl *Method,
+                                        bool HadMultipleCandidates) {
+  // Convert the expression to match the conversion function's implicit object
+  // parameter.
+  ExprResult Exp;
+  if (Method->isExplicitObjectMemberFunction())
+    Exp = InitializeExplicitObjectArgument(*this, E, Method);
+  else
+    Exp = PerformImplicitObjectArgumentInitialization(E, /*Qualifier=*/nullptr,
+                                                      FoundDecl, Method);
+  if (Exp.isInvalid())
+    return true;
+
+  if (Method->getParent()->isLambda() &&
+      Method->getConversionType()->isBlockPointerType()) {
+    // This is a lambda conversion to block pointer; check if the argument
+    // was a LambdaExpr.
+    Expr *SubE = E;
+    CastExpr *CE = dyn_cast<CastExpr>(SubE);
+    if (CE && CE->getCastKind() == CK_NoOp)
+      SubE = CE->getSubExpr();
+    SubE = SubE->IgnoreParens();
+    if (CXXBindTemporaryExpr *BE = dyn_cast<CXXBindTemporaryExpr>(SubE))
+      SubE = BE->getSubExpr();
+    if (isa<LambdaExpr>(SubE)) {
+      // For the conversion to block pointer on a lambda expression, we
+      // construct a special BlockLiteral instead; this doesn't really make
+      // a difference in ARC, but outside of ARC the resulting block literal
+      // follows the normal lifetime rules for block literals instead of being
+      // autoreleased.
+      PushExpressionEvaluationContext(
+          ExpressionEvaluationContext::PotentiallyEvaluated);
+      ExprResult BlockExp = BuildBlockForLambdaConversion(
+          Exp.get()->getExprLoc(), Exp.get()->getExprLoc(), Method, Exp.get());
+      PopExpressionEvaluationContext();
+
+      // FIXME: This note should be produced by a CodeSynthesisContext.
+      if (BlockExp.isInvalid())
+        Diag(Exp.get()->getExprLoc(), diag::note_lambda_to_block_conv);
+      return BlockExp;
+    }
+  }
+  CallExpr *CE;
+  QualType ResultType = Method->getReturnType();
+  ExprValueKind VK = Expr::getValueKindForType(ResultType);
+  ResultType = ResultType.getNonLValueExprType(Context);
+  if (Method->isExplicitObjectMemberFunction()) {
+    ExprResult FnExpr =
+        CreateFunctionRefExpr(*this, Method, FoundDecl, Exp.get(),
+                              HadMultipleCandidates, E->getBeginLoc());
+    if (FnExpr.isInvalid())
+      return ExprError();
+    Expr *ObjectParam = Exp.get();
+    CE = CallExpr::Create(Context, FnExpr.get(), MultiExprArg(&ObjectParam, 1),
+                          ResultType, VK, Exp.get()->getEndLoc(),
+                          CurFPFeatureOverrides());
+  } else {
+    MemberExpr *ME =
+        BuildMemberExpr(Exp.get(), /*IsArrow=*/false, SourceLocation(),
+                        NestedNameSpecifierLoc(), SourceLocation(), Method,
+                        DeclAccessPair::make(FoundDecl, FoundDecl->getAccess()),
+                        HadMultipleCandidates, DeclarationNameInfo(),
+                        Context.BoundMemberTy, VK_PRValue, OK_Ordinary);
+
+    CE = CXXMemberCallExpr::Create(Context, ME, /*Args=*/{}, ResultType, VK,
+                                   Exp.get()->getEndLoc(),
+                                   CurFPFeatureOverrides());
+  }
+
+  if (CheckFunctionCall(Method, CE,
+                        Method->getType()->castAs<FunctionProtoType>()))
+    return ExprError();
+
+  return CheckForImmediateInvocation(CE, CE->getDirectCallee());
+}
+
 /// Create a unary operation that may resolve to an overloaded
 /// operator.
 ///
@@ -13723,14 +14006,17 @@
 
       // Convert the arguments.
       if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(FnDecl)) {
-        CheckMemberOperatorAccess(OpLoc, Args[0], nullptr, Best->FoundDecl);
+        CheckMemberOperatorAccess(OpLoc, Input, nullptr, Best->FoundDecl);
 
-        ExprResult InputRes =
-          PerformObjectArgumentInitialization(Input, /*Qualifier=*/nullptr,
-                                              Best->FoundDecl, Method);
-        if (InputRes.isInvalid())
+        ExprResult InputInit;
+        if (Method->isExplicitObjectMemberFunction())
+          InputInit = InitializeExplicitObjectArgument(*this, Input, Method);
+        else
+          InputInit = PerformImplicitObjectArgumentInitialization(
+              Input, /*Qualifier=*/nullptr, Best->FoundDecl, Method);
+        if (InputInit.isInvalid())
           return ExprError();
-        Base = Input = InputRes.get();
+        Base = Input = InputInit.get();
       } else {
         // Convert the arguments.
         ExprResult InputInit
@@ -13757,6 +14043,7 @@
       ResultTy = ResultTy.getNonLValueExprType(Context);
 
       Args[0] = Input;
+
       CallExpr *TheCall = CXXOperatorCallExpr::Create(
           Context, Op, FnExpr.get(), ArgsArray, ResultTy, VK, OpLoc,
           CurFPFeatureOverrides(), Best->IsADLCandidate);
@@ -14060,13 +14347,16 @@
               if (auto *MD = dyn_cast<CXXMethodDecl>(FnDecl))
                 if (Op == OverloadedOperatorKind::OO_EqualEqual &&
                     !MD->isConst() &&
+                    !MD->hasCXXExplicitFunctionObjectParameter() &&
                     Context.hasSameUnqualifiedType(
-                        MD->getThisObjectType(),
+                        MD->getFunctionObjectParameterType(),
                         MD->getParamDecl(0)->getType().getNonReferenceType()) &&
-                    Context.hasSameUnqualifiedType(MD->getThisObjectType(),
-                                                   Args[0]->getType()) &&
-                    Context.hasSameUnqualifiedType(MD->getThisObjectType(),
-                                                   Args[1]->getType()))
+                    Context.hasSameUnqualifiedType(
+                        MD->getFunctionObjectParameterType(),
+                        Args[0]->getType()) &&
+                    Context.hasSameUnqualifiedType(
+                        MD->getFunctionObjectParameterType(),
+                        Args[1]->getType()))
                   Diag(FnDecl->getLocation(),
                        diag::note_ovl_ambiguous_eqeq_reversed_self_non_const);
             } else {
@@ -14084,19 +14374,22 @@
           // Best->Access is only meaningful for class members.
           CheckMemberOperatorAccess(OpLoc, Args[0], Args[1], Best->FoundDecl);
 
-          ExprResult Arg1 =
-            PerformCopyInitialization(
-              InitializedEntity::InitializeParameter(Context,
-                                                     FnDecl->getParamDecl(0)),
+          ExprResult Arg0, Arg1;
+          unsigned ParamIdx = 0;
+          if (Method->isExplicitObjectMemberFunction()) {
+            Arg0 = InitializeExplicitObjectArgument(*this, Args[0], FnDecl);
+            ParamIdx = 1;
+          } else {
+            Arg0 = PerformImplicitObjectArgumentInitialization(
+                Args[0], /*Qualifier=*/nullptr, Best->FoundDecl, Method);
+          }
+          Arg1 = PerformCopyInitialization(
+              InitializedEntity::InitializeParameter(
+                  Context, FnDecl->getParamDecl(ParamIdx)),
               SourceLocation(), Args[1]);
-          if (Arg1.isInvalid())
+          if (Arg0.isInvalid() || Arg1.isInvalid())
             return ExprError();
 
-          ExprResult Arg0 =
-            PerformObjectArgumentInitialization(Args[0], /*Qualifier=*/nullptr,
-                                                Best->FoundDecl, Method);
-          if (Arg0.isInvalid())
-            return ExprError();
           Base = Args[0] = Arg0.getAs<Expr>();
           Args[1] = RHS = Arg1.getAs<Expr>();
         } else {
@@ -14131,22 +14424,27 @@
         ExprValueKind VK = Expr::getValueKindForType(ResultTy);
         ResultTy = ResultTy.getNonLValueExprType(Context);
 
-        CXXOperatorCallExpr *TheCall = CXXOperatorCallExpr::Create(
+        CallExpr *TheCall;
+        ArrayRef<const Expr *> ArgsArray(Args, 2);
+        const Expr *ImplicitThis = nullptr;
+
+        // We always create a CXXOperatorCallExpr, even for explicit object
+        // members; CodeGen should take care not to emit the this pointer.
+        TheCall = CXXOperatorCallExpr::Create(
             Context, ChosenOp, FnExpr.get(), Args, ResultTy, VK, OpLoc,
             CurFPFeatureOverrides(), Best->IsADLCandidate);
 
-        if (CheckCallReturnType(FnDecl->getReturnType(), OpLoc, TheCall,
-                                FnDecl))
-          return ExprError();
-
-        ArrayRef<const Expr *> ArgsArray(Args, 2);
-        const Expr *ImplicitThis = nullptr;
-        // Cut off the implicit 'this'.
-        if (isa<CXXMethodDecl>(FnDecl)) {
+        if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(FnDecl);
+            Method && Method->isImplicitObjectMemberFunction()) {
+          // Cut off the implicit 'this'.
           ImplicitThis = ArgsArray[0];
           ArgsArray = ArgsArray.slice(1);
         }
 
+        if (CheckCallReturnType(FnDecl->getReturnType(), OpLoc, TheCall,
+                                FnDecl))
+          return ExprError();
+
         // Check for a self move.
         if (Op == OO_Equal)
           DiagnoseSelfMove(Args[0], Args[1], OpLoc);
@@ -14154,7 +14452,7 @@
         if (ImplicitThis) {
           QualType ThisType = Context.getPointerType(ImplicitThis->getType());
           QualType ThisTypeFromDecl = Context.getPointerType(
-              cast<CXXMethodDecl>(FnDecl)->getThisObjectType());
+              cast<CXXMethodDecl>(FnDecl)->getFunctionObjectParameterType());
 
           CheckArgAlignment(OpLoc, FnDecl, "'this'", ThisType,
                             ThisTypeFromDecl);
@@ -14533,8 +14831,15 @@
         SmallVector<Expr *, 2> MethodArgs;
 
         // Handle 'this' parameter if the selected function is not static.
-        if (Method->isInstance()) {
-          ExprResult Arg0 = PerformObjectArgumentInitialization(
+        if (Method->isExplicitObjectMemberFunction()) {
+          ExprResult Res =
+              InitializeExplicitObjectArgument(*this, Args[0], Method);
+          if (Res.isInvalid())
+            return ExprError();
+          Args[0] = Res.get();
+          ArgExpr = Args;
+        } else if (Method->isInstance()) {
+          ExprResult Arg0 = PerformImplicitObjectArgumentInitialization(
               Args[0], /*Qualifier=*/nullptr, Best->FoundDecl, Method);
           if (Arg0.isInvalid())
             return ExprError();
@@ -14736,6 +15041,7 @@
 
   MemberExpr *MemExpr;
   CXXMethodDecl *Method = nullptr;
+  bool HadMultipleCandidates = false;
   DeclAccessPair FoundDecl = DeclAccessPair::make(nullptr, AS_public);
   NestedNameSpecifier *Qualifier = nullptr;
   if (isa<MemberExpr>(NakedMemExpr)) {
@@ -14767,11 +15073,24 @@
     for (UnresolvedMemberExpr::decls_iterator I = UnresExpr->decls_begin(),
            E = UnresExpr->decls_end(); I != E; ++I) {
 
+      QualType ExplicitObjectType = ObjectType;
+
       NamedDecl *Func = *I;
       CXXRecordDecl *ActingDC = cast<CXXRecordDecl>(Func->getDeclContext());
       if (isa<UsingShadowDecl>(Func))
         Func = cast<UsingShadowDecl>(Func)->getTargetDecl();
 
+      bool HasExplicitParameter = false;
+      if (auto *M = dyn_cast<FunctionDecl>(Func);
+          M && M->hasCXXExplicitFunctionObjectParameter())
+        HasExplicitParameter = true;
+      else if (auto *M = dyn_cast<FunctionTemplateDecl>(Func);
+               M &&
+               M->getTemplatedDecl()->hasCXXExplicitFunctionObjectParameter())
+        HasExplicitParameter = true;
+
+      if (HasExplicitParameter)
+        ExplicitObjectType = GetExplicitObjectType(*this, UnresExpr);
 
       // Microsoft supports direct constructor calls.
       if (getLangOpts().MicrosoftExt && isa<CXXConstructorDecl>(Func)) {
@@ -14784,17 +15103,20 @@
         if (TemplateArgs)
           continue;
 
-        AddMethodCandidate(Method, I.getPair(), ActingDC, ObjectType,
+        AddMethodCandidate(Method, I.getPair(), ActingDC, ExplicitObjectType,
                            ObjectClassification, Args, CandidateSet,
                            /*SuppressUserConversions=*/false);
       } else {
-        AddMethodTemplateCandidate(
-            cast<FunctionTemplateDecl>(Func), I.getPair(), ActingDC,
-            TemplateArgs, ObjectType, ObjectClassification, Args, CandidateSet,
-            /*SuppressUserConversions=*/false);
+        AddMethodTemplateCandidate(cast<FunctionTemplateDecl>(Func),
+                                   I.getPair(), ActingDC, TemplateArgs,
+                                   ExplicitObjectType, ObjectClassification,
+                                   Args, CandidateSet,
+                                   /*SuppressUserConversions=*/false);
       }
     }
 
+    HadMultipleCandidates = (CandidateSet.size() > 1);
+
     DeclarationName DeclName = UnresExpr->getMemberName();
 
     UnbridgedCasts.restore();
@@ -14848,10 +15170,14 @@
     if (!Succeeded)
       return BuildRecoveryExpr(chooseRecoveryType(CandidateSet, &Best));
 
-    MemExprE = FixOverloadedFunctionReference(MemExprE, FoundDecl, Method);
+    ExprResult Res =
+        FixOverloadedFunctionReference(MemExprE, FoundDecl, Method);
+    if (Res.isInvalid())
+      return ExprError();
+    MemExprE = Res.get();
 
-    // If overload resolution picked a static member, build a
-    // non-member call based on that function.
+    // If overload resolution picked a static member
+    // build a non-member call based on that function.
     if (Method->isStatic()) {
       return BuildResolvedCallExpr(MemExprE, Method, LParenLoc, Args, RParenLoc,
                                    ExecConfig, IsExecConfig);
@@ -14866,27 +15192,41 @@
 
   assert(Method && "Member call to something that isn't a method?");
   const auto *Proto = Method->getType()->castAs<FunctionProtoType>();
-  CXXMemberCallExpr *TheCall = CXXMemberCallExpr::Create(
-      Context, MemExprE, Args, ResultType, VK, RParenLoc,
-      CurFPFeatureOverrides(), Proto->getNumParams());
 
-  // Check for a valid return type.
-  if (CheckCallReturnType(Method->getReturnType(), MemExpr->getMemberLoc(),
-                          TheCall, Method))
-    return BuildRecoveryExpr(ResultType);
+  CallExpr *TheCall = nullptr;
+  llvm::SmallVector<Expr *, 8> NewArgs;
+  if (Method->isExplicitObjectMemberFunction()) {
+    PrepareExplicitObjectArgument(*this, Method, MemExpr->getBase(), Args,
+                                  NewArgs);
+    // Build the actual expression node.
+    ExprResult FnExpr =
+        CreateFunctionRefExpr(*this, Method, FoundDecl, MemExpr,
+                              HadMultipleCandidates, MemExpr->getExprLoc());
+    if (FnExpr.isInvalid())
+      return ExprError();
 
-  // Convert the object argument (for a non-static member function call).
-  // We only need to do this if there was actually an overload; otherwise
-  // it was done at lookup.
-  if (!Method->isStatic()) {
-    ExprResult ObjectArg =
-      PerformObjectArgumentInitialization(MemExpr->getBase(), Qualifier,
-                                          FoundDecl, Method);
+    TheCall =
+        CallExpr::Create(Context, FnExpr.get(), Args, ResultType, VK, RParenLoc,
+                         CurFPFeatureOverrides(), Proto->getNumParams());
+  } else {
+    // Convert the object argument (for a non-static member function call).
+    // We only need to do this if there was actually an overload; otherwise
+    // it was done at lookup.
+    ExprResult ObjectArg = PerformImplicitObjectArgumentInitialization(
+        MemExpr->getBase(), Qualifier, FoundDecl, Method);
     if (ObjectArg.isInvalid())
       return ExprError();
     MemExpr->setBase(ObjectArg.get());
+    TheCall = CXXMemberCallExpr::Create(Context, MemExprE, Args, ResultType, VK,
+                                        RParenLoc, CurFPFeatureOverrides(),
+                                        Proto->getNumParams());
   }
 
+  // Check for a valid return type.
+  if (CheckCallReturnType(Method->getReturnType(), MemExpr->getMemberLoc(),
+                          TheCall, Method))
+    return BuildRecoveryExpr(ResultType);
+
   // Convert the rest of the arguments
   if (ConvertArgumentsForCall(TheCall, MemExpr, Method, Proto, Args,
                               RParenLoc))
@@ -14915,8 +15255,8 @@
 
   if ((isa<CXXConstructorDecl>(CurContext) ||
        isa<CXXDestructorDecl>(CurContext)) &&
-      TheCall->getMethodDecl()->isPure()) {
-    const CXXMethodDecl *MD = TheCall->getMethodDecl();
+      TheCall->getDirectCallee()->isPure()) {
+    const FunctionDecl *MD = TheCall->getDirectCallee();
 
     if (isa<CXXThisExpr>(MemExpr->getBase()->IgnoreParenCasts()) &&
         MemExpr->performsVirtualDispatch(getLangOpts())) {
@@ -14933,7 +15273,7 @@
   }
 
   if (CXXDestructorDecl *DD =
-          dyn_cast<CXXDestructorDecl>(TheCall->getMethodDecl())) {
+          dyn_cast<CXXDestructorDecl>(TheCall->getDirectCallee())) {
     // a->A::f() doesn't go through the vtable, except in AppleKext mode.
     bool CallCanBeVirtual = !MemExpr->hasQualifier() || getLangOpts().AppleKext;
     CheckVirtualDtorCall(DD, MemExpr->getBeginLoc(), /*IsDelete=*/false,
@@ -14942,7 +15282,7 @@
   }
 
   return CheckForImmediateInvocation(MaybeBindToTemporary(TheCall),
-                                     TheCall->getMethodDecl());
+                                     TheCall->getDirectCallee());
 }
 
 /// BuildCallToObjectOfClassType - Build a call to an object of class
@@ -15167,8 +15507,13 @@
   // Initialize the implicit object parameter if needed.
   // Since C++23, this could also be a call to a static call operator
   // which we emit as a regular CallExpr.
-  if (Method->isInstance()) {
-    ExprResult ObjRes = PerformObjectArgumentInitialization(
+  llvm::SmallVector<Expr *, 8> NewArgs;
+  if (Method->isExplicitObjectMemberFunction()) {
+    // FIXME: we should do that during the definition of the lambda when we can
+    DiagnoseInvalidExplicitObjectParameterInLambda(Method);
+    PrepareExplicitObjectArgument(*this, Method, Obj, Args, NewArgs);
+  } else if (Method->isInstance()) {
+    ExprResult ObjRes = PerformImplicitObjectArgumentInitialization(
         Object.get(), /*Qualifier=*/nullptr, Best->FoundDecl, Method);
     if (ObjRes.isInvalid())
       IsError = true;
@@ -15310,12 +15655,19 @@
 
   // Convert the object parameter.
   CXXMethodDecl *Method = cast<CXXMethodDecl>(Best->Function);
-  ExprResult BaseResult =
-    PerformObjectArgumentInitialization(Base, /*Qualifier=*/nullptr,
-                                        Best->FoundDecl, Method);
-  if (BaseResult.isInvalid())
-    return ExprError();
-  Base = BaseResult.get();
+
+  if (Method->isExplicitObjectMemberFunction()) {
+    ExprResult R = InitializeExplicitObjectArgument(*this, Base, Method);
+    if (R.isInvalid())
+      return ExprError();
+    Base = R.get();
+  } else {
+    ExprResult BaseResult = PerformImplicitObjectArgumentInitialization(
+        Base, /*Qualifier=*/nullptr, Best->FoundDecl, Method);
+    if (BaseResult.isInvalid())
+      return ExprError();
+    Base = BaseResult.get();
+  }
 
   // Build the operator call.
   ExprResult FnExpr = CreateFunctionRefExpr(*this, Method, Best->FoundDecl,
@@ -15326,7 +15678,8 @@
   QualType ResultTy = Method->getReturnType();
   ExprValueKind VK = Expr::getValueKindForType(ResultTy);
   ResultTy = ResultTy.getNonLValueExprType(Context);
-  CXXOperatorCallExpr *TheCall =
+
+  CallExpr *TheCall =
       CXXOperatorCallExpr::Create(Context, OO_Arrow, FnExpr.get(), Base,
                                   ResultTy, VK, OpLoc, CurFPFeatureOverrides());
 
@@ -15492,37 +15845,44 @@
 /// perhaps a '&' around it). We have resolved the overloaded function
 /// to the function declaration Fn, so patch up the expression E to
 /// refer (possibly indirectly) to Fn. Returns the new expr.
-Expr *Sema::FixOverloadedFunctionReference(Expr *E, DeclAccessPair Found,
-                                           FunctionDecl *Fn) {
+ExprResult Sema::FixOverloadedFunctionReference(Expr *E, DeclAccessPair Found,
+                                                FunctionDecl *Fn) {
   if (ParenExpr *PE = dyn_cast<ParenExpr>(E)) {
-    Expr *SubExpr = FixOverloadedFunctionReference(PE->getSubExpr(),
-                                                   Found, Fn);
-    if (SubExpr == PE->getSubExpr())
+    ExprResult SubExpr =
+        FixOverloadedFunctionReference(PE->getSubExpr(), Found, Fn);
+    if (SubExpr.isInvalid())
+      return ExprError();
+    if (SubExpr.get() == PE->getSubExpr())
       return PE;
 
-    return new (Context) ParenExpr(PE->getLParen(), PE->getRParen(), SubExpr);
+    return new (Context)
+        ParenExpr(PE->getLParen(), PE->getRParen(), SubExpr.get());
   }
 
   if (ImplicitCastExpr *ICE = dyn_cast<ImplicitCastExpr>(E)) {
-    Expr *SubExpr = FixOverloadedFunctionReference(ICE->getSubExpr(),
-                                                   Found, Fn);
+    ExprResult SubExpr =
+        FixOverloadedFunctionReference(ICE->getSubExpr(), Found, Fn);
+    if (SubExpr.isInvalid())
+      return ExprError();
     assert(Context.hasSameType(ICE->getSubExpr()->getType(),
-                               SubExpr->getType()) &&
+                               SubExpr.get()->getType()) &&
            "Implicit cast type cannot be determined from overload");
     assert(ICE->path_empty() && "fixing up hierarchy conversion?");
-    if (SubExpr == ICE->getSubExpr())
+    if (SubExpr.get() == ICE->getSubExpr())
       return ICE;
 
     return ImplicitCastExpr::Create(Context, ICE->getType(), ICE->getCastKind(),
-                                    SubExpr, nullptr, ICE->getValueKind(),
+                                    SubExpr.get(), nullptr, ICE->getValueKind(),
                                     CurFPFeatureOverrides());
   }
 
   if (auto *GSE = dyn_cast<GenericSelectionExpr>(E)) {
     if (!GSE->isResultDependent()) {
-      Expr *SubExpr =
+      ExprResult SubExpr =
           FixOverloadedFunctionReference(GSE->getResultExpr(), Found, Fn);
-      if (SubExpr == GSE->getResultExpr())
+      if (SubExpr.isInvalid())
+        return ExprError();
+      if (SubExpr.get() == GSE->getResultExpr())
         return GSE;
 
       // Replace the resulting type information before rebuilding the generic
@@ -15530,7 +15890,7 @@
       ArrayRef<Expr *> A = GSE->getAssocExprs();
       SmallVector<Expr *, 4> AssocExprs(A.begin(), A.end());
       unsigned ResultIdx = GSE->getResultIndex();
-      AssocExprs[ResultIdx] = SubExpr;
+      AssocExprs[ResultIdx] = SubExpr.get();
 
       if (GSE->isExprPredicate())
         return GenericSelectionExpr::Create(
@@ -15560,15 +15920,21 @@
         // Fix the subexpression, which really has to be an
         // UnresolvedLookupExpr holding an overloaded member function
         // or template.
-        Expr *SubExpr = FixOverloadedFunctionReference(UnOp->getSubExpr(),
-                                                       Found, Fn);
-        if (SubExpr == UnOp->getSubExpr())
+        ExprResult SubExpr =
+            FixOverloadedFunctionReference(UnOp->getSubExpr(), Found, Fn);
+        if (SubExpr.isInvalid())
+          return ExprError();
+        if (SubExpr.get() == UnOp->getSubExpr())
           return UnOp;
 
-        assert(isa<DeclRefExpr>(SubExpr)
-               && "fixed to something other than a decl ref");
-        assert(cast<DeclRefExpr>(SubExpr)->getQualifier()
-               && "fixed to a member ref with no nested name qualifier");
+        if (DiagnoseMissingQualifiersInAddressOfOperand(UnOp->getBeginLoc(),
+                                                        SubExpr.get(), Method))
+          return ExprError();
+
+        assert(isa<DeclRefExpr>(SubExpr.get()) &&
+               "fixed to something other than a decl ref");
+        assert(cast<DeclRefExpr>(SubExpr.get())->getQualifier() &&
+               "fixed to a member ref with no nested name qualifier");
 
         // We have taken the address of a pointer to member
         // function. Perform the computation here so that we get the
@@ -15581,18 +15947,22 @@
         if (Context.getTargetInfo().getCXXABI().isMicrosoft())
           (void)isCompleteType(UnOp->getOperatorLoc(), MemPtrType);
 
-        return UnaryOperator::Create(
-            Context, SubExpr, UO_AddrOf, MemPtrType, VK_PRValue, OK_Ordinary,
-            UnOp->getOperatorLoc(), false, CurFPFeatureOverrides());
+        return UnaryOperator::Create(Context, SubExpr.get(), UO_AddrOf,
+                                     MemPtrType, VK_PRValue, OK_Ordinary,
+                                     UnOp->getOperatorLoc(), false,
+                                     CurFPFeatureOverrides());
       }
     }
-    Expr *SubExpr = FixOverloadedFunctionReference(UnOp->getSubExpr(),
-                                                   Found, Fn);
-    if (SubExpr == UnOp->getSubExpr())
+    ExprResult SubExpr =
+        FixOverloadedFunctionReference(UnOp->getSubExpr(), Found, Fn);
+    if (SubExpr.isInvalid())
+      return ExprError();
+    if (SubExpr.get() == UnOp->getSubExpr())
       return UnOp;
 
     // FIXME: This can't currently fail, but in principle it could.
-    return CreateBuiltinUnaryOp(UnOp->getOperatorLoc(), UO_AddrOf, SubExpr)
+    return CreateBuiltinUnaryOp(UnOp->getOperatorLoc(), UO_AddrOf,
+                                SubExpr.get())
         .get();
   }
 
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -752,7 +752,8 @@
     IsEnum = isa_and_nonnull<EnumType>(NNS->getAsType());
 
   if (!MightBeCxx11UnevalField && !isAddressOfOperand && !IsEnum &&
-      isa<CXXMethodDecl>(DC) && cast<CXXMethodDecl>(DC)->isInstance()) {
+      isa<CXXMethodDecl>(DC) &&
+      cast<CXXMethodDecl>(DC)->isImplicitObjectMemberFunction()) {
     QualType ThisType = cast<CXXMethodDecl>(DC)->getThisType();
 
     // Since the 'this' expression is synthesized, we don't need to
@@ -7073,7 +7074,8 @@
       isa<CXXMethodDecl>(DRE->getDecl())) {
     assert((isa<FieldDecl>(DRE->getDecl()) ||
             isa<IndirectFieldDecl>(DRE->getDecl()) ||
-            !cast<CXXMethodDecl>(DRE->getDecl())->isStatic()) &&
+            cast<CXXMethodDecl>(DRE->getDecl())
+                ->isImplicitObjectMemberFunction()) &&
            "Only non-static member pointers can make it here");
 
     // Okay: this is the address of a non-static member, and therefore
@@ -7625,7 +7627,10 @@
         if (DiagnoseUseOfDecl(Fn, Arg->getBeginLoc()))
           return ExprError();
 
-        Arg = FixOverloadedFunctionReference(Arg, FoundResult, Fn);
+        ExprResult Res = FixOverloadedFunctionReference(Arg, FoundResult, Fn);
+        if (Res.isInvalid())
+          return ExprError();
+        Arg = Res.get();
         ArgType = Arg->getType();
       } else
         return ExprError();
@@ -7676,8 +7681,10 @@
                                                                 FoundResult)) {
         if (DiagnoseUseOfDecl(Fn, Arg->getBeginLoc()))
           return ExprError();
-
-        Arg = FixOverloadedFunctionReference(Arg, FoundResult, Fn);
+        ExprResult Res = FixOverloadedFunctionReference(Arg, FoundResult, Fn);
+        if (Res.isInvalid())
+          return ExprError();
+        Arg = Res.get();
         ArgType = Arg->getType();
       } else
         return ExprError();
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -3685,7 +3685,9 @@
       OriginalCallArg OriginalArg = (*OriginalCallArgs)[I];
 
       auto ParamIdx = OriginalArg.ArgIdx;
-      if (ParamIdx >= Specialization->getNumParams())
+      unsigned ExplicitOffset =
+          Specialization->hasCXXExplicitFunctionObjectParameter() ? 1 : 0;
+      if (ParamIdx >= Specialization->getNumParams() - ExplicitOffset)
         // FIXME: This presumably means a pack ended up smaller than we
         // expected while deducing. Should this not result in deduction
         // failure? Can it even happen?
@@ -3695,7 +3697,8 @@
       if (!OriginalArg.DecomposedParam) {
         // P is one of the function parameters, just look up its substituted
         // type.
-        DeducedA = Specialization->getParamDecl(ParamIdx)->getType();
+        DeducedA =
+            Specialization->getParamDecl(ParamIdx + ExplicitOffset)->getType();
       } else {
         // P is a decomposed element of a parameter corresponding to a
         // braced-init-list argument. Substitute back into P to find the
@@ -3745,7 +3748,7 @@
     return {};
 
   if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Fn))
-    if (Method->isInstance()) {
+    if (Method->isImplicitObjectMemberFunction()) {
       // An instance method that's referenced in a form that doesn't
       // look like a member pointer is just invalid.
       if (!R.HasFormOfMemberPointer)
@@ -3874,7 +3877,8 @@
 /// overloaded function set that could not be resolved.
 static bool AdjustFunctionParmAndArgTypesForDeduction(
     Sema &S, TemplateParameterList *TemplateParams, unsigned FirstInnerIndex,
-    QualType &ParamType, QualType &ArgType, Expr *Arg, unsigned &TDF,
+    QualType &ParamType, QualType &ArgType,
+    Expr::Classification ArgClassification, Expr *Arg, unsigned &TDF,
     TemplateSpecCandidateSet *FailedTSC = nullptr) {
   // C++0x [temp.deduct.call]p3:
   //   If P is a cv-qualified type, the top level cv-qualifiers of P's type
@@ -3892,6 +3896,7 @@
   // but there are sometimes special circumstances.  Typically
   // involving a template-id-expr.
   if (ArgType == S.Context.OverloadTy) {
+    assert(Arg && "expected a non-null arg expression");
     ArgType = ResolveOverloadForDeduction(S, TemplateParams, Arg, ParamType,
                                           ParamRefType != nullptr, FailedTSC);
     if (ArgType.isNull())
@@ -3900,14 +3905,16 @@
 
   if (ParamRefType) {
     // If the argument has incomplete array type, try to complete its type.
-    if (ArgType->isIncompleteArrayType())
+    if (ArgType->isIncompleteArrayType()) {
+      assert(Arg && "expected a non-null arg expression");
       ArgType = S.getCompletedType(Arg);
+    }
 
     // C++1z [temp.deduct.call]p3:
     //   If P is a forwarding reference and the argument is an lvalue, the type
     //   "lvalue reference to A" is used in place of A for type deduction.
     if (isForwardingReference(QualType(ParamRefType, 0), FirstInnerIndex) &&
-        Arg->isLValue()) {
+        ArgClassification.isLValue()) {
       if (S.getLangOpts().OpenCL && !ArgType.hasAddressSpace())
         ArgType = S.Context.getAddrSpaceQualType(
             ArgType, S.Context.getDefaultOpenCLPointeeAddrSpace());
@@ -3968,7 +3975,9 @@
 
 static Sema::TemplateDeductionResult DeduceTemplateArgumentsFromCallArgument(
     Sema &S, TemplateParameterList *TemplateParams, unsigned FirstInnerIndex,
-    QualType ParamType, Expr *Arg, TemplateDeductionInfo &Info,
+    QualType ParamType, QualType ArgType,
+    Expr::Classification ArgClassification, Expr *Arg,
+    TemplateDeductionInfo &Info,
     SmallVectorImpl<DeducedTemplateArgument> &Deduced,
     SmallVectorImpl<Sema::OriginalCallArg> &OriginalCallArgs,
     bool DecomposedParam, unsigned ArgIdx, unsigned TDF,
@@ -4013,8 +4022,9 @@
   if (ElTy->isDependentType()) {
     for (Expr *E : ILE->inits()) {
       if (auto Result = DeduceTemplateArgumentsFromCallArgument(
-              S, TemplateParams, 0, ElTy, E, Info, Deduced, OriginalCallArgs, true,
-              ArgIdx, TDF))
+              S, TemplateParams, 0, ElTy, E->getType(),
+              E->Classify(S.getASTContext()), E, Info, Deduced,
+              OriginalCallArgs, true, ArgIdx, TDF))
         return Result;
     }
   }
@@ -4045,23 +4055,25 @@
 ///        single parameter / argument pair.
 static Sema::TemplateDeductionResult DeduceTemplateArgumentsFromCallArgument(
     Sema &S, TemplateParameterList *TemplateParams, unsigned FirstInnerIndex,
-    QualType ParamType, Expr *Arg, TemplateDeductionInfo &Info,
+    QualType ParamType, QualType ArgType,
+    Expr::Classification ArgClassification, Expr *Arg,
+    TemplateDeductionInfo &Info,
     SmallVectorImpl<DeducedTemplateArgument> &Deduced,
     SmallVectorImpl<Sema::OriginalCallArg> &OriginalCallArgs,
     bool DecomposedParam, unsigned ArgIdx, unsigned TDF,
     TemplateSpecCandidateSet *FailedTSC) {
-  QualType ArgType = Arg->getType();
+
   QualType OrigParamType = ParamType;
 
   //   If P is a reference type [...]
   //   If P is a cv-qualified type [...]
-  if (AdjustFunctionParmAndArgTypesForDeduction(S, TemplateParams,
-                                                FirstInnerIndex, ParamType,
-                                                ArgType, Arg, TDF, FailedTSC))
+  if (AdjustFunctionParmAndArgTypesForDeduction(
+          S, TemplateParams, FirstInnerIndex, ParamType, ArgType,
+          ArgClassification, Arg, TDF, FailedTSC))
     return Sema::TDK_Success;
 
   //   If [...] the argument is a non-empty initializer list [...]
-  if (InitListExpr *ILE = dyn_cast<InitListExpr>(Arg))
+  if (InitListExpr *ILE = dyn_cast_if_present<InitListExpr>(Arg))
     return DeduceFromInitializerList(S, TemplateParams, ParamType, ILE, Info,
                                      Deduced, OriginalCallArgs, ArgIdx, TDF);
 
@@ -4070,8 +4082,9 @@
   //
   // Keep track of the argument type and corresponding parameter index,
   // so we can check for compatibility between the deduced A and A.
-  OriginalCallArgs.push_back(
-      Sema::OriginalCallArg(OrigParamType, DecomposedParam, ArgIdx, ArgType));
+  if (Arg)
+    OriginalCallArgs.push_back(
+        Sema::OriginalCallArg(OrigParamType, DecomposedParam, ArgIdx, ArgType));
   return DeduceTemplateArgumentsByTypeMatch(S, TemplateParams, ParamType,
                                             ArgType, Info, Deduced, TDF);
 }
@@ -4106,12 +4119,19 @@
     TemplateArgumentListInfo *ExplicitTemplateArgs, ArrayRef<Expr *> Args,
     FunctionDecl *&Specialization, TemplateDeductionInfo &Info,
     bool PartialOverloading, bool AggregateDeductionCandidate,
+    QualType ObjectType, Expr::Classification ObjectClassification,
     llvm::function_ref<bool(ArrayRef<QualType>)> CheckNonDependent) {
   if (FunctionTemplate->isInvalidDecl())
     return TDK_Invalid;
 
   FunctionDecl *Function = FunctionTemplate->getTemplatedDecl();
   unsigned NumParams = Function->getNumParams();
+  bool HasExplicitObject = false;
+  int ExplicitObjectOffset = 0;
+  if (Function->hasCXXExplicitFunctionObjectParameter()) {
+    HasExplicitObject = true;
+    ExplicitObjectOffset = 1;
+  }
 
   unsigned FirstInnerIndex = getFirstInnerIndex(FunctionTemplate);
 
@@ -4119,9 +4139,11 @@
   //   Template argument deduction is done by comparing each function template
   //   parameter type (call it P) with the type of the corresponding argument
   //   of the call (call it A) as described below.
-  if (Args.size() < Function->getMinRequiredArguments() && !PartialOverloading)
+  if (Args.size() < Function->getMinRequiredExplicitArguments() &&
+      !PartialOverloading)
     return TDK_TooFewArguments;
-  else if (TooManyArguments(NumParams, Args.size(), PartialOverloading)) {
+  else if (TooManyArguments(NumParams, Args.size() + ExplicitObjectOffset,
+                            PartialOverloading)) {
     const auto *Proto = Function->getType()->castAs<FunctionProtoType>();
     if (Proto->isTemplateVariadic())
       /* Do nothing */;
@@ -4157,7 +4179,8 @@
   SmallVector<OriginalCallArg, 8> OriginalCallArgs;
 
   // Deduce an argument of type ParamType from an expression with index ArgIdx.
-  auto DeduceCallArgument = [&](QualType ParamType, unsigned ArgIdx) {
+  auto DeduceCallArgument = [&](QualType ParamType, unsigned ArgIdx,
+                                bool ExplicitObjetArgument) {
     // C++ [demp.deduct.call]p1: (DR1391)
     //   Template argument deduction is done by comparing each function template
     //   parameter that contains template-parameters that participate in
@@ -4165,10 +4188,21 @@
     if (!hasDeducibleTemplateParameters(*this, FunctionTemplate, ParamType))
       return Sema::TDK_Success;
 
+    if (ExplicitObjetArgument) {
+      //   ... with the type of the corresponding argument
+      return DeduceTemplateArgumentsFromCallArgument(
+          *this, TemplateParams, FirstInnerIndex, ParamType, ObjectType,
+          ObjectClassification,
+          /*Arg=*/nullptr, Info, Deduced, OriginalCallArgs,
+          /*Decomposed*/ false, ArgIdx, /*TDF*/ 0);
+    }
+
     //   ... with the type of the corresponding argument
     return DeduceTemplateArgumentsFromCallArgument(
-        *this, TemplateParams, FirstInnerIndex, ParamType, Args[ArgIdx], Info, Deduced,
-        OriginalCallArgs, /*Decomposed*/false, ArgIdx, /*TDF*/ 0);
+        *this, TemplateParams, FirstInnerIndex, ParamType,
+        Args[ArgIdx]->getType(), Args[ArgIdx]->Classify(getASTContext()),
+        Args[ArgIdx], Info, Deduced, OriginalCallArgs, /*Decomposed*/ false,
+        ArgIdx, /*TDF*/ 0);
   };
 
   // Deduce template arguments from the function parameters.
@@ -4182,11 +4216,20 @@
         dyn_cast<PackExpansionType>(ParamType);
     if (!ParamExpansion) {
       // Simple case: matching a function parameter to a function argument.
-      if (ArgIdx >= Args.size())
+      if (ArgIdx >= Args.size() && !(HasExplicitObject && ParamIdx == 0))
         break;
 
       ParamTypesForArgChecking.push_back(ParamType);
-      if (auto Result = DeduceCallArgument(ParamType, ArgIdx++))
+
+      if (ParamIdx == 0 && HasExplicitObject) {
+        if (auto Result = DeduceCallArgument(ParamType, 0,
+                                             /*ExplicitObjetArgument=*/true))
+          return Result;
+        continue;
+      }
+
+      if (auto Result = DeduceCallArgument(ParamType, ArgIdx++,
+                                           /*ExplicitObjetArgument=*/false))
         return Result;
 
       continue;
@@ -4219,7 +4262,8 @@
       for (; ArgIdx < Args.size() && PackScope.hasNextElement();
            PackScope.nextPackElement(), ++ArgIdx) {
         ParamTypesForArgChecking.push_back(ParamPattern);
-        if (auto Result = DeduceCallArgument(ParamPattern, ArgIdx))
+        if (auto Result = DeduceCallArgument(ParamPattern, ArgIdx,
+                                             /*ExplicitObjetArgument=*/false))
           return Result;
       }
     } else {
@@ -4459,11 +4503,10 @@
 /// Deduce template arguments for a templated conversion
 /// function (C++ [temp.deduct.conv]) and, if successful, produce a
 /// conversion function template specialization.
-Sema::TemplateDeductionResult
-Sema::DeduceTemplateArguments(FunctionTemplateDecl *ConversionTemplate,
-                              QualType ToType,
-                              CXXConversionDecl *&Specialization,
-                              TemplateDeductionInfo &Info) {
+Sema::TemplateDeductionResult Sema::DeduceTemplateArguments(
+    FunctionTemplateDecl *ConversionTemplate, QualType ObjectType,
+    Expr::Classification ObjectClassification, QualType ToType,
+    CXXConversionDecl *&Specialization, TemplateDeductionInfo &Info) {
   if (ConversionTemplate->isInvalidDecl())
     return TDK_Invalid;
 
@@ -4558,6 +4601,19 @@
   if ((P->isPointerType() && A->isPointerType()) ||
       (P->isMemberPointerType() && A->isMemberPointerType()))
     TDF |= TDF_IgnoreQualifiers;
+
+  SmallVector<Sema::OriginalCallArg, 1> OriginalCallArgs;
+  if (ConversionGeneric->isExplicitObjectMemberFunction()) {
+    QualType ParamType = ConversionGeneric->getParamDecl(0)->getType();
+    if (TemplateDeductionResult Result =
+            DeduceTemplateArgumentsFromCallArgument(
+                *this, TemplateParams, getFirstInnerIndex(ConversionTemplate),
+                ParamType, ObjectType, ObjectClassification,
+                /*Arg=*/nullptr, Info, Deduced, OriginalCallArgs,
+                /*Decomposed*/ false, 0, /*TDF*/ 0))
+      return Result;
+  }
+
   if (TemplateDeductionResult Result
         = DeduceTemplateArgumentsByTypeMatch(*this, TemplateParams,
                                              P, A, Info, Deduced, TDF))
@@ -4570,7 +4626,8 @@
   TemplateDeductionResult Result;
   runWithSufficientStackSpace(Info.getLocation(), [&] {
     Result = FinishTemplateArgumentDeduction(ConversionTemplate, Deduced, 0,
-                                             ConversionSpecialized, Info);
+                                             ConversionSpecialized, Info,
+                                             &OriginalCallArgs);
   });
   Specialization = cast_or_null<CXXConversionDecl>(ConversionSpecialized);
   return Result;
@@ -4846,7 +4903,8 @@
         if (isa<DesignatedInitExpr>(Init))
           return TDK_Invalid;
         if (auto TDK = DeduceTemplateArgumentsFromCallArgument(
-                *this, TemplateParamsSt.get(), 0, TemplArg, Init, Info, Deduced,
+                *this, TemplateParamsSt.get(), 0, TemplArg, Init->getType(),
+                Init->Classify(getASTContext()), Init, Info, Deduced,
                 OriginalCallArgs, /*Decomposed=*/true,
                 /*ArgIdx=*/0, /*TDF=*/0)) {
           if (TDK == TDK_Inconsistent) {
@@ -4872,7 +4930,8 @@
       assert(!FuncParam.isNull() &&
              "substituting template parameter for 'auto' failed");
       if (auto TDK = DeduceTemplateArgumentsFromCallArgument(
-              *this, TemplateParamsSt.get(), 0, FuncParam, Init, Info, Deduced,
+              *this, TemplateParamsSt.get(), 0, FuncParam, Init->getType(),
+              Init->Classify(getASTContext()), Init, Info, Deduced,
               OriginalCallArgs, /*Decomposed=*/false, /*ArgIdx=*/0, /*TDF=*/0,
               FailedTSC))
         return DeductionFailed(TDK);
@@ -5080,6 +5139,8 @@
   //
   // The standard doesn't say explicitly, but we pick the appropriate kind of
   // reference type based on [over.match.funcs]p4.
+  assert(Method && Method->isImplicitObjectMemberFunction() &&
+         "expected an implicit objet function");
   QualType ArgTy = Context.getTypeDeclType(Method->getParent());
   ArgTy = Context.getQualifiedType(ArgTy, Method->getMethodQualifiers());
   if (Method->getRefQualifier() == RQ_RValue)
@@ -5141,14 +5202,17 @@
 
     unsigned NumComparedArguments = NumCallArguments1;
 
-    if (!Method2 && Method1 && !Method1->isStatic()) {
+    if (!Method2 && Method1 && Method1->isImplicitObjectMemberFunction()) {
       // Compare 'this' from Method1 against first parameter from Method2.
       AddImplicitObjectParameterType(S.Context, Method1, Args1);
       ++NumComparedArguments;
-    } else if (!Method1 && Method2 && !Method2->isStatic()) {
+    } else if (!Method1 && Method2 &&
+               Method2->isImplicitObjectMemberFunction()) {
       // Compare 'this' from Method2 against first parameter from Method1.
       AddImplicitObjectParameterType(S.Context, Method2, Args2);
-    } else if (Method1 && Method2 && Reversed) {
+    } else if (Method1 && Method2 && Reversed &&
+               Method1->isImplicitObjectMemberFunction() &&
+               Method2->isImplicitObjectMemberFunction()) {
       // Compare 'this' from Method1 against second parameter from Method2
       // and 'this' from Method2 against second parameter from Method1.
       AddImplicitObjectParameterType(S.Context, Method1, Args1);
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -2891,6 +2891,8 @@
     NewParm->setUninstantiatedDefaultArg(Arg);
   }
 
+  NewParm->setExplicitObjectParameterLoc(
+      OldParm->getExplicitObjectParamThisLoc());
   NewParm->setHasInheritedDefaultArg(OldParm->hasInheritedDefaultArg());
 
   if (OldParm->isParameterPack() && !NewParm->isParameterPack()) {
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -4423,7 +4423,7 @@
   Qualifiers ThisTypeQuals;
   if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(D)) {
     ThisContext = cast<CXXRecordDecl>(Owner);
-    ThisTypeQuals = Method->getMethodQualifiers();
+    ThisTypeQuals = Method->getFunctionObjectParameterType().getQualifiers();
   }
 
   TypeSourceInfo *NewTInfo = SemaRef.SubstFunctionDeclType(
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -1706,6 +1706,7 @@
   PD->ParmVarDeclBits.HasInheritedDefaultArg = Record.readInt();
   if (Record.readInt()) // hasUninstantiatedDefaultArg.
     PD->setUninstantiatedDefaultArg(Record.readExpr());
+  PD->ExplicitObjectParameterIntroducerLoc = Record.readSourceLocation();
 
   // FIXME: If this is a redeclaration of a function from another module, handle
   // inheritance of default arguments.
diff --git a/clang/lib/Serialization/ASTReaderStmt.cpp b/clang/lib/Serialization/ASTReaderStmt.cpp
--- a/clang/lib/Serialization/ASTReaderStmt.cpp
+++ b/clang/lib/Serialization/ASTReaderStmt.cpp
@@ -596,6 +596,7 @@
   E->DeclRefExprBits.RefersToEnclosingVariableOrCapture = Record.readInt();
   E->DeclRefExprBits.NonOdrUseReason = Record.readInt();
   E->DeclRefExprBits.IsImmediateEscalating = Record.readInt();
+  E->DeclRefExprBits.CapturedByCopyInLambdaWithExplicitObjectParameter = false;
   unsigned NumTemplateArgs = 0;
   if (E->hasTemplateKWAndArgsInfo())
     NumTemplateArgs = Record.readInt();
diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -1162,28 +1162,21 @@
   Record.push_back(D->hasUninstantiatedDefaultArg());
   if (D->hasUninstantiatedDefaultArg())
     Record.AddStmt(D->getUninstantiatedDefaultArg());
+  Record.AddSourceLocation(D->getExplicitObjectParamThisLoc());
   Code = serialization::DECL_PARM_VAR;
 
   // If the assumptions about the DECL_PARM_VAR abbrev are true, use it.  Here
   // we dynamically check for the properties that we optimize for, but don't
   // know are true of all PARM_VAR_DECLs.
-  if (D->getDeclContext() == D->getLexicalDeclContext() &&
-      !D->hasAttrs() &&
-      !D->hasExtInfo() &&
-      !D->isImplicit() &&
-      !D->isUsed(false) &&
-      !D->isInvalidDecl() &&
-      !D->isReferenced() &&
-      D->getAccess() == AS_none &&
-      !D->isModulePrivate() &&
-      D->getStorageClass() == 0 &&
+  if (D->getDeclContext() == D->getLexicalDeclContext() && !D->hasAttrs() &&
+      !D->hasExtInfo() && !D->isImplicit() && !D->isUsed(false) &&
+      !D->isInvalidDecl() && !D->isReferenced() && D->getAccess() == AS_none &&
+      !D->isModulePrivate() && D->getStorageClass() == 0 &&
       D->getInitStyle() == VarDecl::CInit && // Can params have anything else?
-      D->getFunctionScopeDepth() == 0 &&
-      D->getObjCDeclQualifier() == 0 &&
-      !D->isKNRPromoted() &&
-      !D->hasInheritedDefaultArg() &&
-      D->getInit() == nullptr &&
-      !D->hasUninstantiatedDefaultArg())  // No default expr.
+      D->getFunctionScopeDepth() == 0 && D->getObjCDeclQualifier() == 0 &&
+      !D->isKNRPromoted() && !D->isExplicitObjectParameter() &&
+      !D->hasInheritedDefaultArg() && D->getInit() == nullptr &&
+      !D->hasUninstantiatedDefaultArg()) // No default expr.
     AbbrevToUse = Writer.getDeclParmVarAbbrev();
 
   // Check things we know are true of *every* PARM_VAR_DECL, which is more than
diff --git a/clang/lib/StaticAnalyzer/Core/ExprEngine.cpp b/clang/lib/StaticAnalyzer/Core/ExprEngine.cpp
--- a/clang/lib/StaticAnalyzer/Core/ExprEngine.cpp
+++ b/clang/lib/StaticAnalyzer/Core/ExprEngine.cpp
@@ -299,7 +299,7 @@
   }
 
   if (const auto *MD = dyn_cast<CXXMethodDecl>(D)) {
-    if (!MD->isStatic()) {
+    if (MD->isImplicitObjectMemberFunction()) {
       // Precondition: 'this' is always non-null upon entry to the
       // top-level function.  This is our starting assumption for
       // analyzing an "open" program.
@@ -2112,7 +2112,7 @@
       // valid region.
       const Decl *Callee = OCE->getCalleeDecl();
       if (const auto *MD = dyn_cast_or_null<CXXMethodDecl>(Callee)) {
-        if (MD->isInstance()) {
+        if (MD->isImplicitObjectMemberFunction()) {
           ProgramStateRef State = Pred->getState();
           const LocationContext *LCtx = Pred->getLocationContext();
           ProgramStateRef NewState =
@@ -3355,7 +3355,7 @@
 
       // Handle C++ method calls.
       if (const auto *MD = dyn_cast<CXXMethodDecl>(Member)) {
-        if (MD->isInstance())
+        if (MD->isImplicitObjectMemberFunction())
           state = createTemporaryRegionIfNeeded(state, LCtx, BaseExpr);
 
         SVal MDVal = svalBuilder.getFunctionPointer(MD);
diff --git a/clang/lib/StaticAnalyzer/Core/LoopWidening.cpp b/clang/lib/StaticAnalyzer/Core/LoopWidening.cpp
--- a/clang/lib/StaticAnalyzer/Core/LoopWidening.cpp
+++ b/clang/lib/StaticAnalyzer/Core/LoopWidening.cpp
@@ -84,7 +84,7 @@
   // pointer should remain unchanged.  Ignore static methods, since they do not
   // have 'this' pointers.
   const CXXMethodDecl *CXXMD = dyn_cast<CXXMethodDecl>(STC->getDecl());
-  if (CXXMD && !CXXMD->isStatic()) {
+  if (CXXMD && CXXMD->isImplicitObjectMemberFunction()) {
     const CXXThisRegion *ThisR =
         MRMgr.getCXXThisRegion(CXXMD->getThisType(), STC);
     ITraits.setTrait(ThisR,
diff --git a/clang/lib/StaticAnalyzer/Core/SValBuilder.cpp b/clang/lib/StaticAnalyzer/Core/SValBuilder.cpp
--- a/clang/lib/StaticAnalyzer/Core/SValBuilder.cpp
+++ b/clang/lib/StaticAnalyzer/Core/SValBuilder.cpp
@@ -275,7 +275,7 @@
     // We don't need to play a similar trick for static member fields
     // because these are represented as plain VarDecls and not FieldDecls
     // in the AST.
-    if (MD->isStatic())
+    if (!MD->isImplicitObjectMemberFunction())
       return getFunctionPointer(MD);
   }
 
diff --git a/clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct/dcl.fct.def.default/p1.cpp b/clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct/dcl.fct.def.default/p1.cpp
--- a/clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct/dcl.fct.def.default/p1.cpp
+++ b/clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct/dcl.fct.def.default/p1.cpp
@@ -5,10 +5,10 @@
 //   -- not have default arguments
 struct DefArg {
   static DefArg &&make();
-  DefArg(int n = 5) = default; // expected-error {{an explicitly-defaulted constructor cannot have default arguments}}
-  DefArg(const DefArg &DA = make()) = default; // expected-error {{an explicitly-defaulted constructor cannot have default arguments}}
+  DefArg(int n = 5) = default; // expected-error {{an explicitly-defaulted default constructor cannot have default arguments}}
+  DefArg(const DefArg &DA = make()) = default; // expected-error {{an explicitly-defaulted default constructor cannot have default arguments}}
   DefArg(const DefArg &DA, int k = 3) = default; // expected-error {{an explicitly-defaulted copy constructor cannot have default arguments}}
-  DefArg(DefArg &&DA = make()) = default; // expected-error {{an explicitly-defaulted constructor cannot have default arguments}}
+  DefArg(DefArg &&DA = make()) = default; // expected-error {{an explicitly-defaulted default constructor cannot have default arguments}}
   DefArg(DefArg &&DA, int k = 3) = default; // expected-error {{an explicitly-defaulted move constructor cannot have default arguments}}
   DefArg &operator=(const DefArg&, int k = 4) = default; // expected-error {{parameter of overloaded 'operator=' cannot have a default argument}}
   DefArg &operator=(DefArg&&, int k = 4) = default; // expected-error {{parameter of overloaded 'operator=' cannot have a default argument}}
diff --git a/clang/test/CXX/drs/dr25xx.cpp b/clang/test/CXX/drs/dr25xx.cpp
--- a/clang/test/CXX/drs/dr25xx.cpp
+++ b/clang/test/CXX/drs/dr25xx.cpp
@@ -81,6 +81,48 @@
 #endif
 } // namespace dr2521
 
+
+#if __cplusplus >= 202302L
+namespace dr2553 { // dr2553: 16 open
+struct B {
+  virtual void f(this B&);   // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+  static void f(this B&);   // expected-error {{an explicit object parameter cannot appear in a static function}}
+  virtual void g(); // expected-note {{here}}
+};
+struct D : B {
+  void g(this D&); // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+};
+}
+#endif
+
+#if __cplusplus >= 202302L
+namespace dr2554 { // dr2554: 16 open
+struct B {
+  virtual void f(); // expected-note {{here}}
+};
+
+struct D : B {
+  void f(this D&); // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+};
+}
+#endif
+
+#if __cplusplus >= 202302L
+namespace dr2561 { // dr2561: 16 open
+struct C {
+    constexpr C(auto) { }
+};
+void foo() {
+    constexpr auto b = [](this C) { return 1; };
+    constexpr int (*fp)(C) = b;
+    static_assert(fp(1) == 1);
+    static_assert((&decltype(b)::operator())(1) == 1);
+}
+
+}
+#endif
+
+
 namespace dr2565 { // dr2565: 16 open
 #if __cplusplus >= 202002L
   template<typename T>
diff --git a/clang/test/CXX/drs/dr26xx.cpp b/clang/test/CXX/drs/dr26xx.cpp
--- a/clang/test/CXX/drs/dr26xx.cpp
+++ b/clang/test/CXX/drs/dr26xx.cpp
@@ -1,4 +1,6 @@
-// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-unknown %s -verify
+// RUN: %clang_cc1 -std=c++20 -Wno-c++2b-extensions -triple x86_64-unknown-unknown %s -verify
+// RUN: %clang_cc1 -std=c++2b -triple x86_64-unknown-unknown %s -verify
+
 
 namespace dr2621 { // dr2621: yes
 enum class E { a };
@@ -108,6 +110,7 @@
 
 }
 
+#if __cplusplus >= 202302L
 namespace dr2650 { // dr2650: yes
 template <class T, T> struct S {};
 template <class T> int f(S<T, T{}>*); // expected-note {{type 'X' of non-type template parameter is not a structural type}}
@@ -116,6 +119,16 @@
 };
 int i0 = f<X>(0);   //expected-error {{no matching function for call to 'f'}}
 }
+#endif
+
+#if __cplusplus >= 202302L
+namespace dr2653 { // dr2653: 16
+  struct Test { void f(this const auto& = Test{}); };
+  // expected-error@-1 {{an explicit object parameter cannot have a default argument}}
+  auto L =[](this const auto& = Test{}){};
+  // expected-error@-1 {{an explicit object parameter cannot have a default argument}}
+}
+#endif
 
 namespace dr2654 { // dr2654: 16
 void f() {
@@ -150,3 +163,19 @@
 
 J j = { "ghi" };  // expected-error {{no viable constructor or deduction guide}}
 }
+
+#if __cplusplus >= 202302L
+namespace dr2687 { // dr2687: 18
+struct S{
+    void f(int);
+    static void g(int);
+    void h(this const S&, int);
+};
+
+void test() {
+    (&S::f)(1); //expected-error {{called object type 'void (dr2687::S::*)(int)' is not a function or function pointer}}
+    (&S::g)(1);
+    (&S::h)(S(), 1);
+}
+}
+#endif
diff --git a/clang/test/CXX/over/over.load/p2-0x.cpp b/clang/test/CXX/over/over.load/p2-0x.cpp
--- a/clang/test/CXX/over/over.load/p2-0x.cpp
+++ b/clang/test/CXX/over/over.load/p2-0x.cpp
@@ -7,10 +7,10 @@
 // the same template parameter lists cannot be overloaded if any of
 // them, but not all, have a ref-qualifier (8.3.5).
 
-class Y { 
-  void h() &; 
-  void h() const &; 
-  void h() &&; 
+class Y {
+  void h() &;
+  void h() const &;
+  void h() &&;
   void i() &; // expected-note{{previous declaration}}
   void i() const; // expected-error{{cannot overload a member function without a ref-qualifier with a member function with ref-qualifier '&'}}
 
diff --git a/clang/test/CXX/special/class.copy/p20.cpp b/clang/test/CXX/special/class.copy/p20.cpp
--- a/clang/test/CXX/special/class.copy/p20.cpp
+++ b/clang/test/CXX/special/class.copy/p20.cpp
@@ -11,7 +11,7 @@
   NonConstCopy &operator=(NonConstCopy&);
 };
 
-struct VirtualInheritsNonConstCopy : virtual NonConstCopy { 
+struct VirtualInheritsNonConstCopy : virtual NonConstCopy {
   VirtualInheritsNonConstCopy();
   VirtualInheritsNonConstCopy &operator=(const VirtualInheritsNonConstCopy&);
 };
diff --git a/clang/test/CXX/special/class.copy/p25-0x.cpp b/clang/test/CXX/special/class.copy/p25-0x.cpp
--- a/clang/test/CXX/special/class.copy/p25-0x.cpp
+++ b/clang/test/CXX/special/class.copy/p25-0x.cpp
@@ -1,8 +1,16 @@
 // RUN: %clang_cc1 -std=c++11 -verify %s -Wno-deprecated-builtins
 // RUN: %clang_cc1 -std=c++11 -verify %s -Wno-deprecated-builtins -fclang-abi-compat=14 -DCLANG_ABI_COMPAT=14
+// RUN: %clang_cc1 -fsyntax-only -std=c++2b -DDEDUCING_THIS -Wno-deprecated-builtins  %s -verify
 
 // expected-no-diagnostics
 
+#if DEDUCING_THIS
+#define EXPLICIT_PARAMETER(...) this __VA_ARGS__,
+#else
+#define EXPLICIT_PARAMETER(Param)
+#endif
+
+
 template<typename T, bool B> struct trivially_assignable_check {
   static_assert(B == __has_trivial_assign(T), "");
   static_assert(B == __is_trivially_assignable(T&, T), "");
@@ -23,14 +31,18 @@
 
 // A copy/move assignment operator for class X is trivial if it is not user-provided,
 struct UserProvided {
-  UserProvided &operator=(const UserProvided &);
+  UserProvided &operator=(EXPLICIT_PARAMETER(UserProvided&)
+                          const UserProvided &);
 };
 using _ = not_trivially_assignable<UserProvided>;
 
 // its declared parameter type is the same as if it had been implicitly
 // declared,
 struct NonConstCopy {
-  NonConstCopy &operator=(NonConstCopy &) = default;
+  NonConstCopy &operator=(EXPLICIT_PARAMETER(NonConstCopy&) NonConstCopy &) = default;
+#if DEDUCING_THIS
+  NonConstCopy &operator=(EXPLICIT_PARAMETER(NonConstCopy&&) NonConstCopy &) = default;
+#endif
 };
 #if defined(CLANG_ABI_COMPAT) && CLANG_ABI_COMPAT <= 14
 // Up until (and including) Clang 14, non-const copy assignment operators were not trivial because
@@ -50,9 +62,20 @@
 static_assert(!__is_trivially_assignable(NonConstCopy &&, NonConstCopy &&), "");
 
 struct DefaultedSpecialMembers {
-  DefaultedSpecialMembers &operator=(const DefaultedSpecialMembers &) = default;
-  DefaultedSpecialMembers &operator=(DefaultedSpecialMembers &) = default;
-  DefaultedSpecialMembers &operator=(DefaultedSpecialMembers &&) = default;
+  DefaultedSpecialMembers &operator=(EXPLICIT_PARAMETER(DefaultedSpecialMembers&)
+                                     const DefaultedSpecialMembers &) = default;
+  DefaultedSpecialMembers &operator=(EXPLICIT_PARAMETER(DefaultedSpecialMembers&)
+                                     DefaultedSpecialMembers &) = default;
+  DefaultedSpecialMembers &operator=(EXPLICIT_PARAMETER(DefaultedSpecialMembers&)
+                                     DefaultedSpecialMembers &&) = default;
+#if DEDUCING_THIS
+  DefaultedSpecialMembers &operator=(EXPLICIT_PARAMETER(DefaultedSpecialMembers&&)
+                                     const DefaultedSpecialMembers &) = default;
+  DefaultedSpecialMembers &operator=(EXPLICIT_PARAMETER(DefaultedSpecialMembers&&)
+                                     DefaultedSpecialMembers &) = default;
+  DefaultedSpecialMembers &operator=(EXPLICIT_PARAMETER(DefaultedSpecialMembers&&)
+                                     DefaultedSpecialMembers &&) = default;
+#endif
 };
 using _ = trivially_assignable<DefaultedSpecialMembers>;
 #endif
@@ -84,11 +107,16 @@
 
 // Both trivial and non-trivial special members.
 struct TNT {
-  TNT &operator=(const TNT &) = default; // trivial
-  TNT &operator=(TNT &); // non-trivial
-
-  TNT &operator=(TNT &&) = default; // trivial
-  TNT &operator=(const TNT &&); // non-trivial
+  TNT &operator=(EXPLICIT_PARAMETER(TNT&) const TNT &) = default; // trivial
+  TNT &operator=(EXPLICIT_PARAMETER(TNT&) TNT &); // non-trivial
+  TNT &operator=(EXPLICIT_PARAMETER(TNT&) TNT &&) = default; // trivial
+  TNT &operator=(EXPLICIT_PARAMETER(TNT&) const TNT &&); // non-trivial
+#if DEDUCING_THIS
+  TNT &operator=(EXPLICIT_PARAMETER(TNT&&) const TNT &) = default; // trivial
+  TNT &operator=(EXPLICIT_PARAMETER(TNT&&) TNT &); // non-trivial
+  TNT &operator=(EXPLICIT_PARAMETER(TNT&&) TNT &&) = default; // trivial
+  TNT &operator=(EXPLICIT_PARAMETER(TNT&&) const TNT &&); // non-trivial
+#endif
 };
 
 static_assert(!__has_trivial_assign(TNT), "lie deliberately for gcc compatibility");
diff --git a/clang/test/CodeGenCXX/cxx2b-deducing-this.cpp b/clang/test/CodeGenCXX/cxx2b-deducing-this.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx2b-deducing-this.cpp
@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -std=c++2b %s -emit-llvm -triple x86_64-linux -o - | FileCheck %s
+// RUN: %clang_cc1 -std=c++2b %s -emit-llvm -triple x86_64-windows-msvc -o - | FileCheck %s
+
+struct TrivialStruct {
+    void explicit_object_function(this TrivialStruct) {}
+};
+void test() {
+    TrivialStruct s;
+    s.explicit_object_function();
+}
+// CHECK:      define {{.*}}test{{.*}}
+// CHECK-NEXT: entry:
+// CHECK:      {{.*}} = alloca %struct.TrivialStruct, align 1
+// CHECK:      {{.*}} = alloca %struct.TrivialStruct, align 1
+// CHECK:      call void {{.*}}explicit_object_function{{.*}}
+// CHECK-NEXT: ret void
+// CHECK-NEXT: }
+
+// CHECK:      define {{.*}}explicit_object_function{{.*}}
+// CHECK-NEXT: entry:
+// CHECK:        {{.*}} = alloca %struct.TrivialStruct, align 1
+// CHECK:        ret void
+// CHECK-NEXT: }
diff --git a/clang/test/CodeGenCXX/cxx2b-mangle-deducing-this.cpp b/clang/test/CodeGenCXX/cxx2b-mangle-deducing-this.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx2b-mangle-deducing-this.cpp
@@ -0,0 +1,64 @@
+// RUN: %clang_cc1 -std=c++2b -fno-rtti -emit-llvm -triple x86_64-linux -o - %s  2>/dev/null | FileCheck %s
+
+struct S {
+friend void test();
+public:
+    void a(this auto){}
+    void b(this auto&){}
+    void c(this S){}
+    void c(this S, int){}
+private:
+    void d(this auto){}
+    void e(this auto&){}
+    void f(this S){}
+    void f(this S, int){}
+protected:
+    void g(this auto){}
+    void h(this auto&){}
+    void i(this S){}
+    void i(this S, int){}
+};
+
+
+void test() {
+    S s;
+    s.a();
+    // CHECK: call void @_ZNH1S1aIS_EEvT_
+    s.b();
+    // CHECK: call void @_ZNH1S1bIS_EEvRT_
+    s.c();
+    // CHECK: call void @_ZNH1S1cES_
+    s.c(0);
+    // CHECK: call void @_ZNH1S1cES_i
+    s.d();
+    // CHECK: call void @_ZNH1S1dIS_EEvT_
+    s.e();
+    // CHECK: call void @_ZNH1S1eIS_EEvRT_
+    s.f();
+    // CHECK: call void @_ZNH1S1fES_
+    s.f(0);
+    // CHECK: call void @_ZNH1S1fES_i
+    s.g();
+    // CHECK: call void @_ZNH1S1gIS_EEvT_
+    s.h();
+    // CHECK: call void @_ZNH1S1hIS_EEvRT_
+    s.i();
+    // CHECK: call void @_ZNH1S1iES_
+    s.i(0);
+    // CHECK: call void @_ZNH1S1iES_i
+}
+
+struct StaticAndExplicit {
+  static void f(StaticAndExplicit);
+  void f(this StaticAndExplicit);
+};
+
+void test2() {
+    StaticAndExplicit s;
+
+    StaticAndExplicit::f(s);
+    // CHECK: call void @_ZN17StaticAndExplicit1fES_
+
+    s.f();
+    // CHECK: call void @_ZNH17StaticAndExplicit1fES_
+}
diff --git a/clang/test/CodeGenCXX/microsoft-abi-explicit-object-parameters.cpp b/clang/test/CodeGenCXX/microsoft-abi-explicit-object-parameters.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/microsoft-abi-explicit-object-parameters.cpp
@@ -0,0 +1,47 @@
+// RUN: %clang_cc1 -std=c++2b -fno-rtti -emit-llvm -triple=x86_64-pc-win32 -o - %s  2>/dev/null | FileCheck %s
+struct S {
+friend void test();
+public:
+    void a(this auto){}
+    void b(this auto&){}
+    void c(this S){}
+    void c(this S, int){}
+private:
+    void d(this auto){}
+    void e(this auto&){}
+    void f(this S){}
+    void f(this S, int){}
+protected:
+    void g(this auto){}
+    void h(this auto&){}
+    void i(this S){}
+    void i(this S, int){}
+};
+
+void test() {
+    S s;
+    s.a();
+    // CHECK: call void @"??$a@US@@@S@@SAX_VU0@@Z"
+    s.b();
+    // CHECK: call void @"??$b@US@@@S@@SAX_VAEAU0@@Z"
+    s.c();
+    // CHECK: call void @"?c@S@@SAX_VU1@@Z"
+    s.c(0);
+    // CHECK: call void @"?c@S@@SAX_VU1@H@Z"
+    s.d();
+    // CHECK: call void @"??$d@US@@@S@@CAX_VU0@@Z"
+    s.e();
+    // CHECK: call void @"??$e@US@@@S@@CAX_VAEAU0@@Z"
+    s.f();
+    // CHECK: call void @"?f@S@@CAX_VU1@@Z"
+    s.f(0);
+    // CHECK: call void @"?f@S@@CAX_VU1@H@Z"
+    s.g();
+    // CHECK: call void @"??$g@US@@@S@@KAX_VU0@@Z"
+    s.h();
+    // CHECK: call void @"??$h@US@@@S@@KAX_VAEAU0@@Z"
+    s.i();
+    // CHECK: call void @"?i@S@@KAX_VU1@@Z"
+    s.i(0);
+    // CHECK: call void @"?i@S@@KAX_VU1@H@Z"
+}
diff --git a/clang/test/FixIt/fixit-deducing-this.cpp b/clang/test/FixIt/fixit-deducing-this.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/FixIt/fixit-deducing-this.cpp
@@ -0,0 +1,32 @@
+// RUN: %clang_cc1 -verify -std=c++2c %s
+// RUN: cp %s %t
+// RUN: not %clang_cc1 -x c++ -std=c++2c -fixit %t
+// RUN: %clang_cc1 -x c++ -std=c++2c %t
+// RUN: not %clang_cc1 -std=c++2c -x c++ -fsyntax-only -fdiagnostics-parseable-fixits %s 2>&1 | FileCheck %s
+struct S {
+    void f(this S);
+    void g() {
+        (void)&f;  // expected-error {{must explicitly qualify name of member function when taking its address}}
+// CHECK: fix-it:{{.*}}:{9:16-9:16}:"S::"
+    }
+};
+
+struct S2 {
+    void f(this S2 foo) {
+        g(); // expected-error {{call to non-static member function without an object argument}}
+// CHECK: fix-it:{{.*}}:{16:9-16:9}:"foo."
+
+        h(); // expected-error {{call to explicit member function without an object argument}}
+// CHECK: fix-it:{{.*}}:{19:9-19:9}:"foo."
+
+        i();
+
+        var; // expected-error {{invalid use of member 'var' in explicit object member function}}
+// CHECK: fix-it:{{.*}}:{24:9-24:9}:"foo."
+
+    }
+    void g();
+    void h(this S2 s);
+    static void i();
+    int var;
+};
diff --git a/clang/test/SemaCXX/cxx2b-deducing-this-constexpr.cpp b/clang/test/SemaCXX/cxx2b-deducing-this-constexpr.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/SemaCXX/cxx2b-deducing-this-constexpr.cpp
@@ -0,0 +1,56 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++2b %s -verify
+// expected-no-diagnostics
+
+template <typename Base>
+struct Wrap : Base {
+
+};
+
+struct S {
+    constexpr int f(this const S&) {
+        return 42;
+    }
+    constexpr int f(this const S&, auto&&... args) {
+        return (args + ... + 0);
+    }
+    constexpr int operator[](this const S&) {
+        return 42;
+    }
+    constexpr int operator[](this const S& self, int i) {
+        return i + self.base;
+    }
+    constexpr int operator()(this const S&) {
+        return 42;
+    }
+    constexpr int operator()(this const S& self, int i) {
+        return self.base + i;
+    }
+    constexpr bool operator==(this const S& self, auto && test) {
+        return self.base == test;
+    };
+    constexpr int operator*(this const S& self) {
+        return self.base + 22;
+    };
+    constexpr operator Wrap<S> (this const S& self) {
+        return Wrap<S>{self};
+    };
+    constexpr int operator <<(this Wrap<S> self, int i) {
+        return self.base+i;
+    }
+
+    int base = 20;
+};
+
+consteval void test() {
+    constexpr S s;
+    static_assert(s.f() == 42);
+    static_assert(s[] == 42);
+    static_assert(s[22] == 42);
+    static_assert(s.f() == 42);
+    static_assert(s() == 42);
+    static_assert(s(22) == 42);
+    static_assert(s == 20);
+    static_assert(s != 0);
+    static_assert(*s == 42);
+    static_assert((s << 11) == 31);
+}
diff --git a/clang/test/SemaCXX/cxx2b-deducing-this-coro.cpp b/clang/test/SemaCXX/cxx2b-deducing-this-coro.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/SemaCXX/cxx2b-deducing-this-coro.cpp
@@ -0,0 +1,58 @@
+// RUN: %clang_cc1 -std=c++2b %s -verify
+
+
+#include "Inputs/std-coroutine.h"
+
+struct S;
+template <typename T>
+class coro_test {
+public:
+    struct promise_type;
+    using handle = std::coroutine_handle<promise_type>;
+	struct promise_type {
+        promise_type(const promise_type&) = delete; // #copy-ctr
+        promise_type(T);  // #candidate
+        coro_test get_return_object();
+        std::suspend_never initial_suspend();
+	    std::suspend_never final_suspend() noexcept;
+	    void return_void();
+        void unhandled_exception();
+
+
+        template<typename Arg, typename... Args>
+        void* operator new(decltype(0UL) sz, Arg&&, Args&... args) {
+            static_assert(!__is_same(__decay(Arg), S), "Ok"); // expected-error 2{{Ok}}
+        }
+
+    };
+private:
+	handle h;
+};
+
+
+template <typename Ret, typename... P>
+struct std::coroutine_traits<coro_test<S&>, Ret, P...> {
+  using promise_type = coro_test<S&>::promise_type;
+  static_assert(!__is_same(Ret, S&), "Ok"); // expected-error{{static assertion failed due to requirement '!__is_same(S &, S &)': Ok}}
+};
+
+
+struct S {
+
+    coro_test<S&> ok(this S&, int) {
+        co_return; // expected-note {{in instantiation}}
+    }
+
+    coro_test<const S&> ok2(this const S&) { // expected-note {{in instantiation}}
+        co_return;
+    }
+
+    coro_test<int> ko(this const S&) {  // expected-error {{no matching constructor for initialization of 'std::coroutine_traits<coro_test<int>, const S &>::promise_type'}} \
+                                        // expected-note {{in instantiation}} \
+                                       // FIXME: the message below is unhelpful but this is pre-existing
+                                       // expected-note@#candidate {{candidate constructor not viable: requires 1 argument, but 0 were provided}} \
+                                       // expected-note@#copy-ctr  {{candidate constructor not viable}}
+        co_return;
+    }
+
+};
diff --git a/clang/test/SemaCXX/cxx2b-deducing-this.cpp b/clang/test/SemaCXX/cxx2b-deducing-this.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/SemaCXX/cxx2b-deducing-this.cpp
@@ -0,0 +1,492 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++2b -Woverloaded-virtual %s -verify
+
+
+// FIXME: can we improve these diagnostics?
+void f(this); // expected-error{{variable has incomplete type 'void'}} \
+              // expected-error{{invalid use of 'this' outside of a non-static member function}}
+
+void g(this auto); // expected-error{{an explicit object parameter cannot appear in a non-member function}}
+
+auto l1 = [] (this auto) static {}; // expected-error{{an explicit object parameter cannot appear in a static lambda}}
+auto l2 = [] (this auto) mutable {}; // expected-error{{a lambda with an explicit object parameter cannot be mutable}}
+auto l3 = [](this auto...){}; // expected-error {{an explicit object parameter cannot be a function parameter pack}}
+auto l4 = [](int, this auto){}; // expected-error {{an explicit object parameter can only appear as the first parameter of the lambda}}
+
+struct S {
+    static void f(this auto); // expected-error{{an explicit object parameter cannot appear in a static function}}
+    virtual void f(this S); // expected-error{{an explicit object parameter cannot appear in a virtual function}}
+
+    void g(this auto) const; // expected-error{{a function with an explicit object parameter cannot be const}}
+    void h(this auto) &; // expected-error{{a function with an explicit object parameter cannot be reference-qualified}}
+    void i(this auto) &&; // expected-error{{a function with an explicit object parameter cannot be reference-qualified}}
+    void j(this auto) volatile; // expected-error{{a function with an explicit object parameter cannot have qualifiers}}
+    void k(this auto) __restrict; // expected-error{{a function with an explicit object parameter cannot have qualifiers}}
+    void l(this auto) _Nonnull; // expected-error{{a function with an explicit object parameter cannot have qualifiers}}
+
+
+    void variadic(this auto...); // expected-error{{an explicit object parameter cannot be a function parameter pack}}
+    void not_first(int, this auto); // expected-error {{an explicit object parameter can only appear as the first parameter of the function}}
+
+    S(this auto); // expected-error {{an explicit object parameter cannot appear in a constructor}}
+    ~S(this S) {} // expected-error {{an explicit object parameter cannot appear in a destructor}} \
+                  // expected-error {{destructor cannot have any parameters}}
+};
+
+namespace Override {
+struct A {
+    virtual void f(); // expected-note 2{{here}}
+    virtual void g(int); // expected-note {{here}}
+    virtual void h() const; // expected-note 5{{here}}
+};
+
+// CWG2553
+struct B : A {
+    int f(this B&, int); // expected-warning {{hides overloaded virtual function}}
+    int f(this B&);  // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+    int g(this B&); // expected-warning {{hides overloaded virtual function}}
+    int h(this B&); // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+    int h(this B&&); // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+    int h(this const B&&); // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+    int h(this A&); // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+    int h(this int); // expected-error {{an explicit object parameter cannot appear in a virtual function}}
+};
+}
+
+namespace DefaultArgs {
+     struct Test { void f(this const auto& = Test{}); };
+    // expected-error@-1 {{an explicit object parameter cannot have a default argument}}
+    auto L =[](this const auto& = Test{}){};
+    // expected-error@-1 {{an explicit object parameter cannot have a default argument}}
+}
+
+struct CannotUseThis {
+    int fun();
+    int m;
+    void f(this auto) {
+        this->fun(); // expected-error{{invalid use of 'this' in a function with an explicit object parameter}}
+        fun(); // expected-error {{call to non-static member function without an object argument}}
+        m = 0; // expected-error {{invalid use of member 'm' in explicit object member function}}
+    }
+};
+
+namespace ThisInLambdaWithCaptures {
+
+struct Test {
+    Test(auto&&);
+};
+
+void test() {
+
+    [i = 0](this Test) { }();
+    // expected-error@-1 {{invalid explicit object parameter type 'ThisInLambdaWithCaptures::Test' in lambda with capture; the type must be the same as, or derived from, the lambda}}
+
+    struct Derived;
+    auto ok = [i = 0](this const Derived&) {};
+    auto ko = [i = 0](this const Test&) {};
+    // expected-error@-1 {{invalid explicit object parameter type 'ThisInLambdaWithCaptures::Test' in lambda with capture; the type must be the same as, or derived from, the lambda}}
+
+    struct Derived : decltype(ok){};
+    Derived dok{ok};
+    dok();
+
+    struct DerivedErr : decltype(ko){};
+    DerivedErr dko{ko};
+    dko();
+}
+
+}
+
+struct Corresponding {
+    void a(this Corresponding&); // expected-note 2{{here}}
+    void a(); // expected-error{{cannot be redeclared}}
+    void a() &; // expected-error{{cannot be redeclared}}
+    void a(this Corresponding&, int);
+    void a(this Corresponding&, double);
+
+    void b(this const Corresponding&); // expected-note 2{{here}}
+    void b() const; // expected-error{{cannot be redeclared}}
+    void b() const &; // expected-error{{cannot be redeclared}}
+
+    void c(this Corresponding&&); // expected-note {{here}}
+    void c() &&; // expected-error{{cannot be redeclared}}
+
+    void d(this Corresponding&);
+    void d(this Corresponding&&);
+    void d(this const Corresponding&);
+    void d(this const int&);
+    void d(this const int);
+    void d(this int);
+
+    void e(this const Corresponding&&); // expected-note {{here}}
+    void e() const &&; // expected-error{{cannot be redeclared}}
+
+};
+
+template <typename T>
+struct CorrespondingTpl {
+    void a(this CorrespondingTpl&); // expected-note 2{{here}}
+    void a(); // expected-error{{cannot be redeclared}}
+    void a() &; // expected-error{{cannot be redeclared}}
+    void a(this Corresponding&, int);
+    void a(this Corresponding&, double);
+    void a(long);
+
+
+    void b(this const CorrespondingTpl&); // expected-note 2{{here}}
+    void b() const; // expected-error{{cannot be redeclared}}
+    void b() const &; // expected-error{{cannot be redeclared}}
+
+    void c(this CorrespondingTpl&&); // expected-note {{here}}
+    void c() &&; // expected-error{{cannot be redeclared}}
+
+    void d(this Corresponding&);
+    void d(this Corresponding&&);
+    void d(this const Corresponding&);
+    void d(this const int&);
+    void d(this const int);
+    void d(this int);
+
+    void e(this const CorrespondingTpl&&); // expected-note {{here}}
+    void e() const &&; // expected-error{{cannot be redeclared}}
+};
+
+struct C {
+    template <typename T>
+    C(T){}
+};
+
+void func(int i) {
+    (void)[=](this auto&&) { return i; }();
+    (void)[=](this const auto&) { return i; }();
+    (void)[i](this C) { return i; }(); // expected-error{{invalid explicit object parameter type 'C'}}
+    (void)[=](this C) { return i; }(); // expected-error{{invalid explicit object parameter type 'C'}}
+    (void)[](this C) { return 42; }();
+    auto l = [=](this auto&) {};
+    struct D : decltype(l) {};
+    D d{l};
+    d();
+}
+
+void TestMutationInLambda() {
+    [i = 0](this auto &&){ i++; }();
+    [i = 0](this auto){ i++; }();
+    [i = 0](this const auto&){ i++; }();
+    // expected-error@-1 {{cannot assign to a variable captured by copy in a non-mutable lambda}}
+}
+
+struct Over_Call_Func_Example {
+    void a();
+    void b() {
+        a(); // ok, (*this).a()
+    }
+
+    void f(this const Over_Call_Func_Example&); //expected-note {{here}}
+    void g() const {
+        f();       // ok: (*this).f()
+        f(*this);  // expected-error{{too many non-object arguments to function call}}
+        this->f(); // ok
+    }
+
+    static void h() {
+        f();       // expected-error{{call to non-static member function without an object argument}}
+        f(Over_Call_Func_Example{});   // expected-error{{call to non-static member function without an object argument}}
+        Over_Call_Func_Example{}.f();   // ok
+    }
+
+    void k(this int);
+    operator int() const;
+    void m(this const Over_Call_Func_Example& c) {
+        c.k();     // ok
+    }
+};
+
+namespace arity_diagnostics {
+struct S {
+    void f(this auto &&, auto, auto); // expected-note {{requires 2 non-object arguments, but 0 were provided}}
+    void g(this auto &&, auto, auto); // expected-note {{requires 2 non-object arguments, but 3 were provided}}
+    void h(this auto &&, int, int i = 0); // expected-note {{requires at least 1 non-object argument, but 0 were provided}}
+    void i(this S&&, int); // expected-note 2{{declared here}}
+};
+
+int test() {
+    void(*f)(S&&, int, int) = &S::f;
+    f(S{}, 1, 2);
+    f(S{}, 1); // expected-error {{too few arguments to function call, expected 3, have 2}}
+    f(S{}); // expected-error {{too few arguments to function call, expected 3, have 1}}
+    f(S{}, 1, 2, 3); //expected-error {{too many arguments to function call, expected 3, have 4}}
+
+    S{}.f(1, 2);
+    S{}.f(); //  expected-error{{no matching member function for call to 'f'}}
+    S{}.g(1,2,3); // expected-error {{no matching member function for call to 'g'}}
+    S{}.h(); // expected-error {{no matching member function for call to 'h'}}
+    S{}.i(); // expected-error {{too few non-object arguments to function call, expected 1, have 0}}
+    S{}.i(1, 2, 3); // expected-error {{too many non-object arguments to function call, expected 1, have 3}}
+}
+
+}
+
+namespace AddressOf {
+
+struct s {
+    static void f(int);
+    void f(this auto &&) {}
+    void g(this s &&) {};
+
+    void test_qual() {
+        using F = void(s&&);
+        F* a = &f; // expected-error {{must explicitly qualify name of member function when taking its address}}
+        F* b = &g; // expected-error {{must explicitly qualify name of member function when taking its address}}
+        F* c = &s::f;
+        F* d = &s::g;
+    }
+};
+
+void test() {
+    using F = void(s&&);
+    F* a = &s::f;
+    F* b = &s::g;
+    a(s{});
+    b(s{});
+}
+
+}
+
+namespace std {
+  struct strong_ordering {
+    int n;
+    constexpr operator int() const { return n; }
+    static const strong_ordering equal, greater, less;
+  };
+  constexpr strong_ordering strong_ordering::equal = {0};
+  constexpr strong_ordering strong_ordering::greater = {1};
+  constexpr strong_ordering strong_ordering::less = {-1};
+}
+
+namespace operators_deduction {
+
+template <typename T, typename U>
+constexpr bool is_same = false;
+
+template <typename T>
+constexpr bool is_same<T, T> = true;
+
+template <template <typename> typename T>
+struct Wrap {
+void f();
+struct S {
+    operator int(this auto&& self) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return 0;
+    }
+    Wrap* operator->(this auto&& self) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return new Wrap();
+    }
+    int operator[](this auto&& self, int) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return 0;
+    }
+    int operator()(this auto&& self, int) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return 0;
+    }
+    int operator++(this auto&& self, int) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return 0;
+    }
+    int operator++(this auto&& self) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return 0;
+    }
+    int operator--(this auto&& self, int) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return 0;
+    }
+    int operator--(this auto&& self) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return 0;
+    }
+    int operator*(this auto&& self) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return 0;
+    }
+    bool operator==(this auto&& self, int) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return false;
+    }
+    bool operator<=>(this auto&& self, int) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return false;
+    }
+    bool operator<<(this auto&& self, int b) {
+        static_assert(is_same<decltype(self), typename T<S>::type>);
+        return false;
+    }
+};
+};
+
+template <typename T>
+struct lvalue_reference {
+    using type = T&;
+};
+template <typename T>
+struct const_lvalue_reference {
+    using type = const T&;
+};
+template <typename T>
+struct volatile_lvalue_reference {
+    using type = volatile T&;
+};
+template <typename T>
+struct rvalue_reference {
+    using type = T&&;
+};
+template <typename T>
+struct const_rvalue_reference {
+    using type = const T&&;
+};
+
+
+void test() {
+    {
+        Wrap<lvalue_reference>::S s;
+        s++;
+        s.operator++(0);
+        ++s;
+        s.operator++();
+        s--;
+        s.operator--(0);
+        --s;
+        s.operator--();
+        s[0];
+        s.operator[](0);
+        s(0);
+        s.operator()(0);
+        *s;
+        s.operator*();
+        s->f();
+        s.operator->();
+        int i = s;
+        (void)(s << 0);
+        s.operator<<(0);
+        (void)(s == 0);
+        s.operator==(0);
+        (void)(s <=> 0);
+        s.operator<=>(0);
+    }
+    {
+        const Wrap<const_lvalue_reference>::S s;
+        s++;
+        s.operator++(0);
+        ++s;
+        s.operator++();
+        s--;
+        s.operator--(0);
+        --s;
+        s.operator--();
+        s[0];
+        s.operator[](0);
+        s(0);
+        s.operator()(0);
+        *s;
+        s.operator*();
+        s->f();
+        s.operator->();
+        int i = s;
+        (void)(s << 0);
+        s.operator<<(0);
+        (void)(s == 0);
+        s.operator==(0);
+        (void)(s <=> 0);
+        s.operator<=>(0);
+    }
+    {
+        volatile Wrap<volatile_lvalue_reference>::S s;
+        s++;
+        s.operator++(0);
+        ++s;
+        s.operator++();
+        s--;
+        s.operator--(0);
+        --s;
+        s.operator--();
+        s[0];
+        s.operator[](0);
+        s(0);
+        s.operator()(0);
+        *s;
+        s.operator*();
+        s->f();
+        s.operator->();
+        int i = s;
+        (void)(s << 0);
+        s.operator<<(0);
+        (void)(s == 0);
+        s.operator==(0);
+        (void)(s <=> 0);
+        s.operator<=>(0);
+    }
+    {
+        Wrap<rvalue_reference>::S s;
+        using M = Wrap<rvalue_reference>::S&&;
+        ((M)s)++;
+        ((M)s).operator++(0);
+        ++((M)s);
+        ((M)s).operator++();
+        ((M)s)--;
+        ((M)s).operator--(0);
+        --((M)s);
+        ((M)s).operator--();
+        ((M)s)[0];
+        ((M)s).operator[](0);
+        ((M)s)(0);
+        ((M)s).operator()(0);
+        *((M)s);
+        ((M)s).operator*();
+        ((M)s)->f();
+        ((M)s).operator->();
+        int i = ((M)s);
+        (void)(((M)s) << 0);
+        ((M)s).operator<<(0);
+        (void)(((M)s) == 0);
+        ((M)s).operator==(0);
+        (void)(((M)s) <=> 0);
+        ((M)s).operator<=>(0);
+    }
+}
+}
+
+namespace conversions {
+//[over.best.ics]
+struct Y { Y(int); }; //expected-note 3{{candidate}}
+struct A { operator int(this auto&&); };  //expected-note {{candidate}}
+Y y1 = A();   // expected-error{{no viable conversion from 'A' to 'Y'}}
+
+struct X { X(); }; //expected-note 3{{candidate}}
+struct B { operator X(this auto&&); };
+B b;
+X x{{b}}; // expected-error{{no matching constructor for initialization of 'X'}}
+
+struct T{}; // expected-note 2{{candidate constructor}}
+struct C {
+    operator T (this int); // expected-note {{candidate function not viable: no known conversion from 'C' to 'int' for object argument}}
+    operator int() const; // expected-note {{candidate function}}
+};
+
+void foo(C c) {
+   T d = c; // expected-error {{no viable conversion from 'C' to 'T'}}
+}
+
+}
+
+namespace surrogate {
+using fn_t = void();
+struct C {
+    operator fn_t * (this C const &);
+};
+
+void foo(C c) {
+   c();
+}
+
+}
diff --git a/clang/www/cxx_dr_status.html b/clang/www/cxx_dr_status.html
--- a/clang/www/cxx_dr_status.html
+++ b/clang/www/cxx_dr_status.html
@@ -15125,13 +15125,13 @@
     <td><a href="https://cplusplus.github.io/CWG/issues/2553.html">2553</a></td>
     <td>open</td>
     <td>Restrictions on explicit object member functions</td>
-    <td align="center">Not resolved</td>
+    <td class="unreleased" align="center">Clang 16</td>
   </tr>
   <tr class="open" id="2554">
     <td><a href="https://cplusplus.github.io/CWG/issues/2554.html">2554</a></td>
     <td>open</td>
     <td>Overriding virtual functions, also with explicit object parameters</td>
-    <td align="center">Not resolved</td>
+    <td class="unreleased" align="center">Clang 16</td>
   </tr>
   <tr class="open" id="2555">
     <td><a href="https://cplusplus.github.io/CWG/issues/2555.html">2555</a></td>
@@ -15173,7 +15173,7 @@
     <td><a href="https://cplusplus.github.io/CWG/issues/2561.html">2561</a></td>
     <td>open</td>
     <td>Conversion to function pointer for lambda with explicit object parameter</td>
-    <td align="center">Not resolved</td>
+    <td class="unreleased" align="center">Clang 16</td>
   </tr>
   <tr class="open" id="2562">
     <td><a href="https://cplusplus.github.io/CWG/issues/2562.html">2562</a></td>
@@ -15725,7 +15725,7 @@
     <td><a href="https://cplusplus.github.io/CWG/issues/2653.html">2653</a></td>
     <td>WP</td>
     <td>Can an explicit object parameter have a default argument?</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 16</td>
   </tr>
   <tr id="2654">
     <td><a href="https://cplusplus.github.io/CWG/issues/2654.html">2654</a></td>
diff --git a/clang/www/cxx_status.html b/clang/www/cxx_status.html
--- a/clang/www/cxx_status.html
+++ b/clang/www/cxx_status.html
@@ -227,11 +227,11 @@
     <tr>
       <td rowspan=2>Deducing this</td>
       <td><a href="https://wg21.link/P0847R7">P0847R7</a></td>
-      <td class="none" align="center">No</td>
+      <td class="unreleased" align="center">Clang 18</td>
     </tr>
     <tr>
       <td><a href="https://wg21.link/P2797R0">P2797R0</a></td>
-      <td class="none" align="center">No</td>
+      <td class="unreleased" align="center">Clang 18</td>
     </tr>
     <tr>
       <td rowspan=2>Change scope of lambda trailing-return-type</td>