diff --git a/clang/include/clang/AST/ASTContext.h b/clang/include/clang/AST/ASTContext.h
--- a/clang/include/clang/AST/ASTContext.h
+++ b/clang/include/clang/AST/ASTContext.h
@@ -2781,6 +2781,21 @@
     return AddrSpaceMapMangling || isTargetAddressSpace(AS);
   }
 
+  // For two canonically equal types, return a type which has
+  // the common sugar between them. If Unqualified is true,
+  // both types need only be the same unqualified type.
+  // The result will drop the qualifiers which do not occur
+  // in both types.
+  QualType getCommonSugaredType(QualType X, QualType Y,
+                                bool Unqualified = false);
+
+  // Merges two type lists such that the resulting vector will contain
+  // each type (in a canonical sense) only once, in the order they appear
+  // from X to Y. If they occur in both X and Y, the result will contain
+  // the common sugared type between them.
+  void mergeTypeLists(SmallVectorImpl<QualType> &Out, ArrayRef<QualType> X,
+                      ArrayRef<QualType> Y);
+
 private:
   // Helper for integer ordering
   unsigned getIntegerRank(const Type *T) const;
diff --git a/clang/include/clang/AST/Type.h b/clang/include/clang/AST/Type.h
--- a/clang/include/clang/AST/Type.h
+++ b/clang/include/clang/AST/Type.h
@@ -4317,10 +4317,9 @@
   }
 
   using param_type_iterator = const QualType *;
-  using param_type_range = llvm::iterator_range<param_type_iterator>;
 
-  param_type_range param_types() const {
-    return param_type_range(param_type_begin(), param_type_end());
+  ArrayRef<QualType> param_types() const {
+    return llvm::makeArrayRef(param_type_begin(), param_type_end());
   }
 
   param_type_iterator param_type_begin() const {
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
@@ -8727,7 +8727,9 @@
     /// Deduction failed; that's all we know.
     TDK_MiscellaneousDeductionFailure,
     /// CUDA Target attributes do not match.
-    TDK_CUDATargetMismatch
+    TDK_CUDATargetMismatch,
+    /// Some error which was already diagnosed.
+    TDK_AlreadyDiagnosed
   };
 
   TemplateDeductionResult
@@ -8818,21 +8820,11 @@
   TypeSourceInfo *ReplaceAutoTypeSourceInfo(TypeSourceInfo *TypeWithAuto,
                                             QualType Replacement);
 
-  /// Result type of DeduceAutoType.
-  enum DeduceAutoResult {
-    DAR_Succeeded,
-    DAR_Failed,
-    DAR_FailedAlreadyDiagnosed
-  };
-
-  DeduceAutoResult
-  DeduceAutoType(TypeSourceInfo *AutoType, Expr *&Initializer, QualType &Result,
-                 Optional<unsigned> DependentDeductionDepth = None,
-                 bool IgnoreConstraints = false);
-  DeduceAutoResult
-  DeduceAutoType(TypeLoc AutoTypeLoc, Expr *&Initializer, QualType &Result,
-                 Optional<unsigned> DependentDeductionDepth = None,
-                 bool IgnoreConstraints = false);
+  TemplateDeductionResult DeduceAutoType(TypeLoc AutoTypeLoc, Expr *Initializer,
+                                         QualType &Result,
+                                         sema::TemplateDeductionInfo &Info,
+                                         bool DependentDeduction = false,
+                                         bool IgnoreConstraints = false);
   void DiagnoseAutoDeductionFailure(VarDecl *VDecl, Expr *Init);
   bool DeduceReturnType(FunctionDecl *FD, SourceLocation Loc,
                         bool Diagnose = true);
@@ -8854,8 +8846,8 @@
   TypeLoc getReturnTypeLoc(FunctionDecl *FD) const;
 
   bool DeduceFunctionTypeFromReturnExpr(FunctionDecl *FD,
-                                        SourceLocation ReturnLoc,
-                                        Expr *&RetExpr, const AutoType *AT);
+                                        SourceLocation ReturnLoc, Expr *RetExpr,
+                                        const AutoType *AT);
 
   FunctionTemplateDecl *getMoreSpecializedTemplate(
       FunctionTemplateDecl *FT1, FunctionTemplateDecl *FT2, SourceLocation Loc,
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -3204,9 +3204,9 @@
 QualType ASTContext::getFunctionTypeWithoutPtrSizes(QualType T) {
   if (const auto *Proto = T->getAs<FunctionProtoType>()) {
     QualType RetTy = removePtrSizeAddrSpace(Proto->getReturnType());
-    SmallVector<QualType, 16> Args(Proto->param_types());
+    SmallVector<QualType, 16> Args(Proto->param_types().size());
     for (unsigned i = 0, n = Args.size(); i != n; ++i)
-      Args[i] = removePtrSizeAddrSpace(Args[i]);
+      Args[i] = removePtrSizeAddrSpace(Proto->param_types()[i]);
     return getFunctionType(RetTy, Args, Proto->getExtProtoInfo());
   }
 
@@ -12082,6 +12082,482 @@
     return (*AddrSpaceMap)[(unsigned)AS];
 }
 
+static auto getCommonTypeArray(ASTContext &Ctx, ArrayRef<QualType> Xs,
+                               ArrayRef<QualType> Ys,
+                               bool Unqualified = false) {
+  assert(Xs.size() == Ys.size());
+  SmallVector<QualType, 8> Rs(Xs.size());
+  for (size_t I = 0; I < Rs.size(); ++I)
+    Rs[I] = Ctx.getCommonSugaredType(Xs[I], Ys[I], Unqualified);
+  return Rs;
+}
+
+template <class T>
+static SourceLocation getCommonAttrLoc(const T *X, const T *Y) {
+  return X->getAttributeLoc() == Y->getAttributeLoc() ? X->getAttributeLoc()
+                                                      : SourceLocation();
+}
+
+static TemplateArgument getCommonTemplateArgument(ASTContext &Ctx,
+                                                  const TemplateArgument &X,
+                                                  const TemplateArgument &Y) {
+  assert(X.getKind() == Y.getKind());
+  switch (X.getKind()) {
+  case TemplateArgument::ArgKind::Type:
+    return TemplateArgument(
+        Ctx.getCommonSugaredType(X.getAsType(), Y.getAsType()));
+  case TemplateArgument::ArgKind::NullPtr:
+    return TemplateArgument(
+        Ctx.getCommonSugaredType(X.getNullPtrType(), Y.getNullPtrType()), true);
+  case TemplateArgument::ArgKind::Expression:
+    // FIXME: This should work on resolved arguments, and so would mostly never
+    //        deal with expressions directly.
+    return X;
+  default:
+    // FIXME: Handle the other kinds when this is working with resolved
+    //        arguments, per above.
+    llvm_unreachable("Unexpected argument kind");
+  }
+  llvm_unreachable("");
+}
+
+static auto getCommonTemplateArguments(ASTContext &Ctx,
+                                       ArrayRef<TemplateArgument> X,
+                                       ArrayRef<TemplateArgument> Y) {
+  SmallVector<TemplateArgument, 8> R(X.size());
+  for (size_t I = 0; I < R.size(); ++I)
+    R[I] = getCommonTemplateArgument(Ctx, X[I], Y[I]);
+  return R;
+}
+
+template <class T>
+static ElaboratedTypeKeyword getCommonTypeKeyword(const T *X, const T *Y) {
+  return X->getKeyword() == Y->getKeyword() ? X->getKeyword()
+                                            : ElaboratedTypeKeyword::ETK_None;
+}
+
+template <class T>
+static NestedNameSpecifier *getCommonNNS(ASTContext &Ctx, const T *X,
+                                         const T *Y) {
+  // FIXME: Try to keep the common NNS sugar.
+  return X->getQualifier() == Y->getQualifier()
+             ? X->getQualifier()
+             : Ctx.getCanonicalNestedNameSpecifier(X->getQualifier());
+}
+
+template <class T>
+static QualType getCommonElementType(ASTContext &Ctx, const T *X, const T *Y) {
+  return Ctx.getCommonSugaredType(X->getElementType(), Y->getElementType());
+}
+
+template <class T>
+static QualType getCommonPointeeType(ASTContext &Ctx, const T *X, const T *Y) {
+  return Ctx.getCommonSugaredType(X->getPointeeType(), Y->getPointeeType());
+}
+
+template <class T> static auto *getCommonSizeExpr(T *X, T *Y) {
+  assert(X->getSizeExpr() == Y->getSizeExpr());
+  return X->getSizeExpr();
+}
+
+static auto getCommonSizeModifier(const ArrayType *X, const ArrayType *Y) {
+  assert(X->getSizeModifier() == Y->getSizeModifier());
+  return X->getSizeModifier();
+}
+
+static auto getCommonIndexTypeCVRQualifiers(const ArrayType *X,
+                                            const ArrayType *Y) {
+  assert(X->getIndexTypeCVRQualifiers() == Y->getIndexTypeCVRQualifiers());
+  return X->getIndexTypeCVRQualifiers();
+}
+
+static Decl *getCommonDecl(Decl *X, Decl *Y) {
+  if (X == Y)
+    return X;
+  assert(X->getCanonicalDecl() == Y->getCanonicalDecl());
+  // FIXME: Could return the earliest declaration between those two.
+  return X->getCanonicalDecl();
+}
+
+template <class T, std::enable_if_t<std::is_base_of_v<Decl, T>, bool> = true>
+T *getCommonDecl(T *X, T *Y) {
+  return cast_or_null<T>(
+      getCommonDecl(cast_or_null<Decl>(X), cast_or_null<Decl>(Y)));
+}
+
+static TemplateName getCommonTemplateName(ASTContext &Ctx, TemplateName X,
+                                          TemplateName Y) {
+  if (X.getAsVoidPointer() == Y.getAsVoidPointer())
+    return X;
+  // FIXME: There are cases here where we could find a common template name
+  //        with more sugar. For example one could be a SubstTemplateTemplate*
+  //        replacing the other.
+  TemplateName CX = Ctx.getCanonicalTemplateName(X);
+  assert(CX.getAsVoidPointer() ==
+         Ctx.getCanonicalTemplateName(Y).getAsVoidPointer());
+  return CX;
+}
+
+void ASTContext::mergeTypeLists(SmallVectorImpl<QualType> &Out, ArrayRef<QualType> X,
+                          ArrayRef<QualType> Y) {
+  llvm::DenseMap<QualType, unsigned> Found;
+  for (auto Ts : {X, Y})
+    for (QualType T : Ts) {
+      auto Res = Found.try_emplace(getCanonicalType(T), Out.size());
+      if (!Res.second) {
+        QualType &U = Out[Res.first->second];
+        U = getCommonSugaredType(U, T);
+      } else {
+        Out.emplace_back(T);
+      }
+    }
+}
+
+static QualType getCommonType(ASTContext &Ctx, const Type *X, const Type *Y) {
+  Type::TypeClass TC = X->getTypeClass();
+  assert(TC == Y->getTypeClass());
+  switch (TC) {
+#define UNEXPECTED_TYPE(Class, Kind)                                           \
+  case Type::Class:                                                            \
+    llvm_unreachable("Unexpected " Kind ": " #Class);
+
+#define NON_CANONICAL_TYPE(Class, Base) UNEXPECTED_TYPE(Class, "non-canonical")
+#define TYPE(Class, Base)
+#include "clang/AST/TypeNodes.inc"
+
+#define SUGAR_FREE_TYPE(Class) UNEXPECTED_TYPE(Class, "sugar-free")
+    SUGAR_FREE_TYPE(Builtin)
+    SUGAR_FREE_TYPE(Decltype)
+    SUGAR_FREE_TYPE(DeducedTemplateSpecialization)
+    SUGAR_FREE_TYPE(DependentBitInt)
+    SUGAR_FREE_TYPE(Enum)
+    SUGAR_FREE_TYPE(BitInt)
+    SUGAR_FREE_TYPE(ObjCInterface)
+    SUGAR_FREE_TYPE(Record)
+    SUGAR_FREE_TYPE(SubstTemplateTypeParmPack)
+    SUGAR_FREE_TYPE(TemplateTypeParm)
+    SUGAR_FREE_TYPE(UnresolvedUsing)
+#undef SUGAR_FREE_TYPE
+#define NON_UNIQUE_TYPE(Class) UNEXPECTED_TYPE(Class, "non-unique")
+    NON_UNIQUE_TYPE(TypeOfExpr)
+    NON_UNIQUE_TYPE(VariableArray)
+#undef NON_UNIQUE_TYPE
+
+    UNEXPECTED_TYPE(TypeOf, "sugar")
+
+#undef UNEXPECTED_TYPE
+
+  case Type::Auto: {
+    const auto *AX = cast<AutoType>(X), *AY = cast<AutoType>(Y);
+    assert(AX->getDeducedType().isNull());
+    assert(AY->getDeducedType().isNull());
+    assert(AX->getKeyword() == AY->getKeyword());
+    assert(AX->isInstantiationDependentType() ==
+           AY->isInstantiationDependentType());
+    assert(AX->getTypeConstraintConcept() == AY->getTypeConstraintConcept());
+    auto As = getCommonTemplateArguments(Ctx, AX->getTypeConstraintArguments(),
+                                         AY->getTypeConstraintArguments());
+    return Ctx.getAutoType(QualType(), AX->getKeyword(),
+                           AX->isInstantiationDependentType(),
+                           AX->containsUnexpandedParameterPack(),
+                           AX->getTypeConstraintConcept(), As);
+  }
+  case Type::IncompleteArray: {
+    const auto *AX = cast<IncompleteArrayType>(X),
+               *AY = cast<IncompleteArrayType>(Y);
+    return Ctx.getIncompleteArrayType(getCommonElementType(Ctx, AX, AY),
+                                      getCommonSizeModifier(AX, AY),
+                                      getCommonIndexTypeCVRQualifiers(AX, AY));
+  }
+  case Type::DependentSizedArray: {
+    const auto *AX = cast<DependentSizedArrayType>(X),
+               *AY = cast<DependentSizedArrayType>(Y);
+    return Ctx.getDependentSizedArrayType(
+        getCommonElementType(Ctx, AX, AY), getCommonSizeExpr(AX, AY),
+        getCommonSizeModifier(AX, AY), getCommonIndexTypeCVRQualifiers(AX, AY),
+        AX->getBracketsRange() == AY->getBracketsRange()
+            ? AX->getBracketsRange()
+            : SourceRange());
+  }
+  case Type::ConstantArray: {
+    const auto *AX = cast<ConstantArrayType>(X),
+               *AY = cast<ConstantArrayType>(Y);
+    assert(AX->getSize() == AY->getSize());
+    return Ctx.getConstantArrayType(getCommonElementType(Ctx, AX, AY),
+                                    AX->getSize(), getCommonSizeExpr(AX, AY),
+                                    getCommonSizeModifier(AX, AY),
+                                    getCommonIndexTypeCVRQualifiers(AX, AY));
+  }
+  case Type::Atomic: {
+    const auto *AX = cast<AtomicType>(X), *AY = cast<AtomicType>(Y);
+    return Ctx.getAtomicType(
+        Ctx.getCommonSugaredType(AX->getValueType(), AY->getValueType()));
+  }
+  case Type::Complex: {
+    const auto *CX = cast<ComplexType>(X), *CY = cast<ComplexType>(Y);
+    return Ctx.getComplexType(getCommonElementType(Ctx, CX, CY));
+  }
+  case Type::Pointer: {
+    const auto *PX = cast<PointerType>(X), *PY = cast<PointerType>(Y);
+    return Ctx.getPointerType(getCommonPointeeType(Ctx, PX, PY));
+  }
+  case Type::BlockPointer: {
+    const auto *PX = cast<BlockPointerType>(X), *PY = cast<BlockPointerType>(Y);
+    return Ctx.getBlockPointerType(getCommonPointeeType(Ctx, PX, PY));
+  }
+  case Type::ObjCObjectPointer: {
+    const auto *PX = cast<ObjCObjectPointerType>(X),
+               *PY = cast<ObjCObjectPointerType>(Y);
+    return Ctx.getObjCObjectPointerType(getCommonPointeeType(Ctx, PX, PY));
+  }
+  case Type::MemberPointer: {
+    const auto *PX = cast<MemberPointerType>(X),
+               *PY = cast<MemberPointerType>(Y);
+    return Ctx.getMemberPointerType(
+        getCommonPointeeType(Ctx, PX, PY),
+        Ctx.getCommonSugaredType(QualType(PX->getClass(), 0),
+                                 QualType(PY->getClass(), 0))
+            .getTypePtr());
+  }
+  case Type::LValueReference: {
+    const auto *PX = cast<LValueReferenceType>(X),
+               *PY = cast<LValueReferenceType>(Y);
+    return Ctx.getLValueReferenceType(getCommonPointeeType(Ctx, PX, PY),
+                                      PX->isSpelledAsLValue() &&
+                                          PY->isSpelledAsLValue());
+  }
+  case Type::RValueReference: {
+    const auto *PX = cast<RValueReferenceType>(X),
+               *PY = cast<RValueReferenceType>(Y);
+    return Ctx.getRValueReferenceType(getCommonPointeeType(Ctx, PX, PY));
+  }
+  case Type::DependentAddressSpace: {
+    const auto *PX = cast<DependentAddressSpaceType>(X),
+               *PY = cast<DependentAddressSpaceType>(Y);
+    return Ctx.getDependentAddressSpaceType(getCommonPointeeType(Ctx, PX, PY),
+                                            PX->getAddrSpaceExpr(),
+                                            getCommonAttrLoc(PX, PY));
+  }
+  case Type::FunctionNoProto: {
+    const auto *FX = cast<FunctionNoProtoType>(X),
+               *FY = cast<FunctionNoProtoType>(Y);
+    assert(FX->getExtInfo() == FY->getExtInfo());
+    return Ctx.getFunctionNoProtoType(
+        Ctx.getCommonSugaredType(FX->getReturnType(), FY->getReturnType()),
+        FX->getExtInfo());
+  }
+  case Type::FunctionProto: {
+    const auto *FX = cast<FunctionProtoType>(X),
+               *FY = cast<FunctionProtoType>(Y);
+    FunctionProtoType::ExtProtoInfo EPIX = FX->getExtProtoInfo(),
+                                    EPIY = FY->getExtProtoInfo();
+    assert(EPIX.ExceptionSpec.Type == EPIY.ExceptionSpec.Type);
+    assert(EPIX.ExceptionSpec.NoexceptExpr == EPIY.ExceptionSpec.NoexceptExpr);
+
+    if (EPIX.ExceptionSpec.Type == EST_Uninstantiated ||
+        EPIX.ExceptionSpec.Type == EST_Unevaluated)
+      EPIX.ExceptionSpec.SourceDecl = ::getCommonDecl(
+          EPIX.ExceptionSpec.SourceDecl, EPIY.ExceptionSpec.SourceDecl);
+    else
+      assert(!EPIX.ExceptionSpec.SourceDecl && !EPIY.ExceptionSpec.SourceDecl);
+
+    if (EPIX.ExceptionSpec.Type == EST_Uninstantiated)
+      EPIX.ExceptionSpec.SourceTemplate = ::getCommonDecl(
+          EPIX.ExceptionSpec.SourceTemplate, EPIY.ExceptionSpec.SourceTemplate);
+    else
+      assert(!EPIX.ExceptionSpec.SourceTemplate &&
+             !EPIY.ExceptionSpec.SourceTemplate);
+
+    assert(EPIX.ExtInfo == EPIY.ExtInfo);
+    assert(EPIX.ExtParameterInfos == EPIY.ExtParameterInfos);
+    assert(EPIX.RefQualifier == EPIY.RefQualifier);
+    assert(EPIX.TypeQuals == EPIY.TypeQuals);
+    assert(EPIX.Variadic == EPIY.Variadic);
+
+    if (EPIX.EllipsisLoc != EPIY.EllipsisLoc)
+      EPIX.EllipsisLoc = SourceLocation();
+    EPIX.HasTrailingReturn = EPIX.HasTrailingReturn && EPIY.HasTrailingReturn;
+
+    QualType R =
+        Ctx.getCommonSugaredType(FX->getReturnType(), FY->getReturnType());
+    auto P = getCommonTypeArray(Ctx, FX->param_types(), FY->param_types(),
+                                /*Unqualified=*/true);
+
+    SmallVector<QualType, 8> Exceptions;
+    Ctx.mergeTypeLists(Exceptions, EPIX.ExceptionSpec.Exceptions,
+                       EPIY.ExceptionSpec.Exceptions);
+    EPIX.ExceptionSpec.Exceptions = Exceptions;
+    return Ctx.getFunctionType(R, P, EPIX);
+  }
+  case Type::ObjCObject: {
+    const auto *OX = cast<ObjCObjectType>(X), *OY = cast<ObjCObjectType>(Y);
+    assert(llvm::equal(OX->getProtocols(), OY->getProtocols()));
+    auto TAs = getCommonTypeArray(Ctx, OX->getTypeArgsAsWritten(),
+                                  OY->getTypeArgsAsWritten());
+    return Ctx.getObjCObjectType(
+        Ctx.getCommonSugaredType(OX->getBaseType(), OY->getBaseType()), TAs,
+        OX->getProtocols(),
+        OX->isKindOfTypeAsWritten() && OY->isKindOfTypeAsWritten());
+  }
+  case Type::ConstantMatrix: {
+    const auto *MX = cast<ConstantMatrixType>(X),
+               *MY = cast<ConstantMatrixType>(Y);
+    assert(MX->getNumRows() == MY->getNumRows());
+    assert(MX->getNumColumns() == MY->getNumColumns());
+    return Ctx.getConstantMatrixType(getCommonElementType(Ctx, MX, MY),
+                                     MX->getNumRows(), MX->getNumColumns());
+  }
+  case Type::DependentSizedMatrix: {
+    const auto *MX = cast<DependentSizedMatrixType>(X),
+               *MY = cast<DependentSizedMatrixType>(Y);
+    assert(MX->getRowExpr() == MY->getRowExpr());
+    assert(MX->getColumnExpr() == MY->getColumnExpr());
+    return Ctx.getDependentSizedMatrixType(
+        getCommonElementType(Ctx, MX, MY), MX->getRowExpr(),
+        MX->getColumnExpr(), getCommonAttrLoc(MX, MY));
+  }
+  case Type::Vector: {
+    const auto *VX = cast<VectorType>(X), *VY = cast<VectorType>(Y);
+    assert(VX->getNumElements() == VY->getNumElements());
+    assert(VX->getVectorKind() == VY->getVectorKind());
+    return Ctx.getVectorType(getCommonElementType(Ctx, VX, VY),
+                             VX->getNumElements(), VX->getVectorKind());
+  }
+  case Type::ExtVector: {
+    const auto *VX = cast<ExtVectorType>(X), *VY = cast<ExtVectorType>(Y);
+    assert(VX->getNumElements() == VY->getNumElements());
+    return Ctx.getExtVectorType(getCommonElementType(Ctx, VX, VY),
+                                VX->getNumElements());
+  }
+  case Type::DependentSizedExtVector: {
+    const auto *VX = cast<DependentSizedExtVectorType>(X),
+               *VY = cast<DependentSizedExtVectorType>(Y);
+    return Ctx.getDependentSizedExtVectorType(getCommonElementType(Ctx, VX, VY),
+                                              getCommonSizeExpr(VX, VY),
+                                              getCommonAttrLoc(VX, VY));
+  }
+  case Type::DependentVector: {
+    const auto *VX = cast<DependentVectorType>(X),
+               *VY = cast<DependentVectorType>(Y);
+    assert(VX->getVectorKind() == VY->getVectorKind());
+    return Ctx.getDependentVectorType(
+        getCommonElementType(Ctx, VX, VY), getCommonSizeExpr(VX, VY),
+        getCommonAttrLoc(VX, VY), VX->getVectorKind());
+  }
+  case Type::InjectedClassName: {
+    const auto *IX = cast<InjectedClassNameType>(X),
+               *IY = cast<InjectedClassNameType>(Y);
+    return Ctx.getInjectedClassNameType(
+        getCommonDecl(IX->getDecl(), IY->getDecl()),
+        Ctx.getCommonSugaredType(IX->getInjectedSpecializationType(),
+                                 IY->getInjectedSpecializationType()));
+  }
+  case Type::TemplateSpecialization: {
+    const auto *TX = cast<TemplateSpecializationType>(X),
+               *TY = cast<TemplateSpecializationType>(Y);
+    auto As = getCommonTemplateArguments(Ctx, TX->template_arguments(),
+                                         TY->template_arguments());
+    return Ctx.getTemplateSpecializationType(
+        ::getCommonTemplateName(Ctx, TX->getTemplateName(),
+                                TY->getTemplateName()),
+        As, QualType(TX, 0));
+  }
+  case Type::DependentName: {
+    const auto *NX = cast<DependentNameType>(X),
+               *NY = cast<DependentNameType>(Y);
+    assert(NX->getIdentifier() == NY->getIdentifier());
+    return Ctx.getDependentNameType(
+        getCommonTypeKeyword(NX, NY), getCommonNNS(Ctx, NX, NY),
+        NX->getIdentifier(), NX->getCanonicalTypeInternal());
+  }
+  case Type::DependentTemplateSpecialization: {
+    const auto *TX = cast<DependentTemplateSpecializationType>(X),
+               *TY = cast<DependentTemplateSpecializationType>(Y);
+    assert(TX->getIdentifier() == TY->getIdentifier());
+    auto As = getCommonTemplateArguments(Ctx, TX->template_arguments(),
+                                         TY->template_arguments());
+    return Ctx.getDependentTemplateSpecializationType(
+        getCommonTypeKeyword(TX, TY), getCommonNNS(Ctx, TX, TY),
+        TX->getIdentifier(), As);
+  }
+  case Type::UnaryTransform: {
+    const auto *TX = cast<UnaryTransformType>(X),
+               *TY = cast<UnaryTransformType>(Y);
+    assert(TX->getUTTKind() == TY->getUTTKind());
+    return Ctx.getUnaryTransformType(
+        Ctx.getCommonSugaredType(TX->getBaseType(), TY->getBaseType()),
+        Ctx.getCommonSugaredType(TX->getUnderlyingType(),
+                                 TY->getUnderlyingType()),
+        TX->getUTTKind());
+  }
+  case Type::PackExpansion: {
+    const auto *PX = cast<PackExpansionType>(X),
+               *PY = cast<PackExpansionType>(Y);
+    return Ctx.getPackExpansionType(
+        Ctx.getCommonSugaredType(PX->getPattern(), PY->getPattern()),
+        PX->getNumExpansions(), false);
+  }
+  case Type::Pipe: {
+    const auto *PX = cast<PipeType>(X), *PY = cast<PipeType>(Y);
+    assert(PX->isReadOnly() == PY->isReadOnly());
+    auto MP = PX->isReadOnly() ? &ASTContext::getReadPipeType
+                               : &ASTContext::getWritePipeType;
+    return (Ctx.*MP)(getCommonElementType(Ctx, PX, PY));
+  }
+  }
+  llvm_unreachable("Unknown Type Class");
+}
+
+static auto unwrapSugar(SplitQualType &T) {
+  SmallVector<SplitQualType, 8> R;
+  while (true) {
+    QualType NT = T.Ty->getLocallyUnqualifiedSingleStepDesugaredType();
+    if (NT == QualType(T.Ty, 0))
+      break;
+    SplitQualType SplitNT = NT.split();
+    SplitNT.Quals += T.Quals;
+    R.push_back(T);
+    T = SplitNT;
+  }
+  return R;
+}
+
+static bool removeDifferentTopLevelSugar(SplitQualType &SX, SplitQualType &SY) {
+  auto Xs = ::unwrapSugar(SX), Ys = ::unwrapSugar(SY);
+  if (SX.Ty != SY.Ty)
+    return true;
+  while (!Xs.empty() && !Ys.empty() && Xs.back().Ty == Ys.back().Ty) {
+    SX = Xs.pop_back_val();
+    SY = Ys.pop_back_val();
+  }
+  return false;
+}
+
+QualType ASTContext::getCommonSugaredType(QualType X, QualType Y, bool Unqualified) {
+  assert(Unqualified ? hasSameUnqualifiedType(X, Y) : hasSameType(X, Y));
+  if (X == Y)
+    return X;
+  if (!Unqualified) {
+    if (X.isCanonical())
+      return X;
+    if (Y.isCanonical())
+      return Y;
+  }
+
+  SplitQualType SX = X.split(), SY = Y.split();
+  if (::removeDifferentTopLevelSugar(SX, SY))
+    SX.Ty = ::getCommonType(*this, SX.Ty, SY.Ty).getTypePtr();
+
+  if (Unqualified)
+    SX.Quals = Qualifiers::removeCommonQualifiers(SX.Quals, SY.Quals);
+  else
+    assert(SX.Quals == SY.Quals);
+
+  QualType R = getQualifiedType(SX);
+  assert(Unqualified ? hasSameUnqualifiedType(R, X) : hasSameType(R, X));
+  return R;
+}
+
 QualType ASTContext::getCorrespondingSaturatedType(QualType Ty) const {
   assert(Ty->isFixedPointType());
 
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
@@ -4020,10 +4020,9 @@
         // The old declaration provided a function prototype, but the
         // new declaration does not. Merge in the prototype.
         assert(!OldProto->hasExceptionSpec() && "Exception spec in C");
-        SmallVector<QualType, 16> ParamTypes(OldProto->param_types());
-        NewQType =
-            Context.getFunctionType(NewFuncType->getReturnType(), ParamTypes,
-                                    OldProto->getExtProtoInfo());
+        NewQType = Context.getFunctionType(NewFuncType->getReturnType(),
+                                           OldProto->getParamTypes(),
+                                           OldProto->getExtProtoInfo());
         New->setType(NewQType);
         New->setHasInheritedPrototype();
 
@@ -12216,7 +12215,10 @@
                                     Type.getQualifiers());
 
   QualType DeducedType;
-  if (DeduceAutoType(TSI, DeduceInit, DeducedType) == DAR_Failed) {
+  TemplateDeductionInfo Info(DeduceInit->getExprLoc());
+  TemplateDeductionResult Result =
+      DeduceAutoType(TSI->getTypeLoc(), DeduceInit, DeducedType, Info);
+  if (Result != TDK_Success && Result != TDK_AlreadyDiagnosed) {
     if (!IsInitCapture)
       DiagnoseAutoDeductionFailure(VDecl, DeduceInit);
     else if (isa<InitListExpr>(Init))
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
@@ -1506,7 +1506,10 @@
           Diag(Deduce->getBeginLoc(), diag::err_auto_expr_init_paren_braces)
           << ListInitialization << Ty << FullRange);
     QualType DeducedType;
-    if (DeduceAutoType(TInfo, Deduce, DeducedType) == DAR_Failed)
+    TemplateDeductionInfo Info(Deduce->getExprLoc());
+    TemplateDeductionResult Result =
+        DeduceAutoType(TInfo->getTypeLoc(), Deduce, DeducedType, Info);
+    if (Result != TDK_Success && Result != TDK_AlreadyDiagnosed)
       return ExprError(Diag(TyBeginLoc, diag::err_auto_expr_deduction_failure)
                        << Ty << Deduce->getType() << FullRange
                        << Deduce->getSourceRange());
@@ -2045,10 +2048,13 @@
           Diag(Deduce->getBeginLoc(), diag::err_auto_expr_init_paren_braces)
           << Braced << AllocType << TypeRange);
     QualType DeducedType;
-    if (DeduceAutoType(AllocTypeInfo, Deduce, DeducedType) == DAR_Failed)
+    TemplateDeductionInfo Info(Deduce->getExprLoc());
+    TemplateDeductionResult Result =
+        DeduceAutoType(AllocTypeInfo->getTypeLoc(), Deduce, DeducedType, Info);
+    if (Result != TDK_Success && Result != TDK_AlreadyDiagnosed)
       return ExprError(Diag(StartLoc, diag::err_auto_new_deduction_failure)
-                       << AllocType << Deduce->getType()
-                       << TypeRange << Deduce->getSourceRange());
+                       << AllocType << Deduce->getType() << TypeRange
+                       << Deduce->getSourceRange());
     if (DeducedType.isNull())
       return ExprError();
     AllocType = DeducedType;
@@ -6687,12 +6693,8 @@
     // This is the fun case: both exception specifications are dynamic. Form
     // the union of the two lists.
     assert(EST2 == EST_Dynamic && "other cases should already be handled");
-    llvm::SmallPtrSet<QualType, 8> Found;
-    for (auto &Exceptions : {ESI1.Exceptions, ESI2.Exceptions})
-      for (QualType E : Exceptions)
-        if (Found.insert(S.Context.getCanonicalType(E)).second)
-          ExceptionTypeStorage.push_back(E);
-
+    S.Context.mergeTypeLists(ExceptionTypeStorage, ESI1.Exceptions,
+                             ESI2.Exceptions);
     FunctionProtoType::ExceptionSpecInfo Result(EST_Dynamic);
     Result.Exceptions = ExceptionTypeStorage;
     return Result;
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
@@ -684,6 +684,7 @@
 
   case Sema::TDK_Success:
   case Sema::TDK_NonDependentConversionFailure:
+  case Sema::TDK_AlreadyDiagnosed:
     llvm_unreachable("not a deduction failure");
   }
 
@@ -733,6 +734,7 @@
 
   // Unhandled
   case Sema::TDK_MiscellaneousDeductionFailure:
+  case Sema::TDK_AlreadyDiagnosed:
     break;
   }
 }
@@ -770,6 +772,7 @@
 
   // Unhandled
   case Sema::TDK_MiscellaneousDeductionFailure:
+  case Sema::TDK_AlreadyDiagnosed:
     break;
   }
 
@@ -805,6 +808,7 @@
 
   // Unhandled
   case Sema::TDK_MiscellaneousDeductionFailure:
+  case Sema::TDK_AlreadyDiagnosed:
     break;
   }
 
@@ -836,6 +840,7 @@
 
   // Unhandled
   case Sema::TDK_MiscellaneousDeductionFailure:
+  case Sema::TDK_AlreadyDiagnosed:
     break;
   }
 
@@ -867,6 +872,7 @@
 
   // Unhandled
   case Sema::TDK_MiscellaneousDeductionFailure:
+  case Sema::TDK_AlreadyDiagnosed:
     break;
   }
 
@@ -11461,6 +11467,7 @@
   switch ((Sema::TemplateDeductionResult)DFI.Result) {
   case Sema::TDK_Success:
   case Sema::TDK_NonDependentConversionFailure:
+  case Sema::TDK_AlreadyDiagnosed:
     llvm_unreachable("non-deduction failure while diagnosing bad deduction");
 
   case Sema::TDK_Invalid:
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -2308,11 +2308,14 @@
 
       // If the type contained 'auto', deduce the 'auto' to 'id'.
       if (FirstType->getContainedAutoType()) {
-        OpaqueValueExpr OpaqueId(D->getLocation(), Context.getObjCIdType(),
-                                 VK_PRValue);
+        SourceLocation Loc = D->getLocation();
+        OpaqueValueExpr OpaqueId(Loc, Context.getObjCIdType(), VK_PRValue);
         Expr *DeducedInit = &OpaqueId;
-        if (DeduceAutoType(D->getTypeSourceInfo(), DeducedInit, FirstType) ==
-                DAR_Failed)
+        TemplateDeductionInfo Info(Loc);
+        FirstType = QualType();
+        TemplateDeductionResult Result = DeduceAutoType(
+            D->getTypeSourceInfo()->getTypeLoc(), DeducedInit, FirstType, Info);
+        if (Result != TDK_Success && Result != TDK_AlreadyDiagnosed)
           DiagnoseAutoDeductionFailure(D, DeducedInit);
         if (FirstType.isNull()) {
           D->setInvalidDecl();
@@ -2376,10 +2379,16 @@
   // Deduce the type for the iterator variable now rather than leaving it to
   // AddInitializerToDecl, so we can produce a more suitable diagnostic.
   QualType InitType;
-  if ((!isa<InitListExpr>(Init) && Init->getType()->isVoidType()) ||
-      SemaRef.DeduceAutoType(Decl->getTypeSourceInfo(), Init, InitType) ==
-          Sema::DAR_Failed)
+  if (!isa<InitListExpr>(Init) && Init->getType()->isVoidType()) {
     SemaRef.Diag(Loc, DiagID) << Init->getType();
+  } else {
+    TemplateDeductionInfo Info(Init->getExprLoc());
+    Sema::TemplateDeductionResult Result = SemaRef.DeduceAutoType(
+        Decl->getTypeSourceInfo()->getTypeLoc(), Init, InitType, Info);
+    if (Result != Sema::TDK_Success && Result != Sema::TDK_AlreadyDiagnosed)
+      SemaRef.Diag(Loc, DiagID) << Init->getType();
+  }
+
   if (InitType.isNull()) {
     Decl->setInvalidDecl();
     return true;
@@ -3769,17 +3778,13 @@
 /// C++1y [dcl.spec.auto]p6.
 bool Sema::DeduceFunctionTypeFromReturnExpr(FunctionDecl *FD,
                                             SourceLocation ReturnLoc,
-                                            Expr *&RetExpr,
-                                            const AutoType *AT) {
+                                            Expr *RetExpr, const AutoType *AT) {
   // If this is the conversion function for a lambda, we choose to deduce its
   // type from the corresponding call operator, not from the synthesized return
   // statement within it. See Sema::DeduceReturnType.
   if (isLambdaConversionOperator(FD))
     return false;
 
-  TypeLoc OrigResultType = getReturnTypeLoc(FD);
-  QualType Deduced;
-
   if (RetExpr && isa<InitListExpr>(RetExpr)) {
     //  If the deduction is for a return statement and the initializer is
     //  a braced-init-list, the program is ill-formed.
@@ -3799,87 +3804,74 @@
     return false;
   }
 
-  if (RetExpr) {
-    //  Otherwise, [...] deduce a value for U using the rules of template
-    //  argument deduction.
-    DeduceAutoResult DAR = DeduceAutoType(OrigResultType, RetExpr, Deduced);
-
-    if (DAR == DAR_Failed && !FD->isInvalidDecl())
-      Diag(RetExpr->getExprLoc(), diag::err_auto_fn_deduction_failure)
-        << OrigResultType.getType() << RetExpr->getType();
-
-    if (DAR != DAR_Succeeded)
-      return true;
-
-    // If a local type is part of the returned type, mark its fields as
-    // referenced.
-    LocalTypedefNameReferencer Referencer(*this);
-    Referencer.TraverseType(RetExpr->getType());
-  } else {
-    // For a function with a deduced result type to return void,
-    // the result type as written must be 'auto' or 'decltype(auto)',
-    // possibly cv-qualified or constrained, but not ref-qualified.
+  TypeLoc OrigResultType = getReturnTypeLoc(FD);
+  //  In the case of a return with no operand, the initializer is considered
+  //  to be void().
+  CXXScalarValueInitExpr VoidVal(Context.VoidTy, nullptr, SourceLocation());
+  if (!RetExpr) {
+    // For a function with a deduced result type to return with omitted
+    // expression, the result type as written must be 'auto' or
+    // 'decltype(auto)', possibly cv-qualified or constrained, but not
+    // ref-qualified.
     if (!OrigResultType.getType()->getAs<AutoType>()) {
       Diag(ReturnLoc, diag::err_auto_fn_return_void_but_not_auto)
-        << OrigResultType.getType();
+          << OrigResultType.getType();
       return true;
     }
-    // In the case of a return with no operand, the initializer is considered
-    // to be 'void()'.
-    Expr *Dummy = new (Context) CXXScalarValueInitExpr(
-        Context.VoidTy,
-        Context.getTrivialTypeSourceInfo(Context.VoidTy, ReturnLoc), ReturnLoc);
-    DeduceAutoResult DAR = DeduceAutoType(OrigResultType, Dummy, Deduced);
-
-    if (DAR == DAR_Failed && !FD->isInvalidDecl())
-      Diag(ReturnLoc, diag::err_auto_fn_deduction_failure)
-          << OrigResultType.getType() << Dummy->getType();
-
-    if (DAR != DAR_Succeeded)
-      return true;
+    RetExpr = &VoidVal;
   }
 
-  // CUDA: Kernel function must have 'void' return type.
-  if (getLangOpts().CUDA)
-    if (FD->hasAttr<CUDAGlobalAttr>() && !Deduced->isVoidType()) {
-      Diag(FD->getLocation(), diag::err_kern_type_not_void_return)
-          << FD->getType() << FD->getSourceRange();
+  QualType Deduced = AT->getDeducedType();
+  {
+    //  Otherwise, [...] deduce a value for U using the rules of template
+    //  argument deduction.
+    TemplateDeductionInfo Info(RetExpr->getExprLoc());
+    TemplateDeductionResult Res =
+        DeduceAutoType(OrigResultType, RetExpr, Deduced, Info);
+    if (Res != TDK_Success && FD->isInvalidDecl())
       return true;
-    }
-
-  //  If a function with a declared return type that contains a placeholder type
-  //  has multiple return statements, the return type is deduced for each return
-  //  statement. [...] if the type deduced is not the same in each deduction,
-  //  the program is ill-formed.
-  QualType DeducedT = AT->getDeducedType();
-  if (!DeducedT.isNull() && !FD->isInvalidDecl()) {
-    AutoType *NewAT = Deduced->getContainedAutoType();
-    // It is possible that NewAT->getDeducedType() is null. When that happens,
-    // we should not crash, instead we ignore this deduction.
-    if (NewAT->getDeducedType().isNull())
-      return false;
-
-    CanQualType OldDeducedType = Context.getCanonicalFunctionResultType(
-                                   DeducedT);
-    CanQualType NewDeducedType = Context.getCanonicalFunctionResultType(
-                                   NewAT->getDeducedType());
-    if (!FD->isDependentContext() && OldDeducedType != NewDeducedType) {
+    switch (Res) {
+    case TDK_Success:
+      break;
+    case TDK_AlreadyDiagnosed:
+      return true;
+    case TDK_Inconsistent: {
+      //  If a function with a declared return type that contains a placeholder
+      //  type has multiple return statements, the return type is deduced for
+      //  each return statement. [...] if the type deduced is not the same in
+      //  each deduction, the program is ill-formed.
       const LambdaScopeInfo *LambdaSI = getCurLambda();
-      if (LambdaSI && LambdaSI->HasImplicitReturnType) {
+      if (LambdaSI && LambdaSI->HasImplicitReturnType)
         Diag(ReturnLoc, diag::err_typecheck_missing_return_type_incompatible)
-          << NewAT->getDeducedType() << DeducedT
-          << true /*IsLambda*/;
-      } else {
+            << Info.SecondArg << Info.FirstArg << true /*IsLambda*/;
+      else
         Diag(ReturnLoc, diag::err_auto_fn_different_deductions)
-          << (AT->isDecltypeAuto() ? 1 : 0)
-          << NewAT->getDeducedType() << DeducedT;
-      }
+            << (AT->isDecltypeAuto() ? 1 : 0) << Info.SecondArg
+            << Info.FirstArg;
+      return true;
+    }
+    default:
+      Diag(RetExpr->getExprLoc(), diag::err_auto_fn_deduction_failure)
+          << OrigResultType.getType() << Info.SecondArg;
       return true;
     }
-  } else if (!FD->isInvalidDecl()) {
+  }
+
+  // If a local type is part of the returned type, mark its fields as
+  // referenced.
+  LocalTypedefNameReferencer(*this).TraverseType(RetExpr->getType());
+
+  // CUDA: Kernel function must have 'void' return type.
+  if (getLangOpts().CUDA && FD->hasAttr<CUDAGlobalAttr>() &&
+      !Deduced->isVoidType()) {
+    Diag(FD->getLocation(), diag::err_kern_type_not_void_return)
+        << FD->getType() << FD->getSourceRange();
+    return true;
+  }
+
+  if (!FD->isInvalidDecl() && AT->getDeducedType() != Deduced)
     // Update all declarations of the function to have the deduced return type.
     Context.adjustDeducedFunctionResultType(FD, Deduced);
-  }
 
   return false;
 }
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
@@ -6876,7 +6876,6 @@
     // When checking a deduced template argument, deduce from its type even if
     // the type is dependent, in order to check the types of non-type template
     // arguments line up properly in partial ordering.
-    Optional<unsigned> Depth = Param->getDepth() + 1;
     Expr *DeductionArg = Arg;
     if (auto *PE = dyn_cast<PackExpansionExpr>(DeductionArg))
       DeductionArg = PE->getPattern();
@@ -6892,20 +6891,27 @@
           DeduceTemplateSpecializationFromInitializer(TSI, Entity, Kind, Inits);
       if (ParamType.isNull())
         return ExprError();
-    } else if (DeduceAutoType(
-                   TSI, DeductionArg, ParamType, Depth,
-                   // We do not check constraints right now because the
-                   // immediately-declared constraint of the auto type is also
-                   // an associated constraint, and will be checked along with
-                   // the other associated constraints after checking the
-                   // template argument list.
-                   /*IgnoreConstraints=*/true) == DAR_Failed) {
-      Diag(Arg->getExprLoc(),
-           diag::err_non_type_template_parm_type_deduction_failure)
-        << Param->getDeclName() << Param->getType() << Arg->getType()
-        << Arg->getSourceRange();
-      Diag(Param->getLocation(), diag::note_template_param_here);
-      return ExprError();
+    } else {
+      TemplateDeductionInfo Info(DeductionArg->getExprLoc(),
+                                 Param->getDepth() + 1);
+      ParamType = QualType();
+      TemplateDeductionResult Result =
+          DeduceAutoType(TSI->getTypeLoc(), DeductionArg, ParamType, Info,
+                         /*DependentDeduction=*/true,
+                         // We do not check constraints right now because the
+                         // immediately-declared constraint of the auto type is
+                         // also an associated constraint, and will be checked
+                         // along with the other associated constraints after
+                         // checking the template argument list.
+                         /*IgnoreConstraints=*/true);
+      if (Result != TDK_Success && Result != TDK_AlreadyDiagnosed) {
+        Diag(Arg->getExprLoc(),
+             diag::err_non_type_template_parm_type_deduction_failure)
+            << Param->getDeclName() << Param->getType() << Arg->getType()
+            << Arg->getSourceRange();
+        Diag(Param->getLocation(), diag::note_template_param_here);
+        return ExprError();
+      }
     }
     // CheckNonTypeTemplateParameterType will produce a diagnostic if there's
     // an error. The error message normally references the parameter
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
@@ -235,11 +235,13 @@
   case TemplateArgument::Null:
     llvm_unreachable("Non-deduced template arguments handled above");
 
-  case TemplateArgument::Type:
+  case TemplateArgument::Type: {
     // If two template type arguments have the same type, they're compatible.
-    if (Y.getKind() == TemplateArgument::Type &&
-        Context.hasSameType(X.getAsType(), Y.getAsType()))
-      return X;
+    QualType TX = X.getAsType(), TY = Y.getAsType();
+    if (Y.getKind() == TemplateArgument::Type && Context.hasSameType(TX, TY))
+      return DeducedTemplateArgument(Context.getCommonSugaredType(TX, TY),
+                                     X.wasDeducedFromArrayBound() ||
+                                         Y.wasDeducedFromArrayBound());
 
     // If one of the two arguments was deduced from an array bound, the other
     // supersedes it.
@@ -248,6 +250,7 @@
 
     // The arguments are not compatible.
     return DeducedTemplateArgument();
+  }
 
   case TemplateArgument::Integral:
     // If we deduced a constant in one case and either a dependent expression or
@@ -325,7 +328,9 @@
     // If we deduced a null pointer and a dependent expression, keep the
     // null pointer.
     if (Y.getKind() == TemplateArgument::Expression)
-      return X;
+      return TemplateArgument(Context.getCommonSugaredType(
+                                  X.getNullPtrType(), Y.getAsExpr()->getType()),
+                              true);
 
     // If we deduced a null pointer and an integral constant, keep the
     // integral constant.
@@ -334,7 +339,9 @@
 
     // If we deduced two null pointers, they are the same.
     if (Y.getKind() == TemplateArgument::NullPtr)
-      return X;
+      return TemplateArgument(
+          Context.getCommonSugaredType(X.getNullPtrType(), Y.getNullPtrType()),
+          true);
 
     // All other combinations are incompatible.
     return DeducedTemplateArgument();
@@ -4521,42 +4528,9 @@
 
 } // namespace
 
-Sema::DeduceAutoResult
-Sema::DeduceAutoType(TypeSourceInfo *Type, Expr *&Init, QualType &Result,
-                     Optional<unsigned> DependentDeductionDepth,
-                     bool IgnoreConstraints) {
-  return DeduceAutoType(Type->getTypeLoc(), Init, Result,
-                        DependentDeductionDepth, IgnoreConstraints);
-}
-
-/// Attempt to produce an informative diagostic explaining why auto deduction
-/// failed.
-/// \return \c true if diagnosed, \c false if not.
-static bool diagnoseAutoDeductionFailure(Sema &S,
-                                         Sema::TemplateDeductionResult TDK,
-                                         TemplateDeductionInfo &Info,
-                                         ArrayRef<SourceRange> Ranges) {
-  switch (TDK) {
-  case Sema::TDK_Inconsistent: {
-    // Inconsistent deduction means we were deducing from an initializer list.
-    auto D = S.Diag(Info.getLocation(), diag::err_auto_inconsistent_deduction);
-    D << Info.FirstArg << Info.SecondArg;
-    for (auto R : Ranges)
-      D << R;
-    return true;
-  }
-
-  // FIXME: Are there other cases for which a custom diagnostic is more useful
-  // than the basic "types don't match" diagnostic?
-
-  default:
-    return false;
-  }
-}
-
-static Sema::DeduceAutoResult
-CheckDeducedPlaceholderConstraints(Sema &S, const AutoType &Type,
-                                   AutoTypeLoc TypeLoc, QualType Deduced) {
+static bool CheckDeducedPlaceholderConstraints(Sema &S, const AutoType &Type,
+                                               AutoTypeLoc TypeLoc,
+                                               QualType Deduced) {
   ConstraintSatisfaction Satisfaction;
   ConceptDecl *Concept = Type.getTypeConstraintConcept();
   TemplateArgumentListInfo TemplateArgs(TypeLoc.getLAngleLoc(),
@@ -4571,11 +4545,11 @@
   llvm::SmallVector<TemplateArgument, 4> Converted;
   if (S.CheckTemplateArgumentList(Concept, SourceLocation(), TemplateArgs,
                                   /*PartialTemplateArgs=*/false, Converted))
-    return Sema::DAR_FailedAlreadyDiagnosed;
+    return true;
   if (S.CheckConstraintSatisfaction(Concept, {Concept->getConstraintExpr()},
                                     Converted, TypeLoc.getLocalSourceRange(),
                                     Satisfaction))
-    return Sema::DAR_FailedAlreadyDiagnosed;
+    return true;
   if (!Satisfaction.IsSatisfied) {
     std::string Buf;
     llvm::raw_string_ostream OS(Buf);
@@ -4589,11 +4563,11 @@
     OS.flush();
     S.Diag(TypeLoc.getConceptNameLoc(),
            diag::err_placeholder_constraints_not_satisfied)
-         << Deduced << Buf << TypeLoc.getLocalSourceRange();
+        << Deduced << Buf << TypeLoc.getLocalSourceRange();
     S.DiagnoseUnsatisfiedConstraint(Satisfaction);
-    return Sema::DAR_FailedAlreadyDiagnosed;
+    return true;
   }
-  return Sema::DAR_Succeeded;
+  return false;
 }
 
 /// Deduce the type for an auto type-specifier (C++11 [dcl.spec.auto]p6)
@@ -4612,178 +4586,165 @@
 ///        parameter depth at which we should perform 'auto' deduction.
 /// \param IgnoreConstraints Set if we should not fail if the deduced type does
 ///                          not satisfy the type-constraint in the auto type.
-Sema::DeduceAutoResult
-Sema::DeduceAutoType(TypeLoc Type, Expr *&Init, QualType &Result,
-                     Optional<unsigned> DependentDeductionDepth,
-                     bool IgnoreConstraints) {
+Sema::TemplateDeductionResult Sema::DeduceAutoType(TypeLoc Type, Expr *Init,
+                                                   QualType &Result,
+                                                   TemplateDeductionInfo &Info,
+                                                   bool DependentDeduction,
+                                                   bool IgnoreConstraints) {
+  assert(DependentDeduction || Info.getDeducedDepth() == 0);
   if (Init->containsErrors())
-    return DAR_FailedAlreadyDiagnosed;
-  if (Init->getType()->isNonOverloadPlaceholderType()) {
+    return TDK_AlreadyDiagnosed;
+
+  const AutoType *AT = Type.getType()->getContainedAutoType();
+  assert(AT);
+
+  if (Init->getType()->isNonOverloadPlaceholderType() || AT->isDecltypeAuto()) {
     ExprResult NonPlaceholder = CheckPlaceholderExpr(Init);
     if (NonPlaceholder.isInvalid())
-      return DAR_FailedAlreadyDiagnosed;
+      return TDK_AlreadyDiagnosed;
     Init = NonPlaceholder.get();
   }
 
   DependentAuto DependentResult = {
       /*.IsPack = */ (bool)Type.getAs<PackExpansionTypeLoc>()};
 
-  if (!DependentDeductionDepth &&
+  if (!DependentDeduction &&
       (Type.getType()->isDependentType() || Init->isTypeDependent() ||
        Init->containsUnexpandedParameterPack())) {
     Result = SubstituteDeducedTypeTransform(*this, DependentResult).Apply(Type);
     assert(!Result.isNull() && "substituting DependentTy can't fail");
-    return DAR_Succeeded;
+    return TDK_Success;
   }
 
-  // Find the depth of template parameter to synthesize.
-  unsigned Depth = DependentDeductionDepth.value_or(0);
-
-  // If this is a 'decltype(auto)' specifier, do the decltype dance.
-  // Since 'decltype(auto)' can only occur at the top of the type, we
-  // don't need to go digging for it.
-  if (const AutoType *AT = Type.getType()->getAs<AutoType>()) {
-    if (AT->isDecltypeAuto()) {
-      if (isa<InitListExpr>(Init)) {
-        Diag(Init->getBeginLoc(), diag::err_decltype_auto_initializer_list);
-        return DAR_FailedAlreadyDiagnosed;
-      }
-
-      ExprResult ER = CheckPlaceholderExpr(Init);
-      if (ER.isInvalid())
-        return DAR_FailedAlreadyDiagnosed;
-      QualType Deduced = getDecltypeForExpr(ER.get());
-      assert(!Deduced.isNull());
-      if (AT->isConstrained() && !IgnoreConstraints) {
-        auto ConstraintsResult =
-            CheckDeducedPlaceholderConstraints(*this, *AT,
-                                               Type.getContainedAutoTypeLoc(),
-                                               Deduced);
-        if (ConstraintsResult != DAR_Succeeded)
-          return ConstraintsResult;
-      }
-      Result = SubstituteDeducedTypeTransform(*this, Deduced).Apply(Type);
-      if (Result.isNull())
-        return DAR_FailedAlreadyDiagnosed;
-      return DAR_Succeeded;
-    } else if (!getLangOpts().CPlusPlus) {
-      if (isa<InitListExpr>(Init)) {
-        Diag(Init->getBeginLoc(), diag::err_auto_init_list_from_c);
-        return DAR_FailedAlreadyDiagnosed;
-      }
-    }
+  auto *InitList = dyn_cast<InitListExpr>(Init);
+  if (!getLangOpts().CPlusPlus && InitList) {
+    Diag(Init->getBeginLoc(), diag::err_auto_init_list_from_c);
+    return TDK_AlreadyDiagnosed;
   }
 
-  SourceLocation Loc = Init->getExprLoc();
-
-  LocalInstantiationScope InstScope(*this);
-
-  // Build template<class TemplParam> void Func(FuncParam);
-  TemplateTypeParmDecl *TemplParam = TemplateTypeParmDecl::Create(
-      Context, nullptr, SourceLocation(), Loc, Depth, 0, nullptr, false, false,
-      false);
-  QualType TemplArg = QualType(TemplParam->getTypeForDecl(), 0);
-  NamedDecl *TemplParamPtr = TemplParam;
-  FixedSizeTemplateParameterListStorage<1, false> TemplateParamsSt(
-      Context, Loc, Loc, TemplParamPtr, Loc, nullptr);
-
-  QualType FuncParam =
-      SubstituteDeducedTypeTransform(*this, TemplArg, /*UseTypeSugar*/ true)
-          .Apply(Type);
-  assert(!FuncParam.isNull() &&
-         "substituting template parameter for 'auto' failed");
-
   // Deduce type of TemplParam in Func(Init)
   SmallVector<DeducedTemplateArgument, 1> Deduced;
   Deduced.resize(1);
 
-  TemplateDeductionInfo Info(Loc, Depth);
-
   // If deduction failed, don't diagnose if the initializer is dependent; it
   // might acquire a matching type in the instantiation.
-  auto DeductionFailed = [&](TemplateDeductionResult TDK,
-                             ArrayRef<SourceRange> Ranges) -> DeduceAutoResult {
+  auto DeductionFailed = [&](TemplateDeductionResult TDK) {
     if (Init->isTypeDependent()) {
       Result =
           SubstituteDeducedTypeTransform(*this, DependentResult).Apply(Type);
       assert(!Result.isNull() && "substituting DependentTy can't fail");
-      return DAR_Succeeded;
+      return TDK_Success;
     }
-    if (diagnoseAutoDeductionFailure(*this, TDK, Info, Ranges))
-      return DAR_FailedAlreadyDiagnosed;
-    return DAR_Failed;
+    return TDK;
   };
 
   SmallVector<OriginalCallArg, 4> OriginalCallArgs;
 
-  InitListExpr *InitList = dyn_cast<InitListExpr>(Init);
-  if (InitList) {
-    // Notionally, we substitute std::initializer_list<T> for 'auto' and deduce
-    // against that. Such deduction only succeeds if removing cv-qualifiers and
-    // references results in std::initializer_list<T>.
-    if (!Type.getType().getNonReferenceType()->getAs<AutoType>())
-      return DAR_Failed;
-
-    // Resolving a core issue: a braced-init-list containing any designators is
-    // a non-deduced context.
-    for (Expr *E : InitList->inits())
-      if (isa<DesignatedInitExpr>(E))
-        return DAR_Failed;
+  QualType DeducedType;
+  // If this is a 'decltype(auto)' specifier, do the decltype dance.
+  if (AT->isDecltypeAuto()) {
+    if (InitList) {
+      Diag(Init->getBeginLoc(), diag::err_decltype_auto_initializer_list);
+      return TDK_AlreadyDiagnosed;
+    }
 
-    SourceRange DeducedFromInitRange;
-    for (unsigned i = 0, e = InitList->getNumInits(); i < e; ++i) {
-      Expr *Init = InitList->getInit(i);
+    DeducedType = getDecltypeForExpr(Init);
+    assert(!DeducedType.isNull());
 
-      if (auto TDK = DeduceTemplateArgumentsFromCallArgument(
-              *this, TemplateParamsSt.get(), 0, TemplArg, Init,
-              Info, Deduced, OriginalCallArgs, /*Decomposed*/ true,
-              /*ArgIdx*/ 0, /*TDF*/ 0))
-        return DeductionFailed(TDK, {DeducedFromInitRange,
-                                     Init->getSourceRange()});
-
-      if (DeducedFromInitRange.isInvalid() &&
-          Deduced[0].getKind() != TemplateArgument::Null)
-        DeducedFromInitRange = Init->getSourceRange();
+    if (!Result.isNull()) {
+      if (!Context.hasSameType(DeducedType, Result)) {
+        Info.FirstArg = Result;
+        Info.SecondArg = DeducedType;
+        return DeductionFailed(TDK_Inconsistent);
+      }
+      DeducedType = Context.getCommonSugaredType(Result, DeducedType);
     }
   } else {
-    if (!getLangOpts().CPlusPlus && Init->refersToBitField()) {
-      Diag(Loc, diag::err_auto_bitfield);
-      return DAR_FailedAlreadyDiagnosed;
-    }
-
-    if (auto TDK = DeduceTemplateArgumentsFromCallArgument(
-            *this, TemplateParamsSt.get(), 0, FuncParam, Init, Info, Deduced,
-            OriginalCallArgs, /*Decomposed*/ false, /*ArgIdx*/ 0, /*TDF*/ 0))
-      return DeductionFailed(TDK, {});
-  }
-
-  // Could be null if somehow 'auto' appears in a non-deduced context.
-  if (Deduced[0].getKind() != TemplateArgument::Type)
-    return DeductionFailed(TDK_Incomplete, {});
-
-  QualType DeducedType = Deduced[0].getAsType();
+    LocalInstantiationScope InstScope(*this);
+
+    // Build template<class TemplParam> void Func(FuncParam);
+    SourceLocation Loc = Init->getExprLoc();
+    TemplateTypeParmDecl *TemplParam = TemplateTypeParmDecl::Create(
+        Context, nullptr, SourceLocation(), Loc, Info.getDeducedDepth(), 0,
+        nullptr, false, false, false);
+    QualType TemplArg = QualType(TemplParam->getTypeForDecl(), 0);
+    NamedDecl *TemplParamPtr = TemplParam;
+    FixedSizeTemplateParameterListStorage<1, false> TemplateParamsSt(
+        Context, Loc, Loc, TemplParamPtr, Loc, nullptr);
+
+    if (InitList) {
+      // Notionally, we substitute std::initializer_list<T> for 'auto' and
+      // deduce against that. Such deduction only succeeds if removing
+      // cv-qualifiers and references results in std::initializer_list<T>.
+      if (!Type.getType().getNonReferenceType()->getAs<AutoType>())
+        return TDK_Invalid;
+
+      SourceRange DeducedFromInitRange;
+      for (Expr *Init : InitList->inits()) {
+        // Resolving a core issue: a braced-init-list containing any designators
+        // is a non-deduced context.
+        if (isa<DesignatedInitExpr>(Init))
+          return TDK_Invalid;
+        if (auto TDK = DeduceTemplateArgumentsFromCallArgument(
+                *this, TemplateParamsSt.get(), 0, TemplArg, Init, Info, Deduced,
+                OriginalCallArgs, /*Decomposed=*/true,
+                /*ArgIdx=*/0, /*TDF=*/0)) {
+          if (TDK == TDK_Inconsistent) {
+            Diag(Info.getLocation(), diag::err_auto_inconsistent_deduction)
+                << Info.FirstArg << Info.SecondArg << DeducedFromInitRange
+                << Init->getSourceRange();
+            return DeductionFailed(TDK_AlreadyDiagnosed);
+          }
+          return DeductionFailed(TDK);
+        }
 
-  if (InitList) {
-    DeducedType = BuildStdInitializerList(DeducedType, Loc);
-    if (DeducedType.isNull())
-      return DAR_FailedAlreadyDiagnosed;
-  }
+        if (DeducedFromInitRange.isInvalid() &&
+            Deduced[0].getKind() != TemplateArgument::Null)
+          DeducedFromInitRange = Init->getSourceRange();
+      }
+    } else {
+      if (!getLangOpts().CPlusPlus && Init->refersToBitField()) {
+        Diag(Loc, diag::err_auto_bitfield);
+        return TDK_AlreadyDiagnosed;
+      }
+      QualType FuncParam =
+          SubstituteDeducedTypeTransform(*this, TemplArg, /*UseTypeSugar=*/true)
+              .Apply(Type);
+      assert(!FuncParam.isNull() &&
+             "substituting template parameter for 'auto' failed");
+      if (!Result.isNull())
+        Deduced[0] = DeducedTemplateArgument(Result);
+      if (auto TDK = DeduceTemplateArgumentsFromCallArgument(
+              *this, TemplateParamsSt.get(), 0, FuncParam, Init, Info, Deduced,
+              OriginalCallArgs, /*Decomposed=*/false, /*ArgIdx=*/0, /*TDF=*/0))
+        return DeductionFailed(TDK);
+    }
 
-  QualType MaybeAuto = Type.getType().getNonReferenceType();
-  while (MaybeAuto->isPointerType())
-    MaybeAuto = MaybeAuto->getPointeeType();
-  if (const auto *AT = MaybeAuto->getAs<AutoType>()) {
-    if (AT->isConstrained() && !IgnoreConstraints) {
-      auto ConstraintsResult = CheckDeducedPlaceholderConstraints(
-          *this, *AT, Type.getContainedAutoTypeLoc(), DeducedType);
-      if (ConstraintsResult != DAR_Succeeded)
-        return ConstraintsResult;
+    // Could be null if somehow 'auto' appears in a non-deduced context.
+    if (Deduced[0].getKind() != TemplateArgument::Type)
+      return DeductionFailed(TDK_Incomplete);
+    DeducedType = Deduced[0].getAsType();
+
+    if (InitList) {
+      DeducedType = BuildStdInitializerList(DeducedType, Loc);
+      if (DeducedType.isNull())
+        return TDK_AlreadyDiagnosed;
+      if (!Result.isNull() && !Context.hasSameType(Result, DeducedType)) {
+        Info.FirstArg = Result;
+        Info.SecondArg = DeducedType;
+        return DeductionFailed(TDK_Inconsistent);
+      }
     }
   }
 
+  if (AT->isConstrained() && !IgnoreConstraints &&
+      CheckDeducedPlaceholderConstraints(
+          *this, *AT, Type.getContainedAutoTypeLoc(), DeducedType))
+    return TDK_AlreadyDiagnosed;
+
   Result = SubstituteDeducedTypeTransform(*this, DeducedType).Apply(Type);
   if (Result.isNull())
-    return DAR_FailedAlreadyDiagnosed;
+    return TDK_AlreadyDiagnosed;
 
   // Check that the deduced argument type is compatible with the original
   // argument type per C++ [temp.deduct.call]p4.
@@ -4794,11 +4755,11 @@
     if (auto TDK =
             CheckOriginalCallArgDeduction(*this, Info, OriginalArg, DeducedA)) {
       Result = QualType();
-      return DeductionFailed(TDK, {});
+      return DeductionFailed(TDK);
     }
   }
 
-  return DAR_Succeeded;
+  return TDK_Success;
 }
 
 QualType Sema::SubstAutoType(QualType TypeWithAuto,
diff --git a/clang/test/SemaCXX/deduced-return-void.cpp b/clang/test/SemaCXX/deduced-return-void.cpp
--- a/clang/test/SemaCXX/deduced-return-void.cpp
+++ b/clang/test/SemaCXX/deduced-return-void.cpp
@@ -115,7 +115,7 @@
 }
 auto& f5() {
   return i;
-  return void(); // expected-error@-2 {{cannot form a reference to 'void'}}
+  return void(); // expected-error {{deduced as 'int' in earlier return statement}}
 }
 auto& f6() { return 42; } // expected-error {{non-const lvalue reference to type 'int' cannot bind to a temporary of type 'int'}}
 
@@ -130,9 +130,9 @@
   return i;
   return; // expected-error {{cannot deduce return type 'auto &' from omitted return expression}}
 };
-auto l5 = []() -> auto& { // expected-error {{cannot form a reference to 'void'}}
+auto l5 = []() -> auto & {
   return i;
-  return void();
+  return void(); // expected-error {{deduced as 'int' in earlier return statement}}
 };
 auto l6 = []() -> auto& {
   return 42; // expected-error {{non-const lvalue reference to type 'int' cannot bind to a temporary of type 'int'}}
diff --git a/clang/test/SemaCXX/sugared-auto.cpp b/clang/test/SemaCXX/sugared-auto.cpp
--- a/clang/test/SemaCXX/sugared-auto.cpp
+++ b/clang/test/SemaCXX/sugared-auto.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++20
+// RUN: %clang_cc1 -fsyntax-only -verify -xobjective-c++ %s -std=c++20 -fblocks -fobjc-arc -fobjc-runtime-has-weak -fenable-matrix -Wno-dynamic-exception-spec
 
 enum class N {};
 
@@ -9,6 +9,26 @@
 using Man = Animal;
 using Dog = Animal;
 
+using ManPtr = Man *;
+using DogPtr = Dog *;
+
+using SocratesPtr = ManPtr;
+
+using ConstMan = const Man;
+using ConstDog = const Dog;
+
+using Virus = void;
+using SARS = Virus;
+using Ebola = Virus;
+
+using Bacteria = float;
+using Bacilli = Bacteria;
+using Vibrio = Bacteria;
+
+struct Plant;
+using Gymnosperm = Plant;
+using Angiosperm = Plant;
+
 namespace variable {
 
 auto x1 = Animal();
@@ -41,3 +61,123 @@
 N t3 = x3; // expected-error {{lvalue of type 'Animal' (aka 'int')}}
 
 } // namespace function_basic
+
+namespace function_multiple_basic {
+
+N t1 = [] { // expected-error {{rvalue of type 'Animal' (aka 'int')}}
+  if (true)
+    return Man();
+  return Dog();
+}();
+
+N t2 = []() -> decltype(auto) { // expected-error {{rvalue of type 'Animal' (aka 'int')}}
+  if (true)
+    return Man();
+  return Dog();
+}();
+
+N t3 = [] { // expected-error {{rvalue of type 'Animal' (aka 'int')}}
+  if (true)
+    return Dog();
+  auto x = Man();
+  return x;
+}();
+
+N t4 = [] { // expected-error {{rvalue of type 'int'}}
+  if (true)
+    return Dog();
+  return 1;
+}();
+
+N t5 = [] { // expected-error {{rvalue of type 'Virus' (aka 'void')}}
+  if (true)
+    return Ebola();
+  return SARS();
+}();
+
+N t6 = [] { // expected-error {{rvalue of type 'void'}}
+  if (true)
+    return SARS();
+  return;
+}();
+
+} // namespace function_multiple_basic
+
+#define TEST_AUTO(X, A, B) \
+  auto X(A a, B b) {       \
+    if (0)                 \
+      return a;            \
+    if (0)                 \
+      return b;            \
+    return N();            \
+  }
+#define TEST_DAUTO(X, A, B)     \
+  decltype(auto) X(A a, B b) {  \
+    if (0)                      \
+      return static_cast<A>(a); \
+    if (0)                      \
+      return static_cast<B>(b); \
+    return N();                 \
+  }
+
+namespace misc {
+
+TEST_AUTO(t1, ManPtr, DogPtr)      // expected-error {{but deduced as 'Animal *' (aka 'int *')}}
+TEST_AUTO(t2, ManPtr, int *)       // expected-error {{but deduced as 'int *'}}
+TEST_AUTO(t3, SocratesPtr, ManPtr) // expected-error {{but deduced as 'ManPtr' (aka 'int *')}}
+
+TEST_AUTO(t4, _Atomic(Man), _Atomic(Dog)) // expected-error {{but deduced as '_Atomic(Animal)'}}
+
+using block_man = void (^)(Man);
+using block_dog = void (^)(Dog);
+TEST_AUTO(t5, block_man, block_dog) // expected-error {{but deduced as 'void (^__strong)(Animal)'}}
+
+using fp1 = SARS (*)(Man, DogPtr) throw(Vibrio);
+using fp2 = Ebola (*)(Dog, ManPtr) throw(Bacilli);
+TEST_AUTO(t6, fp1, fp2); // expected-error {{but deduced as 'Virus (*)(Animal, Animal *) throw(Bacteria)' (aka 'void (*)(int, int *) throw(Bacteria)')}}
+
+using fp3 = SARS (*)() throw(Man);
+using fp4 = Ebola (*)() throw(Vibrio);
+auto t7(fp3 a, fp4 b) {
+  if (false)
+    return true ? a : b;
+  if (false)
+    return a;
+  return N(); // expected-error {{but deduced as 'SARS (*)() throw(Man, Vibrio)' (aka 'void (*)() throw(Man, Vibrio)')}}
+}
+
+using fp5 = void (*)(const Man);
+using fp6 = void (*)(Dog);
+TEST_AUTO(t8, fp5, fp6); // expected-error {{but deduced as 'void (*)(Animal)' (aka 'void (*)(int)')}}
+
+using fp7 = void (*)(ConstMan);
+using fp8 = void (*)(ConstDog);
+TEST_AUTO(t9, fp7, fp8); // expected-error {{but deduced as 'void (*)(const Animal)' (aka 'void (*)(const int)')}}
+
+using fp9 = void (*)(ConstMan);
+using fp10 = void (*)(const Dog);
+TEST_AUTO(t10, fp9, fp10); // expected-error {{but deduced as 'void (*)(const Animal)' (aka 'void (*)(const int)')}}
+
+using fp11 = void (*)(__strong block_man);
+using fp12 = void (*)(__weak block_dog);
+TEST_AUTO(t11, fp11, fp12); // expected-error {{but deduced as 'void (*)(void (^)(Animal))'}}
+
+TEST_AUTO(t12, Man Angiosperm::*, Dog Gymnosperm::*) // expected-error {{but deduced as 'Animal Plant::*'}}
+
+TEST_DAUTO(t13, const Man &, const Dog &) // expected-error {{but deduced as 'const Animal &' (aka 'const int &')}}
+
+TEST_DAUTO(t14, Man &&, Dog &&) // expected-error {{but deduced as 'Animal &&' (aka 'int &&')}}
+
+using matrix_man = Man __attribute__((matrix_type(4, 4)));
+using matrix_dog = Dog __attribute__((matrix_type(4, 4)));
+TEST_AUTO(t15, matrix_man, matrix_dog) // expected-error {{but deduced as 'Animal __attribute__((matrix_type(4, 4)))'}}
+
+using vector_man = Man __attribute__((vector_size(4)));
+using vector_dog = Dog __attribute__((vector_size(4)));
+TEST_AUTO(t16, vector_man, vector_dog) // expected-error {{but deduced as '__attribute__((__vector_size__(1 * sizeof(Animal)))) Animal' (vector of 1 'Animal' value)}}
+
+using ext_vector_man = Man __attribute__((ext_vector_type(4)));
+using ext_vector_dog = Dog __attribute__((ext_vector_type(4)));
+TEST_AUTO(t17, ext_vector_man, ext_vector_dog) // expected-error {{but deduced as 'Animal __attribute__((ext_vector_type(4)))' (vector of 4 'Animal' values)}}
+
+} // namespace misc
diff --git a/clang/test/SemaTemplate/deduction.cpp b/clang/test/SemaTemplate/deduction.cpp
--- a/clang/test/SemaTemplate/deduction.cpp
+++ b/clang/test/SemaTemplate/deduction.cpp
@@ -162,6 +162,15 @@
 
 } // namespace test4
 
+namespace test5 {
+
+template <bool, int = 0> class a {};
+template <class b> void c(b, b);
+template <bool b> void c(a<b>, a<b>);
+void d() { c(a<true>(), a<true>()); }
+
+} // namespace test5
+
 // Verify that we can deduce enum-typed arguments correctly.
 namespace test14 {
   enum E { E0, E1 };
diff --git a/libcxx/DELETE.ME b/libcxx/DELETE.ME
new file mode 100644
--- /dev/null
+++ b/libcxx/DELETE.ME
@@ -0,0 +1 @@
+D111283