[clang] Implement P2564 "consteval must propagate up"

[aaron.ballman]

clang/include/clang/AST/DeclBase.h

1698

Need to update this as well.

clang/include/clang/Sema/ScopeInfo.h

242

clang/lib/AST/Decl.cpp

3180

Should this be looking at isExplicitlyDefaulted() instead?

3184

Spell out the type.

3185

[aaron.ballman]

clang/lib/AST/Decl.cpp

3199

This code doesn't seem to match the comment (it matches the code below though).

3203–3205

clang/lib/Sema/SemaDeclCXX.cpp

2453

clang/lib/Sema/SemaExpr.cpp

18013

Ouch, this is pretty expensive because it actually evaluates the constant expression (and promptly forgets what it evaluated).

clang/lib/Sema/SemaTemplateDeduction.cpp

5037–5039

[tbaeder]

clang/include/clang/AST/Decl.h

2391

Some documentation on the new API would be useful; seems like most calls to isConsteval() should be using isImmediateFunction() instead?

[cor3ntin]

• Address Aaron's comments
• Track which expressions are immediately escalating to offer better diagnostics

[cor3ntin]

In FunctionScopeInfo, only track that an immediate-escalating expression
has been found rather than which one - as that is recorded on
the expression itself.

[Fznamznon]

clang/lib/Parse/ParseDecl.cpp

2495 ↗

(On Diff #524491)

What is the point for an additional expression evaluation context here?

clang/lib/Sema/SemaDeclCXX.cpp

2477

clang/lib/Sema/SemaExpr.cpp

17982

CallExpr::getCallee() can return nullptr for example for indirect calls. Probably makes sense to check that it is not nullptr before accessing it.

17988

Same is actually for getCurFunction(). Can be a nullptr if there is no function.

18225–18228

18227

clang/test/SemaCXX/cxx2a-consteval-default-params.cpp

1

Should the new behavior only be enabled for c++23?

clang/test/SemaCXX/cxx2b-consteval-propagate.cpp

2

New line is not needed here, I guess.

9

These examples exactly match the ones provided by P2564R3, should they be in a separate test in CXX directory then?

[cor3ntin]

Address some of Mariya's comments

[cor3ntin]

Remove unecessary evaluation context.

[cor3ntin]

clang/include/clang/AST/Decl.h

2391

Yes - I added a comment

clang/lib/AST/Decl.cpp

3180

I don't think so - "defaulted" usually does not imply explicit, unless explicitly stated. the case would be a member init with an immediately escalating expression

clang/lib/Parse/ParseDecl.cpp

2495 ↗

(On Diff #524491)

Unnecessary, good catch! I think i had to add that in an earlier iteration of the patch which checked for immediate escalation in MarkDeclRefReferenced.

clang/lib/Sema/SemaExpr.cpp

17988

There is an assert above , this function should only be called from a function scope

clang/test/SemaCXX/cxx2a-consteval-default-params.cpp

1

Nah, it was voted as a DR

[Fznamznon]

Out of curiosity - does something like these examples - https://godbolt.org/z/Eqb58Wqoo work as expected?

[cor3ntin]

Improve diagnostic message for immediate calls and add Mariya's examples

[cor3ntin]

In D151094#4364133, @Fznamznon wrote:

Out of curiosity - does something like these examples - https://godbolt.org/z/Eqb58Wqoo work as expected?

Yes! I added those as tests, thanks!

[cor3ntin]

clang/test/SemaCXX/cxx2b-consteval-propagate.cpp

9

I don't have a string preference, should we move the paper examples? the whole file?

[Fznamznon]

clang/test/SemaCXX/cxx2b-consteval-propagate.cpp

9

I meant the paper examples. I don't have a strong preference too, so in case it doesn't matter where the test actually is, please ignore this comment.

[shafik]

clang/lib/Parse/ParseDecl.cpp

27 ↗

(On Diff #524674)

This looks like an unrelated change after removing the change below.

[cor3ntin]

Revert unrelated change

[cor3ntin]

@shafik @Fznamznon @erichkeane gentle ping :)

[erichkeane]

I don't understand our consteval implementation enough to approve this, but I didn't see anything to comment on while going through this.

clang/include/clang/Sema/Sema.h

1071–1075

I'm annoyed that we do ExprEvalContexts.back() as often as we do, and don't just have Sema::curExprEvalContext.

There isn't really anything for you to do here, but I wanted to tell someone of this annoyance, as this is the 3rd time its popped up in a week for me :/

[Fznamznon]

Nothing more from me, but I would wait for someone else's approval.

clang/lib/Sema/SemaExpr.cpp

18227

This one seems to be missed.

[cor3ntin]

Remove missed and useless llvm:: specifier

[aaron.ballman]

Generally looks good, but I did have some questions.

clang/lib/AST/ExprConstant.cpp

2165–2172

We can be fancy and reduce a level of indentation, NFC.

clang/lib/AST/TextNodeDumper.cpp

286–287

We should probably also update JSONNodeDumper.cpp similarly so they're somewhat consistent.

clang/lib/CodeGen/CodeGenModule.cpp

4233–4234

6334–6336

Might as well get fancy here too.

clang/lib/Sema/SemaDeclCXX.cpp

2498–2503

Are these for performance reasons, so we don't traverse into them further? If so, a comment would be helpful explaining that.

clang/lib/Sema/SemaExpr.cpp

17980–17987

Should we be asserting the given expression is either a CallExpr (with a valid callee) or a DeclRefExpr? Otherwise, if called with something else, we'll claim the function found an immediately escalating expression but we won't know *what* caused that.

clang/lib/Sema/SemaTemplateDeduction.cpp

5022

I think I'm a bit surprised to see this assert in a function named CheckIf -- I would assume that we'd return false in this case?

clang/lib/Serialization/ASTWriterStmt.cpp

629

Can you explain why this checks that the expression is not immediate escalating? (What test case exercises this?)

clang/test/CodeGenCXX/cxx20-consteval-crash.cpp

20 ↗

(On Diff #528205)

Why is the cast to void added?

clang/test/SemaCXX/cxx2b-consteval-propagate.cpp

9

Because it's voted in as a DR, we should have a test in clang/test/CXX/drs/ with the appropriate DR number markings (and then regenerate the DR status page as well).

[cor3ntin]

clang/lib/Sema/SemaTemplateDeduction.cpp

5022

I think we could but checking whether a non-escalating function is immediate does not make sense to me

clang/lib/Serialization/ASTWriterStmt.cpp

629

Not entirely. My understanding is that we can only use an abbreviation when it's just a reference to a Decl with no additional non-default property, otherwise it needs to be fully serialized (otherwise the "immediate escalating" bit would be lost and deserIAlizing incorrect

clang/test/CodeGenCXX/cxx20-consteval-crash.cpp

20 ↗

(On Diff #528205)

oups, i wasn't supposed to commit that!

[cor3ntin]

clang/test/SemaCXX/cxx2b-consteval-propagate.cpp

9

I'm not sure core makes dr numbers for papers

[aaron.ballman]

clang/lib/Sema/SemaTemplateDeduction.cpp

5022

Okay, that seems reasonable enough.

clang/lib/Serialization/ASTWriterStmt.cpp

629

Ahhhh, okay, that makes sense to me. I was thrown off because it seemed like this was related to ODR captures in lambdas somehow, but I see what you mean now that I look at how other abbreviations are handled. Thanks!

clang/test/SemaCXX/cxx2b-consteval-propagate.cpp

9

Yeah, I think you're right... I can't find a DR number for this either. @Endill -- something to be aware of for DR conformance testing, I guess.

[Endill]

clang/test/SemaCXX/cxx2b-consteval-propagate.cpp

9

Thank you for letting me know!

[cor3ntin]

Address Aaron's feedback

[cor3ntin]

Track immediate escalating construct expressions

[cor3ntin]

clang/lib/Sema/SemaExpr.cpp

17980–17987

We should. Doing that, I realized the case of CXXConstructExpr was not handled so now both DeclRef and CXXConstructExpr track whether they are immediate escalating (which is only useful for diagnostics but it's a pretty useful diagnostic)

[aaron.ballman]

LGTM!

clang/docs/ReleaseNotes.rst

- Implemented `P2036R3: Change scope of lambda trailing-return-type <https://wg21.link/P2036R3>`_
   expressions and how their types are deduced therein, in all C++ language versions.
 - Implemented partial support for `P2448R2: Relaxing some constexpr restrictions <https://wg21.link/p2448r2>`_
   Explicitly defaulted functions no longer have to be constexpr-compatible but merely constexpr suitable.
   We do not support outside of defaulted special memeber functions the change that constexpr functions no
   longer have to be constexpr compatible but rather support a less restricted requirements for constexpr
   functions. Which include allowing non-literal types as return values and parameters, allow calling of
   non-constexpr functions and constructors.
 - Clang now supports `requires cplusplus23` for module maps.
+- Implemented `P2564R3: consteval needs to propagate up <https://wg21.link/P2564R3>`_.
 
 C++2c Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 - Compiler flags ``-std=c++2c`` and ``-std=gnu++2c`` have been added for experimental C++2c implementation work.
 
 Resolutions to C++ Defect Reports
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Implemented `DR2397 <https://wg21.link/CWG2397>`_ which allows ``auto`` specifier for pointers

clang/include/clang/AST/Decl.h

public:
   }
   bool isConstexprSpecified() const {
     return getConstexprKind() == ConstexprSpecKind::Constexpr;
   }
   bool isConsteval() const {
     return getConstexprKind() == ConstexprSpecKind::Consteval;
   }
 
+  void setBodyContainsImmediateEscalatingExpressions(bool Set) {
+    FunctionDeclBits.BodyContainsImmediateEscalatingExpression = Set;
+  }
+
+  bool BodyContainsImmediateEscalatingExpressions() const {
+    return FunctionDeclBits.BodyContainsImmediateEscalatingExpression;
+  }
+
+  bool isImmediateEscalating() const;
+
+  // The function is a C++ immediate function.
+  // This can be either a consteval function, or an immediate escalating
+  // function containing an immediate escalating expression.
+  bool isImmediateFunction() const;
+
   /// Whether the instantiation of this function is pending.
   /// This bit is set when the decision to instantiate this function is made
   /// and unset if and when the function body is created. That leaves out
   /// cases where instantiation did not happen because the template definition
   /// was not seen in this TU. This bit remains set in those cases, under the
   /// assumption that the instantiation will happen in some other TU.
   bool instantiationIsPending() const {
     return FunctionDeclBits.InstantiationIsPending;

clang/include/clang/AST/DeclBase.h

class FunctionDeclBitfields {
     /// [class.mem.special].
     uint64_t IsIneligibleOrNotSelected : 1;
 
     uint64_t HasImplicitReturnZero : 1;
     uint64_t IsLateTemplateParsed : 1;
 
     /// Kind of contexpr specifier as defined by ConstexprSpecKind.
     uint64_t ConstexprKind : 2;
+    uint64_t BodyContainsImmediateEscalatingExpression : 1;
+
     uint64_t InstantiationIsPending : 1;
 
     /// Indicates if the function uses __try.
     uint64_t UsesSEHTry : 1;
 
     /// Indicates if the function was a definition
     /// but its body was skipped.
     uint64_t HasSkippedBody : 1;
class FunctionDeclBitfields {
     uint64_t UsesFPIntrin : 1;
 
     // Indicates this function is a constrained friend, where the constraint
     // refers to an enclosing template for hte purposes of [temp.friend]p9.
     uint64_t FriendConstraintRefersToEnclosingTemplate : 1;
   };
 
   /// Number of non-inherited bits in FunctionDeclBitfields.
-  enum { NumFunctionDeclBits = 29 };
+  enum { NumFunctionDeclBits = 30 };
 
   /// Stores the bits used by CXXConstructorDecl. If modified
   /// NumCXXConstructorDeclBits and the accessor
   /// methods in CXXConstructorDecl should be updated appropriately.
   class CXXConstructorDeclBitfields {
     friend class CXXConstructorDecl;
     /// For the bits in DeclContextBitfields.
     uint64_t : NumDeclContextBits;
     /// For the bits in FunctionDeclBitfields.
     uint64_t : NumFunctionDeclBits;
 
-    /// 22 bits to fit in the remaining available space.
+    /// 21 bits to fit in the remaining available space.
     /// Note that this makes CXXConstructorDeclBitfields take
     /// exactly 64 bits and thus the width of NumCtorInitializers
     /// will need to be shrunk if some bit is added to NumDeclContextBitfields,
     /// NumFunctionDeclBitfields or CXXConstructorDeclBitfields.
-    uint64_t NumCtorInitializers : 19;
+    uint64_t NumCtorInitializers : 18;
     uint64_t IsInheritingConstructor : 1;
 
     /// Whether this constructor has a trail-allocated explicit specifier.
     uint64_t HasTrailingExplicitSpecifier : 1;
     /// If this constructor does't have a trail-allocated explicit specifier.
     /// Whether this constructor is explicit specified.
     uint64_t IsSimpleExplicit : 1;
   };

clang/include/clang/AST/Expr.h

public:
   }
 
   /// Does this DeclRefExpr refer to an enclosing local or a captured
   /// variable?
   bool refersToEnclosingVariableOrCapture() const {
     return DeclRefExprBits.RefersToEnclosingVariableOrCapture;
   }
 
+  bool isImmediateEscalating() const {
+    return DeclRefExprBits.IsImmediateEscalating;
+  }
+
+  void setIsImmediateEscalating(bool Set) {
+    DeclRefExprBits.IsImmediateEscalating = Set;
+  }
+
   static bool classof(const Stmt *T) {
     return T->getStmtClass() == DeclRefExprClass;
   }
 
   // Iterators
   child_range children() {
     return child_range(child_iterator(), child_iterator());
   }

clang/include/clang/AST/Stmt.h

class DeclRefExprBitfields {
     unsigned : NumExprBits;
 
     unsigned HasQualifier : 1;
     unsigned HasTemplateKWAndArgsInfo : 1;
     unsigned HasFoundDecl : 1;
     unsigned HadMultipleCandidates : 1;
     unsigned RefersToEnclosingVariableOrCapture : 1;
     unsigned NonOdrUseReason : 2;
+    unsigned IsImmediateEscalating : 1;
 
     /// The location of the declaration name itself.
     SourceLocation Loc;
   };
 
 
   class FloatingLiteralBitfields {
     friend class FloatingLiteral;

clang/include/clang/Basic/DiagnosticSemaKinds.td

def warn_cxx98_compat_constexpr : Warning<
   "'constexpr' specifier is incompatible with C++98">,
   InGroup<CXX98Compat>, DefaultIgnore;
 // FIXME: Maybe this should also go in -Wc++14-compat?
 def warn_cxx14_compat_constexpr_not_const : Warning<
   "'constexpr' non-static member function will not be implicitly 'const' "
   "in C++14; add 'const' to avoid a change in behavior">,
   InGroup<DiagGroup<"constexpr-not-const">>;
 def err_invalid_consteval_take_address : Error<
-  "cannot take address of consteval %select{function|call operator of}1 %0 outside"
+  "cannot take address of %select{immediate|consteval}2 "
+  "%select{function|call operator of}1 %0 outside"
   " of an immediate invocation">;
 def err_invalid_consteval_call : Error<
-  "call to consteval function %q0 is not a constant expression">;
+  "call to %select{immediate|consteval}1 function "
+  "%q0 is not a constant expression">;
+
+def err_immediate_function_used_before_definition : Error<
+  "immediate function %0 used before it is defined">;
+
+def note_immediate_function_reason : Note<
+  "%0 is an immediate function because its body "
+  "%select{evaluates the address of %select{an immediate|a consteval}2 function %1|"
+  "contains a call to %select{an immediate|a consteval}2 "
+  "function %1 and that call is not a constant expression}3">;
+
 def note_invalid_consteval_initializer : Note<
   "in the default initalizer of %0">;
 def note_invalid_consteval_initializer_here : Note<
   "initialized here %0">;
 def err_invalid_consteval_decl_kind : Error<
   "%0 cannot be declared consteval">;
 def err_invalid_constexpr : Error<
   "%select{function parameter|typedef}0 "

clang/include/clang/Sema/ScopeInfo.h

public:
   /// True only when this function has not already built, or attempted
   /// to build, the initial and final coroutine suspend points
   bool NeedsCoroutineSuspends : 1;
 
   /// An enumeration represeting the kind of the first coroutine statement
   /// in the function. One of co_return, co_await, or co_yield.
   unsigned char FirstCoroutineStmtKind : 2;
 
+  /// Whether we found an immediate-escalating expression.
+  bool FoundImmediateEscalatingExpression : 1;
+
   /// First coroutine statement in the current function.
   /// (ex co_return, co_await, co_yield)
   SourceLocation FirstCoroutineStmtLoc;
 
   /// First 'return' statement in the current function.
   SourceLocation FirstReturnLoc;
 
   /// First C++ 'try' or ObjC @try statement in the current function.
public:
 
   /// A list of parameters which have the nonnull attribute and are
   /// modified in the function.
   llvm::SmallPtrSet<const ParmVarDecl *, 8> ModifiedNonNullParams;
 
   /// The set of GNU address of label extension "&&label".
   llvm::SmallVector<AddrLabelExpr *, 4> AddrLabels;
 
 public:
   /// Represents a simple identification of a weak object.
   ///
   /// Part of the implementation of -Wrepeated-use-of-weak.
   ///
   /// This is used to determine if two weak accesses refer to the same object.
   /// Here are some examples of how various accesses are "profiled":
   ///
   /// Access Expression |     "Base" Decl     |          "Property" Decl
public:
   FunctionScopeInfo(DiagnosticsEngine &Diag)
       : Kind(SK_Function), HasBranchProtectedScope(false),
         HasBranchIntoScope(false), HasIndirectGoto(false), HasMustTail(false),
         HasDroppedStmt(false), HasOMPDeclareReductionCombiner(false),
         HasFallthroughStmt(false), UsesFPIntrin(false),
         HasPotentialAvailabilityViolations(false), ObjCShouldCallSuper(false),
         ObjCIsDesignatedInit(false), ObjCWarnForNoDesignatedInitChain(false),
         ObjCIsSecondaryInit(false), ObjCWarnForNoInitDelegation(false),
-        NeedsCoroutineSuspends(true), ErrorTrap(Diag) {}
+        NeedsCoroutineSuspends(true), FoundImmediateEscalatingExpression(false),
+        ErrorTrap(Diag) {}
 
   virtual ~FunctionScopeInfo();
 
   /// Determine whether an unrecoverable error has occurred within this
   /// function. Note that this may return false even if the function body is
   /// invalid, because the errors may be suppressed if they're caused by prior
   /// invalid declarations.
   ///

clang/include/clang/Sema/Sema.h

class SynthesizedFunctionScope {
     bool PushedCodeSynthesisContext = false;
 
   public:
     SynthesizedFunctionScope(Sema &S, DeclContext *DC)
         : S(S), SavedContext(S, DC) {
       S.PushFunctionScope();
       S.PushExpressionEvaluationContext(
           Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
-      if (auto *FD = dyn_cast<FunctionDecl>(DC))
+      if (auto *FD = dyn_cast<FunctionDecl>(DC)) {
         FD->setWillHaveBody(true);
-      else
+        S.ExprEvalContexts.back().InImmediateFunctionContext =
+            FD->isImmediateFunction();
+        S.ExprEvalContexts.back().InImmediateEscalatingFunctionContext =
+            S.getLangOpts().CPlusPlus20 && FD->isImmediateEscalating();
+      } else
         assert(isa<ObjCMethodDecl>(DC));
     }
 
     void addContextNote(SourceLocation UseLoc) {
       assert(!PushedCodeSynthesisContext);
 
       Sema::CodeSynthesisContext Ctx;
       Ctx.Kind = Sema::CodeSynthesisContext::DefiningSynthesizedFunction;
enum class ExpressionEvaluationContext {
 
     /// The current context is "potentially evaluated" in C++11 terms,
     /// but the expression is evaluated at compile-time (like the values of
     /// cases in a switch statement).
     ConstantEvaluated,
 
     /// In addition of being constant evaluated, the current expression
     /// occurs in an immediate function context - either a consteval function
-    /// or a consteval if function.
+    /// or a consteval if statement.
     ImmediateFunctionContext,
 
     /// The current expression is potentially evaluated at run time,
     /// which means that code may be generated to evaluate the value of the
     /// expression at run time.
     PotentiallyEvaluated,
 
     /// The current expression is potentially evaluated, but any
struct ExpressionEvaluationContextRecord {
     enum ExpressionKind {
       EK_Decltype, EK_TemplateArgument, EK_Other
     } ExprContext;
 
     // A context can be nested in both a discarded statement context and
     // an immediate function context, so they need to be tracked independently.
     bool InDiscardedStatement;
     bool InImmediateFunctionContext;
+    bool InImmediateEscalatingFunctionContext;
 
     bool IsCurrentlyCheckingDefaultArgumentOrInitializer = false;
 
     // When evaluating immediate functions in the initializer of a default
     // argument or default member initializer, this is the declaration whose
     // default initializer is being evaluated and the location of the call
     // or constructor definition.
     struct InitializationContext {
struct ExpressionEvaluationContextRecord {
     ExpressionEvaluationContextRecord(ExpressionEvaluationContext Context,
                                       unsigned NumCleanupObjects,
                                       CleanupInfo ParentCleanup,
                                       Decl *ManglingContextDecl,
                                       ExpressionKind ExprContext)
         : Context(Context), ParentCleanup(ParentCleanup),
           NumCleanupObjects(NumCleanupObjects), NumTypos(0),
           ManglingContextDecl(ManglingContextDecl), ExprContext(ExprContext),
-          InDiscardedStatement(false), InImmediateFunctionContext(false) {}
+          InDiscardedStatement(false), InImmediateFunctionContext(false),
+          InImmediateEscalatingFunctionContext(false) {}
 
     bool isUnevaluated() const {
       return Context == ExpressionEvaluationContext::Unevaluated ||
              Context == ExpressionEvaluationContext::UnevaluatedAbstract ||
              Context == ExpressionEvaluationContext::UnevaluatedList;
     }
 
     bool isConstantEvaluated() const {
public:
   /// a non-trivial destructor, this will return CXXBindTemporaryExpr. Otherwise
   /// it simply returns the passed in expression.
   ExprResult MaybeBindToTemporary(Expr *E);
 
   /// Wrap the expression in a ConstantExpr if it is a potential immediate
   /// invocation.
   ExprResult CheckForImmediateInvocation(ExprResult E, FunctionDecl *Decl);
 
+  bool CheckImmediateEscalatingFunctionDefinition(
+      FunctionDecl *FD, bool HasImmediateEscalatingExpression);
+
+  void MarkExpressionAsImmediateEscalating(Expr *E);
+
+  void DiagnoseImmediateEscalatingReason(const clang::FunctionDecl *FD);
+
   bool CompleteConstructorCall(CXXConstructorDecl *Constructor,
                                QualType DeclInitType, MultiExprArg ArgsPtr,
                                SourceLocation Loc,
                                SmallVectorImpl<Expr *> &ConvertedArgs,
                                bool AllowExplicit = false,
                                bool IsListInitialization = false);
 
   ParsedType getInheritingConstructorName(CXXScopeSpec &SS,
DeduceAutoType(TypeLoc AutoTypeLoc, Expr *Initializer, QualType &Result,
                  sema::TemplateDeductionInfo &Info,
                  bool DependentDeduction = false,
                  bool IgnoreConstraints = false,
                  TemplateSpecCandidateSet *FailedTSC = nullptr);
   void DiagnoseAutoDeductionFailure(VarDecl *VDecl, Expr *Init);
   bool DeduceReturnType(FunctionDecl *FD, SourceLocation Loc,
                         bool Diagnose = true);
 
+  bool CheckIfFunctionSpecializationIsImmediate(FunctionDecl *FD,
+                                                SourceLocation Loc);
+
   /// Declare implicit deduction guides for a class template if we've
   /// not already done so.
   void DeclareImplicitDeductionGuides(TemplateDecl *Template,
                                       SourceLocation Loc);
 
   QualType DeduceTemplateSpecializationFromInitializer(
       TypeSourceInfo *TInfo, const InitializedEntity &Entity,
       const InitializationKind &Kind, MultiExprArg Init);

clang/lib/AST/ASTImporter.cpp

ExpectedStmt ASTNodeImporter::VisitDeclRefExpr(DeclRefExpr *E) {
   }
 
   auto *ToE = DeclRefExpr::Create(
       Importer.getToContext(), ToQualifierLoc, ToTemplateKeywordLoc, ToDecl,
       E->refersToEnclosingVariableOrCapture(), ToLocation, ToType,
       E->getValueKind(), ToFoundD, ToResInfo, E->isNonOdrUse());
   if (E->hadMultipleCandidates())
     ToE->setHadMultipleCandidates(true);
+  ToE->setIsImmediateEscalating(E->isImmediateEscalating());
   return ToE;
 }
 
 ExpectedStmt ASTNodeImporter::VisitImplicitValueInitExpr(ImplicitValueInitExpr *E) {
   ExpectedType TypeOrErr = import(E->getType());
   if (!TypeOrErr)
     return TypeOrErr.takeError();
 

clang/lib/AST/Decl.cpp

FunctionDecl::FunctionDecl(Kind DK, ASTContext &C, DeclContext *DC,
   FunctionDeclBits.IsTrivialForCall = false;
   FunctionDeclBits.IsDefaulted = false;
   FunctionDeclBits.IsExplicitlyDefaulted = false;
   FunctionDeclBits.HasDefaultedFunctionInfo = false;
   FunctionDeclBits.IsIneligibleOrNotSelected = false;
   FunctionDeclBits.HasImplicitReturnZero = false;
   FunctionDeclBits.IsLateTemplateParsed = false;
   FunctionDeclBits.ConstexprKind = static_cast<uint64_t>(ConstexprKind);
+  FunctionDeclBits.BodyContainsImmediateEscalatingExpression = false;
   FunctionDeclBits.InstantiationIsPending = false;
   FunctionDeclBits.UsesSEHTry = false;
   FunctionDeclBits.UsesFPIntrin = UsesFPIntrin;
   FunctionDeclBits.HasSkippedBody = false;
   FunctionDeclBits.WillHaveBody = false;
   FunctionDeclBits.IsMultiVersion = false;
   FunctionDeclBits.IsCopyDeductionCandidate = false;
   FunctionDeclBits.HasODRHash = false;

 }
 
 template<std::size_t Len>
 static bool isNamed(const NamedDecl *ND, const char (&Str)[Len]) {
   IdentifierInfo *II = ND->getIdentifier();
   return II && II->isStr(Str);
 }
 
+bool FunctionDecl::isImmediateEscalating() const {
+  // C++23 [expr.const]/p17
+  // An immediate-escalating function is
+  //  - the call operator of a lambda that is not declared with the consteval
+  //  specifier,
+  if (isLambdaCallOperator(this) && !isConsteval())
+    return true;
+  // - a defaulted special member function that is not declared with the
+  // consteval specifier,
+  if (isDefaulted() && !isConsteval())
+    return true;
+  // - a function that results from the instantiation of a templated entity
+  // defined with the constexpr specifier.
+  TemplatedKind TK = getTemplatedKind();
+  if (TK != TK_NonTemplate && TK != TK_DependentNonTemplate &&
+      isConstexprSpecified())
+    return true;
+  return false;
+}
+
+bool FunctionDecl::isImmediateFunction() const {
+  // C++23 [expr.const]/p18
+  // An immediate function is a function or constructor that is
+  // - declared with the consteval specifier
+  if (isConsteval())
+    return true;
+  // - an immediate-escalating function F whose function body contains an
+  // immediate-escalating expression
+  if (isImmediateEscalating() && BodyContainsImmediateEscalatingExpressions())
+    return true;
+
+  if (const auto *MD = dyn_cast<CXXMethodDecl>(this);
+      MD && MD->isLambdaStaticInvoker())
+    return MD->getParent()->getLambdaCallOperator()->isImmediateFunction();
+
+  return false;
+}
+
 bool FunctionDecl::isMain() const {
   const TranslationUnitDecl *tunit =
     dyn_cast<TranslationUnitDecl>(getDeclContext()->getRedeclContext());
   return tunit &&
          !tunit->getASTContext().getLangOpts().Freestanding &&
          isNamed(this, "main");
 }
 

clang/lib/AST/DeclPrinter.cpp

if (!Policy.SuppressSpecifiers) {
     }
 
     if (D->isInlineSpecified())  Out << "inline ";
     if (D->isVirtualAsWritten()) Out << "virtual ";
     if (D->isModulePrivate())    Out << "__module_private__ ";
     if (D->isConstexprSpecified() && !D->isExplicitlyDefaulted())
       Out << "constexpr ";
     if (D->isConsteval())        Out << "consteval ";
+    else if (D->isImmediateFunction())
+      Out << "immediate ";
     ExplicitSpecifier ExplicitSpec = ExplicitSpecifier::getFromDecl(D);
     if (ExplicitSpec.isSpecified())
       printExplicitSpecifier(ExplicitSpec, Out, Policy, Indentation, Context);
   }
 
   PrintingPolicy SubPolicy(Policy);
   SubPolicy.SuppressSpecifiers = false;
   std::string Proto;

clang/lib/AST/Expr.cpp

DeclRefExpr::DeclRefExpr(const ASTContext &Ctx, ValueDecl *D,
     : Expr(DeclRefExprClass, T, VK, OK_Ordinary), D(D), DNLoc(LocInfo) {
   DeclRefExprBits.HasQualifier = false;
   DeclRefExprBits.HasTemplateKWAndArgsInfo = false;
   DeclRefExprBits.HasFoundDecl = false;
   DeclRefExprBits.HadMultipleCandidates = false;
   DeclRefExprBits.RefersToEnclosingVariableOrCapture =
       RefersToEnclosingVariableOrCapture;
   DeclRefExprBits.NonOdrUseReason = NOUR;
+  DeclRefExprBits.IsImmediateEscalating = false;
   DeclRefExprBits.Loc = L;
   setDependence(computeDependence(this, Ctx));
 }
 
 DeclRefExpr::DeclRefExpr(const ASTContext &Ctx,
                          NestedNameSpecifierLoc QualifierLoc,
                          SourceLocation TemplateKWLoc, ValueDecl *D,
                          bool RefersToEnclosingVariableOrCapture,
getTrailingObjects<ASTTemplateKWAndArgsInfo>()->initializeFrom(
         TemplateKWLoc, *TemplateArgs, getTrailingObjects<TemplateArgumentLoc>(),
         Deps);
     assert(!(Deps & TemplateArgumentDependence::Dependent) &&
            "built a DeclRefExpr with dependent template args");
   } else if (TemplateKWLoc.isValid()) {
     getTrailingObjects<ASTTemplateKWAndArgsInfo>()->initializeFrom(
         TemplateKWLoc);
   }
+  DeclRefExprBits.IsImmediateEscalating = false;
   DeclRefExprBits.HadMultipleCandidates = 0;
   setDependence(computeDependence(this, Ctx));
 }
 
 DeclRefExpr *DeclRefExpr::Create(const ASTContext &Context,
                                  NestedNameSpecifierLoc QualifierLoc,
                                  SourceLocation TemplateKWLoc, ValueDecl *D,
                                  bool RefersToEnclosingVariableOrCapture,

clang/lib/AST/ExprConstant.cpp

if (isTemplateArgument(Kind)) {
 
     if (InvalidBaseKind != -1) {
       Info.FFDiag(Loc, diag::note_constexpr_invalid_template_arg)
           << IsReferenceType << !Designator.Entries.empty() << InvalidBaseKind
           << Ident;
       return false;
     }
   }
 
-  if (auto *FD = dyn_cast_or_null<FunctionDecl>(BaseVD)) {
-    if (FD->isConsteval()) {
-      Info.FFDiag(Loc, diag::note_consteval_address_accessible)
-          << !Type->isAnyPointerType();
-      Info.Note(FD->getLocation(), diag::note_declared_at);
-      return false;
-    }
-  }
+  if (auto *FD = dyn_cast_or_null<FunctionDecl>(BaseVD);
+      FD && FD->isImmediateFunction()) {
 
   // Check that the object is a global. Note that the fake 'this' object we
   // manufacture when checking potential constant expressions is conservatively
   // assumed to be global here.
   if (!IsGlobalLValue(Base)) {
     if (Info.getLangOpts().CPlusPlus11) {
       Info.FFDiag(Loc, diag::note_constexpr_non_global, 1)
           << IsReferenceType << !Designator.Entries.empty() << !!BaseVD
static bool CheckMemberPointerConstantExpression(EvalInfo &Info,
                                                  SourceLocation Loc,
                                                  QualType Type,
                                                  const APValue &Value,
                                                  ConstantExprKind Kind) {
   const ValueDecl *Member = Value.getMemberPointerDecl();
   const auto *FD = dyn_cast_or_null<CXXMethodDecl>(Member);
   if (!FD)
     return true;
-  if (FD->isConsteval()) {
+  if (FD->isImmediateFunction()) {
     Info.FFDiag(Loc, diag::note_consteval_address_accessible) << /*pointer*/ 0;
     Info.Note(FD->getLocation(), diag::note_declared_at);
     return false;
   }
   return isForManglingOnly(Kind) || FD->isVirtual() ||
          !FD->hasAttr<DLLImportAttr>();
 }
 

clang/lib/AST/JSONNodeDumper.cpp

void JSONNodeDumper::VisitFunctionDecl(const FunctionDecl *FD) {
   if (SC != SC_None)
     JOS.attribute("storageClass", VarDecl::getStorageClassSpecifierString(SC));
   attributeOnlyIfTrue("inline", FD->isInlineSpecified());
   attributeOnlyIfTrue("virtual", FD->isVirtualAsWritten());
   attributeOnlyIfTrue("pure", FD->isPure());
   attributeOnlyIfTrue("explicitlyDeleted", FD->isDeletedAsWritten());
   attributeOnlyIfTrue("constexpr", FD->isConstexpr());
   attributeOnlyIfTrue("variadic", FD->isVariadic());
+  attributeOnlyIfTrue("immediate", FD->isImmediateFunction());
 
   if (FD->isDefaulted())
     JOS.attribute("explicitlyDefaulted",
                   FD->isDeleted() ? "deleted" : "default");
 }
 
 void JSONNodeDumper::VisitEnumDecl(const EnumDecl *ED) {
   VisitNamedDecl(ED);
if (DRE->getDecl() != DRE->getFoundDecl())
     JOS.attribute("foundReferencedDecl",
                   createBareDeclRef(DRE->getFoundDecl()));
   switch (DRE->isNonOdrUse()) {
   case NOUR_None: break;
   case NOUR_Unevaluated: JOS.attribute("nonOdrUseReason", "unevaluated"); break;
   case NOUR_Constant: JOS.attribute("nonOdrUseReason", "constant"); break;
   case NOUR_Discarded: JOS.attribute("nonOdrUseReason", "discarded"); break;
   }
+  attributeOnlyIfTrue("isImmediateEscalating", DRE->isImmediateEscalating());
 }
 
 void JSONNodeDumper::VisitSYCLUniqueStableNameExpr(
     const SYCLUniqueStableNameExpr *E) {
   JOS.attribute("typeSourceInfo",
                 createQualType(E->getTypeSourceInfo()->getType()));
 }
 

clang/lib/AST/TextNodeDumper.cpp

void TextNodeDumper::Visit(const Decl *D) {
 
   if (D->isInvalidDecl())
     OS << " invalid";
   if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D)) {
     if (FD->isConstexprSpecified())
       OS << " constexpr";
     if (FD->isConsteval())
       OS << " consteval";
+    else if (FD->isImmediateFunction())
+      OS << " immediate";
     if (FD->isMultiVersion())
       OS << " multiversion";
   }
 
   if (!isa<FunctionDecl>(*D)) {
     const auto *MD = dyn_cast<ObjCMethodDecl>(D);
     if (!MD || !MD->isThisDeclarationADefinition()) {
       const auto *DC = dyn_cast<DeclContext>(D);
if (Node->getDecl() != Node->getFoundDecl()) {
     OS << ")";
   }
   switch (Node->isNonOdrUse()) {
   case NOUR_None: break;
   case NOUR_Unevaluated: OS << " non_odr_use_unevaluated"; break;
   case NOUR_Constant: OS << " non_odr_use_constant"; break;
   case NOUR_Discarded: OS << " non_odr_use_discarded"; break;
   }
+  if (Node->isImmediateEscalating())
+    OS << " immediate-escalating";
 }
 
 void TextNodeDumper::VisitUnresolvedLookupExpr(
     const UnresolvedLookupExpr *Node) {
   OS << " (";
   if (!Node->requiresADL())
     OS << "no ";
   OS << "ADL) = '" << Node->getName() << '\'';

clang/lib/AST/VTableBuilder.cpp

assert((LHS.first != RHS.first || LHS.second == RHS.second) &&
            "Different thunks should have unique indices!");
     return LHS.first < RHS.first;
   });
 }
 
 VTableLayout::~VTableLayout() { }
 
 bool VTableContextBase::hasVtableSlot(const CXXMethodDecl *MD) {
-  return MD->isVirtual() && !MD->isConsteval();
+  return MD->isVirtual() && !MD->isImmediateFunction();
 }
 
 ItaniumVTableContext::ItaniumVTableContext(
     ASTContext &Context, VTableComponentLayout ComponentLayout)
     : VTableContextBase(/*MS=*/false), ComponentLayout(ComponentLayout) {}
 
 ItaniumVTableContext::~ItaniumVTableContext() {}
 

clang/lib/CodeGen/CodeGenModule.cpp

 /// GetAddrOfFunction - Return the address of the given function.  If Ty is
 /// non-null, then this function will use the specified type if it has to
 /// create it (this occurs when we see a definition of the function).
 llvm::Constant *CodeGenModule::GetAddrOfFunction(GlobalDecl GD,
                                                  llvm::Type *Ty,
                                                  bool ForVTable,
                                                  bool DontDefer,
                                               ForDefinition_t IsForDefinition) {
-  assert(!cast<FunctionDecl>(GD.getDecl())->isConsteval() &&
-         "consteval function should never be emitted");
+  assert(!cast<FunctionDecl>(GD.getDecl())->isImmediateFunction() &&
+         "an immediate function should never be emitted");
   // If there was no specific requested type, just convert it now.
   if (!Ty) {
     const auto *FD = cast<FunctionDecl>(GD.getDecl());
     Ty = getTypes().ConvertType(FD->getType());
   }
 
   // Devirtualized destructor calls may come through here instead of via
   // getAddrOfCXXStructor. Make sure we use the MS ABI base destructor instead

 }
 
 /// EmitTopLevelDecl - Emit code for a single top level declaration.
 void CodeGenModule::EmitTopLevelDecl(Decl *D) {
   // Ignore dependent declarations.
   if (D->isTemplated())
     return;
 
   // Consteval function shouldn't be emitted.
-  if (auto *FD = dyn_cast<FunctionDecl>(D))
-    if (FD->isConsteval())
-      return;
+  if (auto *FD = dyn_cast<FunctionDecl>(D); FD && FD->isImmediateFunction())
 
   switch (D->getKind()) {
   case Decl::CXXConversion:
   case Decl::CXXMethod:
   case Decl::Function:
     EmitGlobal(cast<FunctionDecl>(D));
     // Always provide some coverage mapping
     // even for the functions that aren't emitted.

clang/lib/Frontend/InitPreprocessor.cpp

static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
   if (LangOpts.CPlusPlus20) {
     Builder.defineMacro("__cpp_aggregate_paren_init", "201902L");
 
     // P0848 is implemented, but we're still waiting for other concepts
     // issues to be addressed before bumping __cpp_concepts up to 202002L.
     // Refer to the discussion of this at https://reviews.llvm.org/D128619.
     Builder.defineMacro("__cpp_concepts", "201907L");
     Builder.defineMacro("__cpp_conditional_explicit", "201806L");
-    //Builder.defineMacro("__cpp_consteval", "201811L");
+    // Builder.defineMacro("__cpp_consteval", "202211L");
     Builder.defineMacro("__cpp_constexpr_dynamic_alloc", "201907L");
     Builder.defineMacro("__cpp_constinit", "201907L");
     Builder.defineMacro("__cpp_impl_coroutine", "201902L");
     Builder.defineMacro("__cpp_designated_initializers", "201707L");
     Builder.defineMacro("__cpp_impl_three_way_comparison", "201907L");
     //Builder.defineMacro("__cpp_modules", "201907L");
     Builder.defineMacro("__cpp_using_enum", "201907L");
   }

clang/lib/Parse/ParseDeclCXX.cpp

assert(Tok.isOneOf(tok::equal, tok::l_brace) &&
          "Data member initializer not starting with '=' or '{'");
 
   EnterExpressionEvaluationContext Context(
       Actions,
       isa_and_present<FieldDecl>(D)
           ? Sema::ExpressionEvaluationContext::PotentiallyEvaluatedIfUsed
           : Sema::ExpressionEvaluationContext::PotentiallyEvaluated,
       D);
+  Actions.ExprEvalContexts.back().InImmediateEscalatingFunctionContext = true;
   if (TryConsumeToken(tok::equal, EqualLoc)) {
     if (Tok.is(tok::kw_delete)) {
       // In principle, an initializer of '= delete p;' is legal, but it will
       // never type-check. It's better to diagnose it as an ill-formed
       // expression than as an ill-formed deleted non-function member. An
       // initializer of '= delete p, foo' will never be parsed, because a
       // top-level comma always ends the initializer expression.
       const Token &Next = NextToken();

clang/lib/Sema/ScopeInfo.cpp

void FunctionScopeInfo::Clear() {
   ObjCShouldCallSuper = false;
   ObjCIsDesignatedInit = false;
   ObjCWarnForNoDesignatedInitChain = false;
   ObjCIsSecondaryInit = false;
   ObjCWarnForNoInitDelegation = false;
   FirstReturnLoc = SourceLocation();
   FirstCXXOrObjCTryLoc = SourceLocation();
   FirstSEHTryLoc = SourceLocation();
+  FoundImmediateEscalatingExpression = false;
 
   // Coroutine state
   FirstCoroutineStmtLoc = SourceLocation();
   CoroutinePromise = nullptr;
   CoroutineParameterMoves.clear();
   NeedsCoroutineSuspends = true;
   CoroutineSuspends.first = nullptr;
   CoroutineSuspends.second = nullptr;

clang/lib/Sema/SemaDecl.cpp

Decl *Sema::ActOnStartOfFunctionDef(Scope *FnBodyScope, Decl *D,
   // Each ExpressionEvaluationContextRecord also keeps track of whether the
   // context is nested in an immediate function context, so smaller contexts
   // that appear inside immediate functions (like variable initializers) are
   // considered to be inside an immediate function context even though by
   // themselves they are not immediate function contexts. But when a new
   // function is entered, we need to reset this tracking, since the entered
   // function might be not an immediate function.
   ExprEvalContexts.back().InImmediateFunctionContext = FD->isConsteval();
+  ExprEvalContexts.back().InImmediateEscalatingFunctionContext =
+      getLangOpts().CPlusPlus20 && FD->isImmediateEscalating();
 
   // Check for defining attributes before the check for redefinition.
   if (const auto *Attr = FD->getAttr<AliasAttr>()) {
     Diag(Attr->getLocation(), diag::err_alias_is_definition) << FD << 0;
     FD->dropAttr<AliasAttr>();
     FD->setInvalidDecl();
   }
   if (const auto *Attr = FD->getAttr<IFuncAttr>()) {
Decl *Sema::ActOnFinishFunctionBody(Decl *dcl, Stmt *Body,
   if (getLangOpts().Coroutines && FSI->isCoroutine())
     CheckCompletedCoroutineBody(FD, Body);
 
   {
     // Do not call PopExpressionEvaluationContext() if it is a lambda because
     // one is already popped when finishing the lambda in BuildLambdaExpr().
     // This is meant to pop the context added in ActOnStartOfFunctionDef().
     ExitFunctionBodyRAII ExitRAII(*this, isLambdaCallOperator(FD));
-
     if (FD) {
       FD->setBody(Body);
       FD->setWillHaveBody(false);
+      CheckImmediateEscalatingFunctionDefinition(
+          FD, FSI->FoundImmediateEscalatingExpression);
 
       if (getLangOpts().CPlusPlus14) {
         if (!FD->isInvalidDecl() && Body && !FD->isDependentContext() &&
             FD->getReturnType()->isUndeducedType()) {
           // 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.
           if (!FD->getReturnType()->getAs<AutoType>()) {

clang/lib/Sema/SemaDeclCXX.cpp

for (size_t I = 0, N = Diags.size(); I != N; ++I)
       SemaRef.Diag(Diags[I].first, Diags[I].second);
     // Don't return false here: we allow this for compatibility in
     // system headers.
   }
 
   return true;
 }
 
+bool Sema::CheckImmediateEscalatingFunctionDefinition(
+    FunctionDecl *FD, bool HasImmediateEscalatingExpression) {
+  if (!FD->hasBody() || !getLangOpts().CPlusPlus20 ||
+      !FD->isImmediateEscalating())
+    return true;
+  FD->setBodyContainsImmediateEscalatingExpressions(
+      HasImmediateEscalatingExpression);
+  if (HasImmediateEscalatingExpression) {
+    auto it = UndefinedButUsed.find(FD->getCanonicalDecl());
+    if (it != UndefinedButUsed.end()) {
+      Diag(it->second, diag::err_immediate_function_used_before_definition)
+          << it->first;
+      Diag(FD->getLocation(), diag::note_defined_here) << FD;
+      if (FD->isImmediateFunction() && !FD->isConsteval())
+        DiagnoseImmediateEscalatingReason(FD);
+      return false;
+    }
+  }
+  return true;
+}
+
+void Sema::DiagnoseImmediateEscalatingReason(const FunctionDecl *FD) {
+  assert(FD->isImmediateEscalating() && !FD->isConsteval() &&
+         "expected an immediate function");
+  assert(FD->hasBody() && "expected the function to have a body");
+  struct ImmediateEscalatingExpressionsVisitor
+      : public RecursiveASTVisitor<ImmediateEscalatingExpressionsVisitor> {
+    using Base = RecursiveASTVisitor<ImmediateEscalatingExpressionsVisitor>;
+    Sema &SemaRef;
+    const FunctionDecl *FD;
+    ImmediateEscalatingExpressionsVisitor(Sema &SemaRef, const FunctionDecl *FD)
+        : SemaRef(SemaRef), FD(FD) {}
+
+    bool shouldVisitImplicitCode() const { return true; }
+    bool shouldVisitLambdaBody() const { return false; }
+
+    bool TraverseCallExpr(CallExpr *E) {
+      if (const auto *DR =
+              dyn_cast<DeclRefExpr>(E->getCallee()->IgnoreImplicit());
+          DR && DR->isImmediateEscalating()) {
+        SemaRef.Diag(E->getBeginLoc(), diag::note_immediate_function_reason)
+            << FD << E->getDirectCallee() << E->getDirectCallee()->isConsteval()
+            << 1 << E->getSourceRange();
+      }
+      for (auto A : E->arguments()) {
+        getDerived().TraverseStmt(A);
+      }
+      return true;
+    }
+    bool VisitDeclRefExpr(DeclRefExpr *E) {
+      if (const auto *ReferencedFn = dyn_cast<FunctionDecl>(E->getDecl());
+          ReferencedFn && E->isImmediateEscalating()) {
+        SemaRef.Diag(E->getBeginLoc(), diag::note_immediate_function_reason)
+            << FD << ReferencedFn << ReferencedFn->isConsteval() << 0
+            << E->getSourceRange();
+      }
+      return true;
+    }
+    // These nodes can never contain an immediate escalating expression,
+    // we can skip them to avoid unecessary work.
+    bool TraverseDecl(Decl *D) {
+      if (isa<FunctionDecl, RecordDecl>(D))
+        return true;
+      return Base::TraverseDecl(D);
+    }
+    bool TraverseType(QualType T) { return true; }
+    bool VisitBlockExpr(BlockExpr *T) { return true; }
+
+  } Visitor(*this, FD);
+  Visitor.TraverseStmt(FD->getBody());
+}
+
 /// Get the class that is directly named by the current context. This is the
 /// class for which an unqualified-id in this scope could name a constructor
 /// or destructor.
 ///
 /// If the scope specifier denotes a class, this will be that class.
 /// If the scope specifier is empty, this will be the class whose
 /// member-specification we are currently within. Otherwise, there
 /// is no such class.

clang/lib/Sema/SemaExpr.cpp

assert(!InitWithCleanup->getNumObjects() &&
            "default argument expression has capturing blocks?");
   }
   // C++ [expr.const]p15.1:
   //   An expression or conversion is in an immediate function context if it is
   //   potentially evaluated and [...] its innermost enclosing non-block scope
   //   is a function parameter scope of an immediate function.
   EnterExpressionEvaluationContext EvalContext(
       *this,
-      FD->isConsteval() ? ExpressionEvaluationContext::ImmediateFunctionContext
-                        : ExpressionEvaluationContext::PotentiallyEvaluated,
+      FD->isImmediateFunction()
+          ? ExpressionEvaluationContext::ImmediateFunctionContext
       Param);
   ExprEvalContexts.back().IsCurrentlyCheckingDefaultArgumentOrInitializer =
       SkipImmediateInvocations;
   runWithSufficientStackSpace(CallLoc, [&] {
     MarkDeclarationsReferencedInExpr(Init, /*SkipLocalVariables=*/true);
   });
   return false;
 }
 
 struct ImmediateCallVisitor : public RecursiveASTVisitor<ImmediateCallVisitor> {
+  const ASTContext &Context;
+  ImmediateCallVisitor(const ASTContext &Ctx) : Context(Ctx) {}
+
   bool HasImmediateCalls = false;
+  bool IsImmediateInvocation = false;
 
   bool shouldVisitImplicitCode() const { return true; }
 
   bool VisitCallExpr(CallExpr *E) {
-    if (const FunctionDecl *FD = E->getDirectCallee())
-      HasImmediateCalls |= FD->isConsteval();
+    if (const FunctionDecl *FD = E->getDirectCallee()) {
+      HasImmediateCalls |= FD->isImmediateFunction();
+      if (FD->isConsteval() && !E->isCXX11ConstantExpr(Context))
+        IsImmediateInvocation = true;
+    }
+
     return RecursiveASTVisitor<ImmediateCallVisitor>::VisitStmt(E);
   }
 
   // SourceLocExpr are not immediate invocations
   // but CXXDefaultInitExpr/CXXDefaultArgExpr containing a SourceLocExpr
   // need to be rebuilt so that they refer to the correct SourceLocation and
   // DeclContext.
   bool VisitSourceLocExpr(SourceLocExpr *E) {
if (!Init && !Param->hasUnparsedDefaultArg()) {
     // If we are instantiating a template we won't have to
     // retransform immediate calls.
     // C++ [expr.const]p15.1:
     //   An expression or conversion is in an immediate function context if it
     //   is potentially evaluated and [...] its innermost enclosing non-block
     //   scope is a function parameter scope of an immediate function.
     EnterExpressionEvaluationContext EvalContext(
         *this,
-        FD->isConsteval()
+        FD->isImmediateFunction()
             ? ExpressionEvaluationContext::ImmediateFunctionContext
             : ExpressionEvaluationContext::PotentiallyEvaluated,
         Param);
 
     if (Param->hasUninstantiatedDefaultArg()) {
       if (InstantiateDefaultArgument(CallLoc, FD, Param))
         return ExprError();
     }
     // CWG2631
     // An immediate invocation that is not evaluated where it appears is
     // evaluated and checked for whether it is a constant expression at the
     // point where the enclosing initializer is used in a function call.
-    ImmediateCallVisitor V;
+    ImmediateCallVisitor V(getASTContext());
     if (!NestedDefaultChecking)
       V.TraverseDecl(Param);
     if (V.HasImmediateCalls) {
       ExprEvalContexts.back().DelayedDefaultInitializationContext = {
           CallLoc, Param, CurContext};
       EnsureImmediateInvocationInDefaultArgs Immediate(*this);
       ExprResult Res;
       runWithSufficientStackSpace(CallLoc, [&] {
if (!Field->getInClassInitializer()) {
     }
   }
 
   // CWG2631
   // An immediate invocation that is not evaluated where it appears is
   // evaluated and checked for whether it is a constant expression at the
   // point where the enclosing initializer is used in a [...] a constructor
   // definition, or an aggregate initialization.
-  ImmediateCallVisitor V;
+  ImmediateCallVisitor V(getASTContext());
   if (!NestedDefaultChecking)
     V.TraverseDecl(Field);
   if (V.HasImmediateCalls) {
+    // C++23 [expr.const]/p15
+    // An aggregate initialization is an immediate invocation
+    // if it evaluates a default member initializer that has a subexpression
+    // that is an immediate-escalating expression.
+    ExprEvalContexts.back().InImmediateFunctionContext |=
+        V.IsImmediateInvocation;
+
     ExprEvalContexts.back().DelayedDefaultInitializationContext = {Loc, Field,
                                                                    CurContext};
     ExprEvalContexts.back().IsCurrentlyCheckingDefaultArgumentOrInitializer =
         NestedDefaultChecking;
 
     EnsureImmediateInvocationInDefaultArgs Immediate(*this);
     ExprResult Res;
     runWithSufficientStackSpace(Loc, [&] {
ExprEvalContexts.emplace_back(NewContext, ExprCleanupObjects.size(), Cleanup,
                                 LambdaContextDecl, ExprContext);
 
   // Discarded statements and immediate contexts nested in other
   // discarded statements or immediate context are themselves
   // a discarded statement or an immediate context, respectively.
   ExprEvalContexts.back().InDiscardedStatement =
       ExprEvalContexts[ExprEvalContexts.size() - 2]
           .isDiscardedStatementContext();
+
+  // C++23 [expr.const]/p15
+  // An expression or conversion is in an immediate function context if [...]
+  // it is a subexpression of a manifestly constant-evaluated expression or
+  // conversion.
+  const auto &Prev = ExprEvalContexts[ExprEvalContexts.size() - 2];
   ExprEvalContexts.back().InImmediateFunctionContext =
-      ExprEvalContexts[ExprEvalContexts.size() - 2]
-          .isImmediateFunctionContext();
+      Prev.isImmediateFunctionContext() || Prev.isConstantEvaluated();
+
+  ExprEvalContexts.back().InImmediateEscalatingFunctionContext =
+      Prev.InImmediateEscalatingFunctionContext;
 
   Cleanup.reset();
   if (!MaybeODRUseExprs.empty())
     std::swap(MaybeODRUseExprs, ExprEvalContexts.back().SavedMaybeODRUseExprs);
 }
 
 void
 Sema::PushExpressionEvaluationContext(
void Sema::CheckUnusedVolatileAssignment(Expr *E) {
   if (auto *BO = dyn_cast<BinaryOperator>(E->IgnoreParenImpCasts())) {
     if (BO->getOpcode() == BO_Assign) {
       auto &LHSs = ExprEvalContexts.back().VolatileAssignmentLHSs;
       llvm::erase_value(LHSs, BO->getLHS());
     }
   }
 }
 
+void Sema::MarkExpressionAsImmediateEscalating(Expr *E) {
+  assert(!FunctionScopes.empty() && "Expected a function scope");
+  assert(getLangOpts().CPlusPlus20 &&
+         ExprEvalContexts.back().InImmediateEscalatingFunctionContext &&
+         "Cannot mark an immediate escalating expression outside of an "
+         "immediate escalating context");
+  if (auto *Call = dyn_cast<CallExpr>(E->IgnoreImplicit());
+      Call && Call->getCallee()) {
+    if (auto *DeclRef =
+            dyn_cast<DeclRefExpr>(Call->getCallee()->IgnoreImplicit()))
+      DeclRef->setIsImmediateEscalating(true);
+  } else if (auto *DeclRef = dyn_cast<DeclRefExpr>(E->IgnoreImplicit())) {
+    DeclRef->setIsImmediateEscalating(true);
+  }
+  getCurFunction()->FoundImmediateEscalatingExpression = true;
+}
+
 ExprResult Sema::CheckForImmediateInvocation(ExprResult E, FunctionDecl *Decl) {
   if (isUnevaluatedContext() || !E.isUsable() || !Decl ||
-      !Decl->isConsteval() || isConstantEvaluated() ||
+      !Decl->isImmediateFunction() || isConstantEvaluated() ||
       isCheckingDefaultArgumentOrInitializer() ||
       RebuildingImmediateInvocation || isImmediateFunctionContext())
     return E;
 
   /// Opportunistically remove the callee from ReferencesToConsteval if we can.
   /// It's OK if this fails; we'll also remove this in
   /// HandleImmediateInvocations, but catching it here allows us to avoid
   /// walking the AST looking for it in simple cases.
   if (auto *Call = dyn_cast<CallExpr>(E.get()->IgnoreImplicit()))
     if (auto *DeclRef =
             dyn_cast<DeclRefExpr>(Call->getCallee()->IgnoreImplicit()))
       ExprEvalContexts.back().ReferenceToConsteval.erase(DeclRef);
 
+  // C++23 [expr.const]/p16
+  // An expression or conversion is immediate-escalating if it is not initially
+  // in an immediate function context and it is [...] an immediate invocation
+  // that is not a constant expression and is not a subexpression of an
+  // immediate invocation.
+  APValue Cached;
+  auto CheckConstantExpressionAndKeepResult = [&]() {
+    llvm::SmallVector<PartialDiagnosticAt, 8> Notes;
+    Expr::EvalResult Eval;
+    Eval.Diag = &Notes;
+    bool Res = E.get()->EvaluateAsConstantExpr(
+        Eval, getASTContext(), ConstantExprKind::ImmediateInvocation);
+    if (Res && Notes.empty()) {
+      Cached = std::move(Eval.Val);
+      return true;
+    }
+    return false;
+  };
+
+  if (!E.get()->isValueDependent() &&
+      ExprEvalContexts.back().InImmediateEscalatingFunctionContext &&
+      !CheckConstantExpressionAndKeepResult()) {
+    MarkExpressionAsImmediateEscalating(E.get());
+    return E;
+  }
+
   E = MaybeCreateExprWithCleanups(E);
 
   ConstantExpr *Res = ConstantExpr::Create(
       getASTContext(), E.get(),
       ConstantExpr::getStorageKind(Decl->getReturnType().getTypePtr(),
                                    getASTContext()),
       /*IsImmediateInvocation*/ true);
+  if (Cached.hasValue())
+    Res->MoveIntoResult(Cached, getASTContext());
   /// Value-dependent constant expressions should not be immediately
   /// evaluated until they are instantiated.
   if (!Res->isValueDependent())
     ExprEvalContexts.back().ImmediateInvocationCandidates.emplace_back(Res, 0);
   return Res;
 }
 
 static void EvaluateAndDiagnoseImmediateInvocation(
if (auto *FunctionalCast = dyn_cast<CXXFunctionalCastExpr>(InnerExpr))
       InnerExpr = FunctionalCast->getSubExpr();
     FunctionDecl *FD = nullptr;
     if (auto *Call = dyn_cast<CallExpr>(InnerExpr))
       FD = cast<FunctionDecl>(Call->getCalleeDecl());
     else if (auto *Call = dyn_cast<CXXConstructExpr>(InnerExpr))
       FD = Call->getConstructor();
     else
       llvm_unreachable("unhandled decl kind");
-    assert(FD && FD->isConsteval());
-    SemaRef.Diag(CE->getBeginLoc(), diag::err_invalid_consteval_call) << FD;
+    assert(FD && FD->isImmediateFunction());
+    SemaRef.Diag(CE->getBeginLoc(), diag::err_invalid_consteval_call)
+        << FD << FD->isConsteval();
     if (auto Context =
             SemaRef.InnermostDeclarationWithDelayedImmediateInvocations()) {
       SemaRef.Diag(Context->Loc, diag::note_invalid_consteval_initializer)
           << Context->Decl;
       SemaRef.Diag(Context->Decl->getBeginLoc(), diag::note_declared_at);
     }
+    if (!FD->isConsteval())
+      SemaRef.DiagnoseImmediateEscalatingReason(FD);
     for (auto &Note : Notes)
       SemaRef.Diag(Note.first, Note.second);
     return;
   }
   CE->MoveIntoResult(Eval.Val, SemaRef.getASTContext());
 }
 
 static void RemoveNestedImmediateInvocation(
if (Rec.ImmediateInvocationCandidates.size() > 1 ||
     } Visitor(Rec.ReferenceToConsteval);
     Visitor.TraverseStmt(
         Rec.ImmediateInvocationCandidates.front().getPointer()->getSubExpr());
   }
   for (auto CE : Rec.ImmediateInvocationCandidates)
     if (!CE.getInt())
       EvaluateAndDiagnoseImmediateInvocation(SemaRef, CE);
   for (auto *DR : Rec.ReferenceToConsteval) {
-    NamedDecl *ND = cast<FunctionDecl>(DR->getDecl());
-    if (auto *MD = llvm::dyn_cast<CXXMethodDecl>(ND);
+    const auto *FD = cast<FunctionDecl>(DR->getDecl());
+    const NamedDecl *ND = FD;
+    if (const auto *MD = dyn_cast<CXXMethodDecl>(ND);
         MD && (MD->isLambdaStaticInvoker() || isLambdaCallOperator(MD)))
       ND = MD->getParent();
+
+    // C++23 [expr.const]/p16
+    // An expression or conversion is immediate-escalating if it is not
+    // initially in an immediate function context and it is [...] a
+    // potentially-evaluated id-expression that denotes an immediate function
+    // that is not a subexpression of an immediate invocation.
+    bool ImmediateEscalating = false;
+    bool IsPotentiallyEvaluated =
+        Rec.Context ==
+            Sema::ExpressionEvaluationContext::PotentiallyEvaluated ||
+        Rec.Context ==
+            Sema::ExpressionEvaluationContext::PotentiallyEvaluatedIfUsed;
+    if (SemaRef.inTemplateInstantiation() && IsPotentiallyEvaluated)
+      ImmediateEscalating = Rec.InImmediateEscalatingFunctionContext;
+
+    if (!Rec.InImmediateEscalatingFunctionContext ||
+        (SemaRef.inTemplateInstantiation() && !ImmediateEscalating)) {
       SemaRef.Diag(DR->getBeginLoc(), diag::err_invalid_consteval_take_address)
-        << ND << isa<CXXRecordDecl>(ND);
+          << ND << isa<CXXRecordDecl>(ND) << FD->isConsteval();
       SemaRef.Diag(ND->getLocation(), diag::note_declared_at);
+      if (FD->isImmediateEscalating() && !FD->isConsteval())
+        SemaRef.DiagnoseImmediateEscalatingReason(FD);
+    } else {
+      SemaRef.MarkExpressionAsImmediateEscalating(DR);
+    }
   }
 }
 
 void Sema::PopExpressionEvaluationContext() {
   ExpressionEvaluationContextRecord& Rec = ExprEvalContexts.back();
   unsigned NumTypos = Rec.NumTypos;
 
   if (!Rec.Lambdas.empty()) {
void Sema::MarkDeclRefReferenced(DeclRefExpr *E, const Expr *Base) {
   // C++11 defect. The address of a pure member should not be an ODR use, even
   // if it's a qualified reference.
   bool OdrUse = true;
   if (const CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(E->getDecl()))
     if (Method->isVirtual() &&
         !Method->getDevirtualizedMethod(Base, getLangOpts().AppleKext))
       OdrUse = false;
 
-  if (auto *FD = dyn_cast<FunctionDecl>(E->getDecl()))
+  if (auto *FD = dyn_cast<FunctionDecl>(E->getDecl())) {
     if (!isUnevaluatedContext() && !isConstantEvaluated() &&
         !isImmediateFunctionContext() &&
-        !isCheckingDefaultArgumentOrInitializer() && FD->isConsteval() &&
-        !RebuildingImmediateInvocation && !FD->isDependentContext())
+        !isCheckingDefaultArgumentOrInitializer() &&
+        FD->isImmediateFunction() && !RebuildingImmediateInvocation &&
+        !FD->isDependentContext())
       ExprEvalContexts.back().ReferenceToConsteval.insert(E);
+  }
   MarkExprReferenced(*this, E->getLocation(), E->getDecl(), E, OdrUse,
                      RefsMinusAssignments);
 }
 
 /// Perform reference-marking and odr-use handling for a MemberExpr.
 void Sema::MarkMemberReferenced(MemberExpr *E) {
   // C++11 [basic.def.odr]p2:
   //   A non-overloaded function whose name appears as a potentially-evaluated

clang/lib/Sema/SemaLambda.cpp

void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
   }
 
   // Enter a new evaluation context to insulate the lambda from any
   // cleanups from the enclosing full-expression.
   PushExpressionEvaluationContext(
       LSI->CallOperator->isConsteval()
           ? ExpressionEvaluationContext::ImmediateFunctionContext
           : ExpressionEvaluationContext::PotentiallyEvaluated);
+  ExprEvalContexts.back().InImmediateFunctionContext =
+      LSI->CallOperator->isConsteval();
+  ExprEvalContexts.back().InImmediateEscalatingFunctionContext =
+      getLangOpts().CPlusPlus20 && LSI->CallOperator->isImmediateEscalating();
 }
 
 void Sema::ActOnLambdaError(SourceLocation StartLoc, Scope *CurScope,
                             bool IsInstantiation) {
   LambdaScopeInfo *LSI = cast<LambdaScopeInfo>(FunctionScopes.back());
 
   // Leave the expression-evaluation context.
   DiscardCleanupsInEvaluationContext();

clang/lib/Sema/SemaStmt.cpp

if (std::get<0>(LHC)) {
                    diag::warn_attributes_likelihood_ifstmt_conflict)
           << ThenAttr << ThenAttr->getRange();
       Diags.Report(ElseAttr->getLocation(), diag::note_conflicting_attribute)
           << ElseAttr << ElseAttr->getRange();
     }
   }
 
   if (ConstevalOrNegatedConsteval) {
-    bool Immediate = isImmediateFunctionContext();
+    bool Immediate = ExprEvalContexts.back().Context ==
+                     ExpressionEvaluationContext::ImmediateFunctionContext;
     if (CurContext->isFunctionOrMethod()) {
       const auto *FD =
           dyn_cast<FunctionDecl>(Decl::castFromDeclContext(CurContext));
-      if (FD && FD->isConsteval())
+      if (FD && FD->isImmediateFunction())
         Immediate = true;
     }
     if (isUnevaluatedContext() || Immediate)
       Diags.Report(IfLoc, diag::warn_consteval_if_always_true) << Immediate;
   }
 
   return BuildIfStmt(IfLoc, StatementKind, LParenLoc, InitStmt, Cond, RParenLoc,
                      thenStmt, ElseLoc, elseStmt);
void Sema::ActOnCapturedRegionStart(SourceLocation Loc, Scope *CurScope,
 
   if (CurScope)
     PushDeclContext(CurScope, CD);
   else
     CurContext = CD;
 
   PushExpressionEvaluationContext(
       ExpressionEvaluationContext::PotentiallyEvaluated);
+  ExprEvalContexts.back().InImmediateEscalatingFunctionContext = false;
 }
 
 void Sema::ActOnCapturedRegionStart(SourceLocation Loc, Scope *CurScope,
                                     CapturedRegionKind Kind,
                                     ArrayRef<CapturedParamNameType> Params,
                                     unsigned OpenMPCaptureLevel) {
   CapturedDecl *CD = nullptr;
   RecordDecl *RD = CreateCapturedStmtRecordDecl(CD, Loc, Params.size());

clang/lib/Sema/SemaTemplateDeduction.cpp

Sema::TemplateDeductionResult Sema::DeduceTemplateArguments(
 
   // If the function has a deduced return type, deduce it now, so we can check
   // that the deduced function type matches the requested type.
   if (HasDeducedReturnType &&
       Specialization->getReturnType()->isUndeducedType() &&
       DeduceReturnType(Specialization, Info.getLocation(), false))
     return TDK_MiscellaneousDeductionFailure;
 
+  if (IsAddressOfFunction && getLangOpts().CPlusPlus20 &&
+      Specialization->isImmediateEscalating() &&
+      CheckIfFunctionSpecializationIsImmediate(Specialization,
+                                               Info.getLocation()))
+    return TDK_MiscellaneousDeductionFailure;
+
   // If the function has a dependent exception specification, resolve it now,
   // so we can check that the exception specification matches.
   auto *SpecializationFPT =
       Specialization->getType()->castAs<FunctionProtoType>();
   if (getLangOpts().CPlusPlus17 &&
       isUnresolvedExceptionSpec(SpecializationFPT->getExceptionSpecType()) &&
       !ResolveExceptionSpec(Info.getLocation(), SpecializationFPT))
     return TDK_MiscellaneousDeductionFailure;
bool Sema::DeduceReturnType(FunctionDecl *FD, SourceLocation Loc,
   if (StillUndeduced && Diagnose && !FD->isInvalidDecl()) {
     Diag(Loc, diag::err_auto_fn_used_before_defined) << FD;
     Diag(FD->getLocation(), diag::note_callee_decl) << FD;
   }
 
   return StillUndeduced;
 }
 
+bool Sema::CheckIfFunctionSpecializationIsImmediate(FunctionDecl *FD,
+                                                    SourceLocation Loc) {
+  assert(FD->isImmediateEscalating());
+
+  if (isLambdaConversionOperator(FD)) {
+    CXXRecordDecl *Lambda = cast<CXXMethodDecl>(FD)->getParent();
+    FunctionDecl *CallOp = Lambda->getLambdaCallOperator();
+
+    // For a generic lambda, instantiate the call operator if needed.
+    if (auto *Args = FD->getTemplateSpecializationArgs()) {
+      CallOp = InstantiateFunctionDeclaration(
+          CallOp->getDescribedFunctionTemplate(), Args, Loc);
+      if (!CallOp || CallOp->isInvalidDecl())
+        return true;
+      runWithSufficientStackSpace(
+          Loc, [&] { InstantiateFunctionDefinition(Loc, CallOp); });
+    }
+    return CallOp->isInvalidDecl();
+  }
+
+  if (FD->getTemplateInstantiationPattern()) {
+    runWithSufficientStackSpace(
+        Loc, [&] { InstantiateFunctionDefinition(Loc, FD); });
+  }
+  return false;
+}
+
 /// If this is a non-static member function,
 static void
 AddImplicitObjectParameterType(ASTContext &Context,
                                CXXMethodDecl *Method,
                                SmallVectorImpl<QualType> &ArgTypes) {
   // C++11 [temp.func.order]p3:
   //   [...] The new parameter is of type "reference to cv A," where cv are
   //   the cv-qualifiers of the function template (if any) and A is

clang/lib/Serialization/ASTReaderStmt.cpp

void ASTStmtReader::VisitDeclRefExpr(DeclRefExpr *E) {
   VisitExpr(E);
 
   E->DeclRefExprBits.HasQualifier = Record.readInt();
   E->DeclRefExprBits.HasFoundDecl = Record.readInt();
   E->DeclRefExprBits.HasTemplateKWAndArgsInfo = Record.readInt();
   E->DeclRefExprBits.HadMultipleCandidates = Record.readInt();
   E->DeclRefExprBits.RefersToEnclosingVariableOrCapture = Record.readInt();
   E->DeclRefExprBits.NonOdrUseReason = Record.readInt();
+  E->DeclRefExprBits.IsImmediateEscalating = Record.readInt();
   unsigned NumTemplateArgs = 0;
   if (E->hasTemplateKWAndArgsInfo())
     NumTemplateArgs = Record.readInt();
 
   if (E->hasQualifier())
     new (E->getTrailingObjects<NestedNameSpecifierLoc>())
         NestedNameSpecifierLoc(Record.readNestedNameSpecifierLoc());
 
while (true) {
     case EXPR_PREDEFINED:
       S = PredefinedExpr::CreateEmpty(
           Context,
           /*HasFunctionName*/ Record[ASTStmtReader::NumExprFields]);
       break;
 
     case EXPR_DECL_REF:
       S = DeclRefExpr::CreateEmpty(
           Context,
           /*HasQualifier=*/Record[ASTStmtReader::NumExprFields],
           /*HasFoundDecl=*/Record[ASTStmtReader::NumExprFields + 1],
           /*HasTemplateKWAndArgsInfo=*/Record[ASTStmtReader::NumExprFields + 2],
-        /*NumTemplateArgs=*/Record[ASTStmtReader::NumExprFields + 2] ?
-          Record[ASTStmtReader::NumExprFields + 6] : 0);
+          /*NumTemplateArgs=*/
+          Record[ASTStmtReader::NumExprFields + 2]
+              ? Record[ASTStmtReader::NumExprFields + 7]
+              : 0);
       break;
 
     case EXPR_INTEGER_LITERAL:
       S = IntegerLiteral::Create(Context, Empty);
       break;
 
     case EXPR_FIXEDPOINT_LITERAL:
       S = FixedPointLiteral::Create(Context, Empty);

clang/lib/Serialization/ASTWriterDecl.cpp

if (D->hasExtInfo()) {
     Record.AddStmt(Info->TrailingRequiresClause);
   }
   // The location information is deferred until the end of the record.
   Record.AddTypeRef(D->getTypeSourceInfo() ? D->getTypeSourceInfo()->getType()
                                            : QualType());
 }
 
 void ASTDeclWriter::VisitFunctionDecl(FunctionDecl *D) {
-  static_assert(DeclContext::NumFunctionDeclBits == 29,
+  static_assert(DeclContext::NumFunctionDeclBits == 30,
                 "You need to update the serializer after you change the "
                 "FunctionDeclBits");
 
   VisitRedeclarable(D);
 
   Record.push_back(D->getTemplatedKind());
   switch (D->getTemplatedKind()) {
   case FunctionDecl::TK_NonTemplate:
if (D->getDeclContext() == D->getLexicalDeclContext() &&
       D->hasWrittenPrototype() &&
       D->getTemplatedKind() == FunctionDecl::TK_NonTemplate)
     AbbrevToUse = Writer.getDeclCXXMethodAbbrev();
 
   Code = serialization::DECL_CXX_METHOD;
 }
 
 void ASTDeclWriter::VisitCXXConstructorDecl(CXXConstructorDecl *D) {
-  static_assert(DeclContext::NumCXXConstructorDeclBits == 22,
+  static_assert(DeclContext::NumCXXConstructorDeclBits == 21,
                 "You need to update the serializer after you change the "
                 "CXXConstructorDeclBits");
 
   Record.push_back(D->getTrailingAllocKind());
   addExplicitSpecifier(D->getExplicitSpecifier(), Record);
   if (auto Inherited = D->getInheritedConstructor()) {
     Record.AddDeclRef(Inherited.getShadowDecl());
     Record.AddDeclRef(Inherited.getConstructor());
void ASTWriter::WriteDeclAbbrevs() {
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 3)); //GetObjectKind
   //DeclRefExpr
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); //HasQualifier
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); //GetDeclFound
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); //ExplicitTemplateArgs
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); //HadMultipleCandidates
   Abv->Add(BitCodeAbbrevOp(0)); // RefersToEnclosingVariableOrCapture
   Abv->Add(BitCodeAbbrevOp(0)); // NonOdrUseReason
+  Abv->Add(BitCodeAbbrevOp(0)); // IsImmediateEscalating
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // DeclRef
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // Location
   DeclRefExprAbbrev = Stream.EmitAbbrev(std::move(Abv));
 
   // Abbreviation for EXPR_INTEGER_LITERAL
   Abv = std::make_shared<BitCodeAbbrev>();
   Abv->Add(BitCodeAbbrevOp(serialization::EXPR_INTEGER_LITERAL));
   //Stmt

clang/lib/Serialization/ASTWriterStmt.cpp

void ASTStmtWriter::VisitDeclRefExpr(DeclRefExpr *E) {
   VisitExpr(E);
 
   Record.push_back(E->hasQualifier());
   Record.push_back(E->getDecl() != E->getFoundDecl());
   Record.push_back(E->hasTemplateKWAndArgsInfo());
   Record.push_back(E->hadMultipleCandidates());
   Record.push_back(E->refersToEnclosingVariableOrCapture());
   Record.push_back(E->isNonOdrUse());
+  Record.push_back(E->isImmediateEscalating());
 
   if (E->hasTemplateKWAndArgsInfo()) {
     unsigned NumTemplateArgs = E->getNumTemplateArgs();
     Record.push_back(NumTemplateArgs);
   }
 
   DeclarationName::NameKind nk = (E->getDecl()->getDeclName().getNameKind());
 
   if ((!E->hasTemplateKWAndArgsInfo()) && (!E->hasQualifier()) &&
       (E->getDecl() == E->getFoundDecl()) &&
       nk == DeclarationName::Identifier &&
-      !E->refersToEnclosingVariableOrCapture() && !E->isNonOdrUse()) {
+      !E->refersToEnclosingVariableOrCapture() && !E->isNonOdrUse() &&
+      !E->isImmediateEscalating()) {
     AbbrevToUse = Writer.getDeclRefExprAbbrev();
   }
 
   if (E->hasQualifier())
     Record.AddNestedNameSpecifierLoc(E->getQualifierLoc());
 
   if (E->getDecl() != E->getFoundDecl())
     Record.AddDeclRef(E->getFoundDecl());

clang/test/SemaCXX/cxx2a-consteval-default-params.cpp

 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++23 %s
 
-consteval int undefined();  // expected-note 4 {{declared here}}
+consteval int undefined();  // expected-note 2 {{declared here}}
 
 void check_lambdas_unused(
-    int a = []
-    {
-        // The body of a lambda is not a subexpression of the lambda
-        // so this is immediately evaluated even if the parameter
-        // is never used.
-        return undefined();  // expected-error {{not a constant expression}} \
-                             // expected-note  {{undefined function 'undefined'}}
-    }(),
-    int b = [](int no_error = undefined()) {
+    int a = [](int no_error = undefined()) {
         return no_error;
     }(0),
-    int c = [](int defaulted = undefined()) {
+    int b = [](int defaulted = undefined()) {
         return defaulted;
     }()
 ) {}
 
 int check_lambdas_used(
     int b = [](int no_error = undefined()) {
         return no_error;
     }(0),
     int c = [](int defaulted = undefined()) { // expected-error {{not a constant expression}} \
                               // expected-note  {{declared here}} \
                               // expected-note  {{undefined function 'undefined'}}
         return defaulted;
     }(),  // expected-note {{in the default initalizer of 'defaulted'}}
     int d = [](int defaulted = sizeof(undefined())) {
         return defaulted;
     }()
 ) {
     return 0;
 }
 
 int test_check_lambdas_used = check_lambdas_used();
 
 struct UnusedInitWithLambda {
     int a = [] {
-        return undefined();  // expected-error {{not a constant expression}} \
-                             // expected-note  {{undefined function 'undefined'}}
+        return undefined(); // never evaluated because immediate escalating
     }();
     // UnusedInitWithLambda is never constructed, so the initializer
     // of b and undefined() are never evaluated.
     int b = [](int no_error = undefined()) {
         return no_error;
     }();
 };
 
-consteval int ub(int n) {
-    return 0/n; // expected-note  {{division}}
+consteval int ub(int n) { // expected-note {{declared here}}
+    return 0/n;
 }
 
 struct InitWithLambda {
-    int b = [](int error = undefined()) { // expected-error {{not a constant expression}} \
-                              // expected-note  {{declared here}} \
-                              // expected-note  {{undefined function 'undefined'}}
+    int b = [](int error = undefined()) { // expected-error {{cannot take address of consteval function 'undefined' outside of an immediate invocation}}
         return error;
-    }(); // expected-note {{in the default initalizer of 'error'}}
-    int c = [](int error = sizeof(undefined()) + ub(0)) { // expected-error {{'ub' is not a constant expression}} \
-                                                          // expected-note  {{declared here}} \
-                                                          // expected-note {{in call to 'ub(0)}}
+    }();
+    int c = [](int error = sizeof(undefined()) + ub(0)) { // expected-error {{cannot take address of consteval function 'ub' outside of an immediate invocation}}
+
         return error;
-    }(); // expected-note {{in the default initalizer of 'error'}}
-} i; // expected-note {{in implicit default constructor}}
+    }();
+} i;
 
 namespace ShouldNotCrash {
     template<typename T>
     struct F {
         template<typename U>
         F(const U&) {}
     };
     struct A {

clang/test/SemaCXX/cxx2a-consteval.cpp

 consteval int f_eval(int i) {
   return f(i);
 }
 
 auto l0 = [](int i) consteval {
   return f(i);
 };
 
-auto l1 = [](int i) constexpr {
-// expected-note@-1 {{declared here}}
+auto l1 = [](int i) constexpr { // expected-error{{cannot take address of immediate call operator}} \
+                                // expected-note {{declared here}}
   int t = f(i);
-// expected-error@-1 {{is not a constant expression}}
-// expected-note@-2 {{function parameter}}
-  return f(0);  
+  return f(0);
 };
 
+int(*test)(int)  = l1;
+
 }
 
 namespace std {
 
 template <typename T> struct remove_reference { using type = T; };
 template <typename T> struct remove_reference<T &> { using type = T; };
 template <typename T> struct remove_reference<T &&> { using type = T; };
 

clang/test/SemaCXX/cxx2b-consteval-propagate.cpp

+// RUN: %clang_cc1 -std=c++2a -emit-llvm-only -Wno-unused-value %s -verify
+// RUN: %clang_cc1 -std=c++2b -emit-llvm-only -Wno-unused-value %s -verify
+
+consteval int id(int i) { return i; }
+constexpr char id(char c) { return c; }
+
+template <typename T>
+constexpr int f(T t) { // expected-note {{declared here}}
+    return t + id(t);  // expected-note 2{{'f<int>' is an immediate function because its body contains a call to a consteval function 'id' and that call is not a constant expression}}
+}
+
+namespace examples {
+
+auto a = &f<char>; // ok, f<char> is not an immediate function
+auto b = &f<int>;  // expected-error {{cannot take address of immediate function 'f<int>' outside of an immediate invocation}}
+
+static_assert(f(3) == 6); // ok
+
+template <typename T>
+constexpr int g(T t) {    // g<int> is not an immediate function
+    return t + id(42);    // because id(42) is already a constant
+}
+
+template <typename T, typename F>
+constexpr bool is_not(T t, F f) {
+    return not f(t);
+}
+
+consteval bool is_even(int i) { return i % 2 == 0; }
+
+static_assert(is_not(5, is_even));
+
+int x = 0; // expected-note {{declared here}}
+
+template <typename T>
+constexpr T h(T t = id(x)) { // expected-note {{read of non-const variable 'x' is not allowed in a constant expression}} \
+                             // expected-note 2{{'hh<int>' is an immediate function because its body contains a call to a consteval function 'id' and that call is not a constant expression}}
+    return t;
+}
+
+template <typename T>
+constexpr T hh() {           // hh<int> is an immediate function
+    [[maybe_unused]] auto x = h<T>();
+    return h<T>();
+}
+
+int i = hh<int>(); // expected-error {{call to immediate function 'examples::hh<int>' is not a constant expression}} \
+                   // expected-note {{in call to 'hh()'}}
+
+struct A {
+  int x;
+  int y = id(x);
+};
+
+template <typename T>
+constexpr int k(int) {
+  return A(42).y;
+}
+
+}
+
+namespace nested {
+
+template <typename T>
+constexpr int fdupe(T t) {
+    return id(t);
+}
+
+struct a {
+  constexpr a(int) { }
+};
+
+a aa(fdupe<int>((f<int>(7))));
+
+template <typename T>
+constexpr int foo(T t);     // expected-note {{declared here}}
+
+a bb(f<int>(foo<int>(7))); // expected-error{{call to immediate function 'f<int>' is not a constant expression}} \
+                           // expected-note{{undefined function 'foo<int>' cannot be used in a constant expression}}
+
+}
+
+namespace e2{
+template <typename T>
+constexpr int f(T t);
+auto a = &f<char>;
+auto b = &f<int>;
+}
+
+namespace forward_declare_constexpr{
+template <typename T>
+constexpr int f(T t);
+
+auto a = &f<char>;
+auto b = &f<int>;
+
+template <typename T>
+constexpr int f(T t) {
+    return id(0);
+}
+}
+
+namespace forward_declare_consteval{
+template <typename T>
+constexpr int f(T t);  // expected-note {{'f<int>' defined here}}
+
+auto a = &f<char>;
+auto b = &f<int>; // expected-error {{immediate function 'f<int>' used before it is defined}} \
+                  // expected-note {{in instantiation of function template specialization}}
+
+template <typename T>
+constexpr int f(T t) {
+    return id(t); // expected-note {{'f<int>' is an immediate function because its body contains a call to a consteval function 'id' and that call is not a constant expression}}
+}
+}
+
+namespace constructors {
+consteval int f(int) {
+  return 0;
+}
+struct S {
+  constexpr S(auto i) {
+    f(i);
+  }
+};
+constexpr void g(auto i) {
+  [[maybe_unused]] S s{i};
+}
+void test() {
+  g(0);
+}
+}
+
+namespace aggregate {
+consteval int f(int);
+struct S{
+    int a = 0;
+    int b = f(a);
+};
+
+constexpr bool test(auto i) {
+    S s{i};
+    return s.b == 2 *i;
+}
+consteval int f(int i) {
+    return 2 * i;
+}
+
+void test() {
+    static_assert(test(42));
+}
+
+}

clang/www/cxx_status.html

</tr>
     <tr>
       <td>Permitting static constexpr variables in constexpr functions</td>
       <td><a href="https://wg21.link/P2647R1">P2647R1</a></td>
       <td class="full" align="center">Clang 16</td>
     </tr>
     <tr>
       <td>consteval needs to propagate up</td>
       <td><a href="https://wg21.link/P2564R3">P2564R3</a> (<a href="#dr">DR</a>)</td>
-      <td class="none" align="center">No</td>
+      <td class="unreleased" align="center">Clang 17</td>
     </tr>
     <tr>
       <td>Lifetime extension in range-based for loops</td>
       <td><a href="https://wg21.link/P2718R0">P2718R0</a></td>
       <td class="none" align="center">No</td>
     </tr>
     <!--Issaquah 2023 papers-->
     <tr>