[C++20] add Basic consteval specifier

[Tyker]

Summary

this revision adds Lexing, Parsing and Basic Semantic for the consteval specifier as specified by http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1073r3.html

with this patch, the consteval specifier is treated as constexpr but can only be applied to function declaration.

Changes:

• add the consteval keyword.
• add parsing of consteval specifier for normal declarations and lambdas expressions.
• add the whether a declaration is constexpr is now represented by and enum everywhere except for variable because they can't be consteval.
• adapt diagnostic about constexpr to print constexpr or consteval depending on the case.
• add tests for basic semantic.

[martong]

ASTImporter.cpp looks good to me.

[Tyker]

code-gen was adapted to consteval.

i have a question. what C++ code generate a complex value in llvm's representation ?

[efriedma]

"complex" in this context is the C99 _Complex, which is supported in C++ as an extension, using the same syntax as C. You can just declare a variable with type _Complex float. If you need to manipulate the real and imaginary parts of a variable, you can use the gcc extension __real__/__imag__, although I don't think you need to.

[rsmith]

Thanks for working on this!

Comments on the general approach:

• We should only evaluate each immediate invocation once (this will become essential once we start supporting reflection -- and particularly operations that mutate the AST -- inside consteval functions).
• Each time an immediate invocation is formed, you should create a ConstantExpr AST node wrapping that call to reflect that it is a constant.
• You should change ConstantExpr to store an APValue representing the evaluated value of the expression.

Most of the above should be done as a separate preparatory change; we should be able to remove a lot of the existing ad-hoc caching of evaluated values (eg, for enumerators and the initializers of variables) at the same time.

Please also split this into smaller pieces. If you split off a patch to just add the keyword, parsing, and AST representation for the consteval specifier (but treat it identically to constexpr for constant evaluation purposes), that'd be a good first patch for this feature.

clang/include/clang/AST/Decl.h

2102

Please rename this to isConstexprSpecified() (compare this to how we model inline, which has similar behavior: it can be explicit, or can be implied by other properties of the function).

2109

Both the constexpr and consteval specifiers make a function a "constexpr function", so I think this should be called simply isConstexpr; callers that want to distinguish constexpr from consteval can use isConsteval() (or we can add a getConstexprKind() or similar function).

clang/include/clang/AST/DeclCXX.h

2115

Instead of the three bool flags in a row here (which will make call sites hard to read), consider using an enum, perhaps:

enum class ConstexprSpecifierKind { None, Constexpr, Consteval };

clang/include/clang/Basic/DiagnosticSemaKinds.td

2297–2300

It would be useful in these diagnostics to explain why the call is required to be constant evaluated; please also use the standard terminology "constant expression" here. (Eg, "call to consteval function %0 is not a constant expression")

2308–2314

In these diagnostics (including the existing one for tags), we should say "declared constexpr" not "marked constexpr".

clang/include/clang/Basic/TokenKinds.def

391–392

This and char8_t are both adopted C++2a features now (they'e not just proposals any more); please change the comment to just "C++2a keywords".

clang/include/clang/Sema/DeclSpec.h

401

I think it would be preferable to track only one location here (for the constexpr / consteval specifier) and store a constexpr specifier kind instead of Constexpr_specified, like we do for the other kinds of specifier for which we allow only one of a set of keywords.

clang/include/clang/Sema/Sema.h

985–986

An "immediate function context" is also "potentially evaluated"; I think what we want to say here is something like "The current context is "potentially evaluated", but will only ever be constant evaluated; runtime code will never be generated for it. (Eg, the case values of a switch, or the body of a consteval function.)"

4052–4053

Please don't use the "FunctionName - " style in new code.

4054

return -> \return

4055–4056

Drop the "Invalid" from this function name; it doesn't add anything. But I think this is the wrong interface anyway -- I'll get to that later.

clang/lib/AST/ExprConstant.cpp

4474–4475

We should not need to do this; instead, when the evaluator hits a ConstantExpr node, it should use the already-evaluated value of that node.

clang/lib/Parse/ParseDecl.cpp

2480–2487

Do not pass random English words into diagnostics; this makes translation impossible. Use a %select or two different diagnostics instead. (Passing in C++ syntax such as constexpr or consteval is OK, since they don't need to be translated.)

clang/lib/Parse/ParseExprCXX.cpp

1073–1075

It doesn't seem worthwhile to me to support consteval in earlier language modes only for lambdas; if we really want to allow consteval as an extension in earlier language modes, the better approach would be to add a __consteval keyword. But regardless of whether we do that, please remove this.

1303–1305

Please follow the formatting of the adjacent code.

clang/lib/Sema/DeclSpec.cpp

1326–1332

This should be handled by SetConstevalSpec instead; please look for calls to BadSpecifier to see how we handle this for other kinds of specifier.

clang/lib/Sema/SemaChecking.cpp

10471 ↗

(On Diff #199428)

Are the changes to this file really specific to consteval evaluation? They look more general than that; I think this would fix the evaluation of std::is_constant_evaluated() in any constant-evaluated context that we analyzed. Can this be split out of this patch into a separate change?

clang/lib/Sema/SemaDeclCXX.cpp

640–648

This will produce diagnostics such as "non-constexpr declaration follows constexpr declaration" for a consteval declaration following a constexpr declaration, which seems misleading since a consteval declaration does declare a constexpr function. I think the order of these two checks should be reversed, so that that case will instead be diagnosed as "consteval declaration follows non-consteval declaration".

6655

The diagnostic text here is:

error: defaulted definition of <thing> is not constexpr

which might be confusing if the user wrote consteval thing = default;. Maybe change the diagnostic to:

"defaulted definition of <thing> cannot be declared %select{constexpr|consteval}1 because its implicit definition would not be constexpr"

or something like that?

clang/lib/Sema/SemaExpr.cpp

5720–5769

This checking doesn't make sense: we should be checking that the invocation as a whole is a constant expression, not whether the arguments (evaluated in isolation) are constant expressions. This check should be applied to the Expr representing the call, and should wrap it in a ConstantExpr holding the evaluated value if evaluation succeeds.

13417–13435

This isn't correct: you can take the address of a consteval function within an immediate invocation. In general, we need to delay this checking until we've finished processing any potentially-enclosing immediate invocations. Trying to capture all of the places where we might form a reference to a consteval function name without forming an immediate invocation is also fragile: you'll miss some, and as Clang is extended, there'll be more cases introduced that you miss.

Here's how I was intending to handle this:

• Change Sema::MarkFunctionReferenced to take an Expr* instead of a location, and update all of its callers. (We'll need a separate function for marking destructors referenced, because destructor calls typically don't have expressions, but destructors can't be consteval so that's OK.)
• In Sema::MarkFunctionReferenced, if Func is a consteval function and our current context is not a consteval function, add it to a list on the ExpressionEvaluationContext.
• When forming an immediate invocation, walk all of its subexpressions and remove them all from the list on the ExpressionEvaluationContext
• When popping the ExpressionEvaluationContext, diagnose any remaining expressions in its list.

(The first step should be done in a separate patch to keep the diffs smaller and easier to review.)

[Tyker]

previous revision was spitted in multiple part as requested
consteval specific semantic and code-gen have been removed from the patch.
other parts:

• fixing semantic checking in constant context : https://reviews.llvm.org/D62009
• adding storage for APValue in ConstantExpr : https://reviews.llvm.org/D62399

some remarks weren't fixed because the changes are not present in this patch.

[Tyker]

clang/lib/Sema/SemaChecking.cpp

10471 ↗

(On Diff #199428)

this can be splited in and other patch. https://reviews.llvm.org/D62009.

clang/lib/Sema/SemaExpr.cpp

5720–5769

the idea behind this check was "if the function satisfies the rules of constexpr and all it arguments can be constant evaluated. this function call can be constant evaluated". this was a easy way of checking that the call could be evaluated without evaluating it.

this will be added in later patch after https://reviews.llvm.org/D62399.

13417–13435

delayed to later patch.

[Tyker]

@rsmith ping

[rsmith]

Thank you!

clang/include/clang/AST/DeclBase.h

1504

"kind" -> "Kind"
"Contexpr" -> "constexpr"

clang/lib/AST/DeclPrinter.cpp

615

The only way a defaulted function can be consteval is if consteval was literally written on the declaration, so I think we should print out the consteval keyword for all consteval functions regardless of whether they're defaulted.

clang/lib/Parse/ParseDecl.cpp

2490–2492

Should this say which specifier was used? Or do we somehow reject eg. sizeof(consteval int) before we get here?

clang/lib/Parse/ParseExprCXX.cpp

1155–1157

We should produce a -Wc++17-compat diagnostic similar to this for uses of consteval.

clang/lib/Sema/DeclSpec.cpp

1293–1294

For consteval we should produce an "incompatible with C++ standards before C++2a" diagnostic, not an "incompatible with C++98" diagnostic.

clang/lib/Sema/SemaDecl.cpp

4300

Please capitalize this variable name.

6664

Please consider renaming this diagnostic; err_constexpr_no_declarators doesn't describe the problem here. Maybe err_constexpr_wrong_decl_kind or something?

clang/lib/Sema/SemaDeclCXX.cpp

6655

I don't think the change here has really addressed the problem. We now say:

error: defaulted definition of <thing> is not consteval

in this case, which doesn't explain what's wrong, and appears to directly contradict what the user wrote. The problem is that the implicit definition would not be constexpr, so we should say that.

[Tyker]

fixed requested changes except some i commented upon.

clang/lib/Parse/ParseDecl.cpp

2490–2492

sizeof(consteval int) was rejected before this point but the diagnostics was bad. so i improved it.

clang/lib/Parse/ParseExprCXX.cpp

1155–1157

a consteval keyword can only be lexed when we are in C++2a because it is a C++2a keyword so the warning would never fire.

clang/lib/Sema/DeclSpec.cpp

1293–1294

same as previous comment. the consteval keyword cannot be lexed unless we are in c++2a mode so the warning would never fire.

[rsmith]

Only minor comments remain (other than the -Wc++17-compat warning). In the interest of incremental progress, let's leave the -Wc++17-compat warning for a later patch; feel free to commit this after fixing up the other things.

(If you don't have commit access yet, now would be a good time to request it; please read http://llvm.org/docs/DeveloperPolicy.html for details.)

Thanks again!

clang/include/clang/Basic/DiagnosticSemaKinds.td

1598

Should this say consteval rather than virtual?

Actually... looks like this new diagnostic is not used, and could just be removed?

7860

Remove stray "be" here.

clang/lib/AST/ExprConstant.cpp

4561

The Definition && [...] Definition->isInvalidDecl() case was handled a few lines above; this change appears to have no effect.

clang/lib/Parse/ParseExprCXX.cpp

1155–1157

The purpose of this warning is to warn about code parsed in C++2a mode that would not be valid in C++17. Try (eg) enabling -Wc++98-compat and building some C++11 code to see what's supposed to happen.

clang/lib/Sema/DeclSpec.cpp

1293–1294

(See previous reply.)

[Tyker]

fixed requested changes.
also adapted lldb to AST change.
I will commit this when i have access.

[dyung]

Hi, the test cxx2a-consteval.cpp that you added in this commit is failing on the PS4 Windows bot.

http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast/builds/9/steps/test/logs/stdio

FAIL: Clang :: SemaCXX/cxx2a-consteval.cpp (13131 of 50204)
******************** TEST 'Clang :: SemaCXX/cxx2a-consteval.cpp' FAILED ********************
Script:
--
: 'RUN: at line 1';   c:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.obj\bin\clang.exe -cc1 -internal-isystem c:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.obj\lib\clang\9.0.0\include -nostdsysteminc -std=c++2a -fsyntax-only C:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.src\tools\clang\test\SemaCXX\cxx2a-consteval.cpp -verify
--
Exit Code: 1

Command Output (stdout):
--
$ ":" "RUN: at line 1"
$ "c:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.obj\bin\clang.exe" "-cc1" "-internal-isystem" "c:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.obj\lib\clang\9.0.0\include" "-nostdsysteminc" "-std=c++2a" "-fsyntax-only" "C:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.src\tools\clang\test\SemaCXX\cxx2a-consteval.cpp" "-verify"
# command stderr:
error: 'error' diagnostics expected but not seen: 

  File C:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.src\tools\clang\test\SemaCXX\cxx2a-consteval.cpp Line 9 (directive at C:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.src\tools\clang\test\SemaCXX\cxx2a-consteval.cpp:10): cannot combine

  File C:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.src\tools\clang\test\SemaCXX\cxx2a-consteval.cpp Line 30: destructor cannot be marked consteval

  File C:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.src\tools\clang\test\SemaCXX\cxx2a-consteval.cpp Line 33: struct cannot be marked consteval

  File C:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.src\tools\clang\test\SemaCXX\cxx2a-consteval.cpp Line 35: typedef cannot be consteval

  File C:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.src\tools\clang\test\SemaCXX\cxx2a-consteval.cpp Line 40: consteval can only be used in function declarations

  File C:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.src\tools\clang\test\SemaCXX\cxx2a-consteval.cpp Line 42: consteval can only be used in function declarations

error: 'error' diagnostics seen but not expected: 

  File C:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.src\tools\clang\test\SemaCXX\cxx2a-consteval.cpp Line 30: destructor cannot be marked constexpr

  File C:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.src\tools\clang\test\SemaCXX\cxx2a-consteval.cpp Line 33: struct cannot be marked constexpr

  File C:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.src\tools\clang\test\SemaCXX\cxx2a-consteval.cpp Line 35: typedef cannot be constexpr

  File C:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.src\tools\clang\test\SemaCXX\cxx2a-consteval.cpp Line 42: constexpr can only be used in variable and function declarations

error: 'warning' diagnostics seen but not expected: 

  File C:\ps4-buildslave2\llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast\llvm.src\tools\clang\test\SemaCXX\cxx2a-consteval.cpp Line 9: duplicate 'constexpr' declaration specifier

11 errors generated.


error: command failed with exit status: 1

Can you take a look?

[rnk]

cfe/trunk/include/clang/Sema/DeclSpec.h

366 ↗

(On Diff #204723)

Please always use unsigned for bitfields to avoid sign extensions under MSVC. I've fixed this in r363450. This makes the test you added pass on Windows (@dyung's comment).

clang/include/clang/AST/Decl.h

private:
 
   /// State that an ODRHash has been stored.
   void setHasODRHash(bool B = true) { FunctionDeclBits.HasODRHash = B; }
 
 protected:
   FunctionDecl(Kind DK, ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
                const DeclarationNameInfo &NameInfo, QualType T,
                TypeSourceInfo *TInfo, StorageClass S, bool isInlineSpecified,
-               bool isConstexprSpecified);
+               ConstexprSpecKind ConstexprKind);
 
   using redeclarable_base = Redeclarable<FunctionDecl>;
 
   FunctionDecl *getNextRedeclarationImpl() override {
     return getNextRedeclaration();
   }
 
   FunctionDecl *getPreviousDeclImpl() override {
public:
 
   using redeclarable_base::redecls_begin;
   using redeclarable_base::redecls_end;
   using redeclarable_base::redecls;
   using redeclarable_base::getPreviousDecl;
   using redeclarable_base::getMostRecentDecl;
   using redeclarable_base::isFirstDecl;
 
-  static FunctionDecl *Create(ASTContext &C, DeclContext *DC,
-                              SourceLocation StartLoc, SourceLocation NLoc,
-                              DeclarationName N, QualType T,
-                              TypeSourceInfo *TInfo,
-                              StorageClass SC,
-                              bool isInlineSpecified = false,
-                              bool hasWrittenPrototype = true,
-                              bool isConstexprSpecified = false) {
+  static FunctionDecl *
+  Create(ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
+         SourceLocation NLoc, DeclarationName N, QualType T,
+         TypeSourceInfo *TInfo, StorageClass SC, bool isInlineSpecified = false,
+         ConstexprSpecKind ConstexprKind = CSK_unspecified) {
     DeclarationNameInfo NameInfo(N, NLoc);
-    return FunctionDecl::Create(C, DC, StartLoc, NameInfo, T, TInfo,
-                                SC,
+    return FunctionDecl::Create(C, DC, StartLoc, NameInfo, T, TInfo, SC,
                                 isInlineSpecified, hasWrittenPrototype,
-                                isConstexprSpecified);
+                                ConstexprKind);
   }
 
   static FunctionDecl *Create(ASTContext &C, DeclContext *DC,
                               SourceLocation StartLoc,
-                              const DeclarationNameInfo &NameInfo,
-                              QualType T, TypeSourceInfo *TInfo,
-                              StorageClass SC,
-                              bool isInlineSpecified,
-                              bool hasWrittenPrototype,
-                              bool isConstexprSpecified = false);
+                              const DeclarationNameInfo &NameInfo, QualType T,
+                              TypeSourceInfo *TInfo, StorageClass SC,
+                              bool isInlineSpecified, bool hasWrittenPrototype,
+                              ConstexprSpecKind ConstexprKind);
 
   static FunctionDecl *CreateDeserialized(ASTContext &C, unsigned ID);
 
   DeclarationNameInfo getNameInfo() const {
     return DeclarationNameInfo(getDeclName(), getLocation(), DNLoc);
   }
 
   void getNameForDiagnostic(raw_ostream &OS, const PrintingPolicy &Policy,
public:
   }
 
   /// State that this function inherited its prototype from a
   /// previous declaration.
   void setHasInheritedPrototype(bool P = true) {
     FunctionDeclBits.HasInheritedPrototype = P;
   }
 
   /// Whether this is a (C++11) constexpr function or constexpr constructor.
-  bool isConstexpr() const { return FunctionDeclBits.IsConstexpr; }
-  void setConstexpr(bool IC) { FunctionDeclBits.IsConstexpr = IC; }
+  bool isConstexpr() const {
+    return FunctionDeclBits.ConstexprKind != CSK_unspecified;
+  }
+  void setConstexprKind(ConstexprSpecKind CSK) {
+    FunctionDeclBits.ConstexprKind = CSK;
+  }
+  ConstexprSpecKind getConstexprKind() const {
+    return static_cast<ConstexprSpecKind>(FunctionDeclBits.ConstexprKind);
+  }
+  bool isConstexprSpecified() const {
+    return FunctionDeclBits.ConstexprKind == CSK_constexpr;
+  }
+  bool isConsteval() const {
+    return FunctionDeclBits.ConstexprKind == CSK_consteval;
+  }
 
   /// 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 {

clang/include/clang/AST/DeclBase.h

class FunctionDeclBitfields {
     uint64_t IsTrivialForCall : 1;
 
     /// Used by CXXMethodDecl
     uint64_t IsDefaulted : 1;
     /// Used by CXXMethodDecl
     uint64_t IsExplicitlyDefaulted : 1;
     uint64_t HasImplicitReturnZero : 1;
     uint64_t IsLateTemplateParsed : 1;
-    uint64_t IsConstexpr : 1;
+
+    /// Kind of contexpr specifier as defined by ConstexprSpecKind.
+    uint64_t ConstexprKind : 2;
     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 {
     /// deduction candidate' (is used during overload resolution).
     uint64_t IsCopyDeductionCandidate : 1;
 
     /// Store the ODRHash after first calculation.
     uint64_t HasODRHash : 1;
   };
 
   /// Number of non-inherited bits in FunctionDeclBitfields.
-  enum { NumFunctionDeclBits = 24 };
+  enum { NumFunctionDeclBits = 25 };
 
   /// 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;
 
     /// 24 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 : 24;
+    uint64_t NumCtorInitializers : 23;
     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/DeclCXX.h

class CXXDeductionGuideDecl : public FunctionDecl {
   void anchor() override;
 
 private:
   CXXDeductionGuideDecl(ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
                         ExplicitSpecifier ES,
                         const DeclarationNameInfo &NameInfo, QualType T,
                         TypeSourceInfo *TInfo, SourceLocation EndLocation)
       : FunctionDecl(CXXDeductionGuide, C, DC, StartLoc, NameInfo, T, TInfo,
-                     SC_None, false, false),
+                     SC_None, false, CSK_unspecified),
         ExplicitSpec(ES) {
     if (EndLocation.isValid())
       setRangeEnd(EndLocation);
     setIsCopyDeductionCandidate(false);
   }
 
   ExplicitSpecifier ExplicitSpec;
   void setExplicitSpecifier(ExplicitSpecifier ES) { ExplicitSpec = ES; }

 /// In the terminology of the C++ Standard, these are the (static and
 /// non-static) member functions, whether virtual or not.
 class CXXMethodDecl : public FunctionDecl {
   void anchor() override;
 
 protected:
   CXXMethodDecl(Kind DK, ASTContext &C, CXXRecordDecl *RD,
                 SourceLocation StartLoc, const DeclarationNameInfo &NameInfo,
-                QualType T, TypeSourceInfo *TInfo,
-                StorageClass SC, bool isInline,
-                bool isConstexpr, SourceLocation EndLocation)
-    : FunctionDecl(DK, C, RD, StartLoc, NameInfo, T, TInfo,
-                   SC, isInline, isConstexpr) {
+                QualType T, TypeSourceInfo *TInfo, StorageClass SC,
+                bool isInline, ConstexprSpecKind ConstexprKind,
+                SourceLocation EndLocation)
+      : FunctionDecl(DK, C, RD, StartLoc, NameInfo, T, TInfo, SC, isInline,
+                     ConstexprKind) {
     if (EndLocation.isValid())
       setRangeEnd(EndLocation);
   }
 
 public:
   static CXXMethodDecl *Create(ASTContext &C, CXXRecordDecl *RD,
                                SourceLocation StartLoc,
-                               const DeclarationNameInfo &NameInfo,
-                               QualType T, TypeSourceInfo *TInfo,
-                               StorageClass SC,
-                               bool isInline,
-                               bool isConstexpr,
+                               const DeclarationNameInfo &NameInfo, QualType T,
+                               TypeSourceInfo *TInfo, StorageClass SC,
+                               bool isInline, ConstexprSpecKind ConstexprKind,
                                SourceLocation EndLocation);
 
   static CXXMethodDecl *CreateDeserialized(ASTContext &C, unsigned ID);
 
   bool isStatic() const;
   bool isInstance() const { return !isStatic(); }
 
   /// Returns true if the given operator is implicitly static in a record
class CXXConstructorDecl final
   /// \name Support for base and member initializers.
   /// \{
   /// The arguments used to initialize the base or member.
   LazyCXXCtorInitializersPtr CtorInitializers;
 
   CXXConstructorDecl(ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
                      const DeclarationNameInfo &NameInfo, QualType T,
                      TypeSourceInfo *TInfo, ExplicitSpecifier ES, bool isInline,
-                     bool isImplicitlyDeclared, bool isConstexpr,
+                     bool isImplicitlyDeclared, ConstexprSpecKind ConstexprKind,
                      InheritedConstructor Inherited);
 
   void anchor() override;
 
   size_t numTrailingObjects(OverloadToken<InheritedConstructor>) const {
     return CXXConstructorDeclBits.IsInheritingConstructor;
   }
   size_t numTrailingObjects(OverloadToken<ExplicitSpecifier>) const {
public:
   friend TrailingObjects;
 
   static CXXConstructorDecl *CreateDeserialized(ASTContext &C, unsigned ID,
                                                 uint64_t AllocKind);
   static CXXConstructorDecl *
   Create(ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
          const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
          ExplicitSpecifier ES, bool isInline, bool isImplicitlyDeclared,
-         bool isConstexpr,
+         ConstexprSpecKind ConstexprKind,
          InheritedConstructor Inherited = InheritedConstructor());
 
   ExplicitSpecifier getExplicitSpecifier() {
     return getExplicitSpecifierInternal();
   }
   const ExplicitSpecifier getExplicitSpecifier() const {
     return getExplicitSpecifierInternal();
   }
class CXXDestructorDecl : public CXXMethodDecl {
   friend class ASTDeclWriter;
 
   // FIXME: Don't allocate storage for these except in the first declaration
   // of a virtual destructor.
   FunctionDecl *OperatorDelete = nullptr;
   Expr *OperatorDeleteThisArg = nullptr;
 
   CXXDestructorDecl(ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
-                    const DeclarationNameInfo &NameInfo,
-                    QualType T, TypeSourceInfo *TInfo,
-                    bool isInline, bool isImplicitlyDeclared)
+                    const DeclarationNameInfo &NameInfo, QualType T,
+                    TypeSourceInfo *TInfo, bool isInline,
+                    bool isImplicitlyDeclared)
       : CXXMethodDecl(CXXDestructor, C, RD, StartLoc, NameInfo, T, TInfo,
-                    SC_None, isInline, /*isConstexpr=*/false, SourceLocation())
-  {
+                      SC_None, isInline, CSK_unspecified, SourceLocation()) {
     setImplicit(isImplicitlyDeclared);
   }
 
   void anchor() override;
 
 public:
   static CXXDestructorDecl *Create(ASTContext &C, CXXRecordDecl *RD,
                                    SourceLocation StartLoc,

 /// public:
 ///   operator bool();
 /// };
 /// \endcode
 class CXXConversionDecl : public CXXMethodDecl {
   CXXConversionDecl(ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
                     const DeclarationNameInfo &NameInfo, QualType T,
                     TypeSourceInfo *TInfo, bool isInline, ExplicitSpecifier ES,
-                    bool isConstexpr, SourceLocation EndLocation)
+                    ConstexprSpecKind ConstexprKind, SourceLocation EndLocation)
       : CXXMethodDecl(CXXConversion, C, RD, StartLoc, NameInfo, T, TInfo,
-                      SC_None, isInline, isConstexpr, EndLocation),
+                      SC_None, isInline, ConstexprKind, EndLocation),
         ExplicitSpec(ES) {}
   void anchor() override;
 
   ExplicitSpecifier ExplicitSpec;
 
   void setExplicitSpecifier(ExplicitSpecifier ES) { ExplicitSpec = ES; }
 
 public:
   friend class ASTDeclReader;
   friend class ASTDeclWriter;
 
   static CXXConversionDecl *
   Create(ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
          const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
-         bool isInline, ExplicitSpecifier ES, bool isConstexpr,
+         bool isInline, ExplicitSpecifier ES, ConstexprSpecKind ConstexprKind,
          SourceLocation EndLocation);
   static CXXConversionDecl *CreateDeserialized(ASTContext &C, unsigned ID);
 
   ExplicitSpecifier getExplicitSpecifier() {
     return getCanonicalDecl()->ExplicitSpec;
   }
 
   const ExplicitSpecifier getExplicitSpecifier() const {

clang/include/clang/Basic/DiagnosticParseKinds.td

 // Declarations.
 def err_typename_requires_specqual : Error<
   "type name requires a specifier or qualifier">;
 def err_typename_invalid_storageclass : Error<
   "type name does not allow storage class to be specified">;
 def err_typename_invalid_functionspec : Error<
   "type name does not allow function specifier to be specified">;
 def err_typename_invalid_constexpr : Error<
-  "type name does not allow constexpr specifier to be specified">;
+  "type name does not allow %select{constexpr|consteval}0 specifier to be specified">;
 def err_typename_identifiers_only : Error<
   "typename is allowed for identifiers only">;
 
 def err_friend_invalid_in_context : Error<
   "'friend' used outside of class">;
 def err_templated_using_directive_declaration : Error<
   "cannot template a using %select{directive|declaration}0">;
 def err_unexpected_colon_in_nested_name_spec : Error<

 def err_expected_capture : Error<
   "expected variable name or 'this' in lambda capture list">;
 def err_expected_lambda_body : Error<"expected body of lambda expression">;
 def warn_cxx98_compat_lambda : Warning<
   "lambda expressions are incompatible with C++98">,
   InGroup<CXX98Compat>, DefaultIgnore;
 def err_lambda_missing_parens : Error<
   "lambda requires '()' before %select{'mutable'|return type|"
-  "attribute specifier|'constexpr'}0">;
+  "attribute specifier|'constexpr'|'consteval'}0">;
 def err_lambda_decl_specifier_repeated : Error<
-  "%select{'mutable'|'constexpr'}0 cannot appear multiple times in a lambda declarator">;
+  "%select{'mutable'|'constexpr'|'consteval'}0 cannot appear multiple times in a lambda declarator">;
 def err_lambda_capture_misplaced_ellipsis : Error<
   "ellipsis in pack %select{|init-}0capture must appear %select{after|before}0 "
   "the name of the capture">;
 def err_lambda_capture_multiple_ellipses : Error<
   "multiple ellipses in pack capture">;
 // C++17 lambda expressions
 def err_expected_star_this_capture : Error<
   "expected 'this' following '*' in lambda capture list">;

clang/include/clang/Basic/DiagnosticSemaKinds.td

 def err_static_main : Error<"'main' is not allowed to be declared static">;
 def err_inline_main : Error<"'main' is not allowed to be declared inline">;
 def ext_variadic_main : ExtWarn<
   "'main' is not allowed to be declared variadic">, InGroup<Main>;
 def ext_noreturn_main : ExtWarn<
   "'main' is not allowed to be declared _Noreturn">, InGroup<Main>;
 def note_main_remove_noreturn : Note<"remove '_Noreturn'">;
 def err_constexpr_main : Error<
-  "'main' is not allowed to be declared constexpr">;
+  "'main' is not allowed to be declared %select{constexpr|consteval}0">;
 def err_deleted_main : Error<"'main' is not allowed to be deleted">;
 def err_mainlike_template_decl : Error<"%0 cannot be a template">;
 def err_main_returns_nonint : Error<"'main' must return 'int'">;
 def ext_main_returns_nonint : ExtWarn<"return type of 'main' is not 'int'">,
     InGroup<MainReturnType>;
 def note_main_change_return_type : Note<"change return type to 'int'">;
 def err_main_surplus_args : Error<"too many parameters (%0) for 'main': "
     "must be 0, 2, or 3">;

 def ext_mutable_reference : ExtWarn<
   "'mutable' on a reference type is a Microsoft extension">,
   InGroup<MicrosoftMutableReference>;
 def err_mutable_const : Error<"'mutable' and 'const' cannot be mixed">;
 def err_mutable_nonmember : Error<
   "'mutable' can only be applied to member variables">;
 def err_virtual_in_union : Error<
   "unions cannot have virtual functions">;
+def err_consteval_non_function : Error<
+  "'virtual' can only appear on functions and constructors">;
 def err_virtual_non_function : Error<
   "'virtual' can only appear on non-static member functions">;
 def err_virtual_out_of_class : Error<
   "'virtual' can only be specified inside the class definition">;
 def err_virtual_member_function_template : Error<
   "'virtual' cannot be specified on member function templates">;
 def err_static_overrides_virtual : Error<
   "'static' member function %0 overrides a virtual function in a base class">;
def warn_for_range_const_reference_copy : Warning<
   "| is initialized with a value of a different type}1,2 resulting in a copy">,
   InGroup<RangeLoopAnalysis>, DefaultIgnore;
 def note_use_type_or_non_reference : Note<
   "use non-reference type %0 to keep the copy or type %1 to prevent copying">;
 def warn_for_range_variable_always_copy : Warning<
   "loop variable %0 is always a copy because the range of type %1 does not "
   "return a reference">,
   InGroup<RangeLoopAnalysis>, DefaultIgnore;
 def note_use_non_reference_type : Note<"use non-reference type %0">;
 def warn_for_range_copy : Warning<
   "loop variable %0 of type %1 creates a copy from type %2">,
   InGroup<RangeLoopAnalysis>, DefaultIgnore;
 def note_use_reference_type : Note<"use reference type %0 to prevent copying">;
 def err_objc_for_range_init_stmt : Error<
   "initialization statement is not supported when iterating over Objective-C "
   "collection">;
 
 // C++11 constexpr
 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_constexpr : Error<
   "%select{function parameter|typedef|non-static data member}0 "
-  "cannot be constexpr">;
+  "cannot be %select{constexpr|consteval}1">;
 def err_invalid_constexpr_member : Error<"non-static data member cannot be "
   "constexpr%select{; did you intend to make it %select{const|static}0?|}1">;
 def err_constexpr_tag : Error<
-  "%select{class|struct|interface|union|enum}0 cannot be marked constexpr">;
-def err_constexpr_dtor : Error<"destructor cannot be marked constexpr">;
-def err_constexpr_no_declarators : Error<
-  "constexpr can only be used in variable and function declarations">;
+  "%select{class|struct|interface|union|enum}0 "
+  "cannot be marked %select{constexpr|consteval}1">;
+def err_constexpr_dtor : Error<
+  "destructor cannot be marked %select{constexpr|consteval}0">;
+def err_constexpr_wrong_decl_kind : Error<
+  "%select{constexpr|consteval}0 can only be used "
+  "in %select{variable and |}0function declarations">;
 def err_invalid_constexpr_var_decl : Error<
   "constexpr variable declaration must be a definition">;
 def err_constexpr_static_mem_var_requires_init : Error<
   "declaration of constexpr static data member %0 requires an initializer">;
 def err_constexpr_var_non_literal : Error<
   "constexpr variable cannot have non-literal type %0">;
 def err_constexpr_var_requires_const_init : Error<
   "constexpr variable %0 must be initialized by a constant expression">;
 def err_constexpr_redecl_mismatch : Error<
-  "%select{non-constexpr declaration of %0 follows constexpr declaration"
-  "|constexpr declaration of %0 follows non-constexpr declaration}1">;
+  "%select{non-constexpr|constexpr|consteval}1 declaration of %0"
+  " follows %select{non-constexpr|constexpr|consteval}2 declaration">;
 def err_constexpr_virtual : Error<"virtual function cannot be constexpr">;
 def warn_cxx17_compat_constexpr_virtual : Warning<
   "virtual constexpr functions are incompatible with "
   "C++ standards before C++2a">, InGroup<CXXPre2aCompat>, DefaultIgnore;
 def err_constexpr_virtual_base : Error<
   "constexpr %select{member function|constructor}0 not allowed in "
   "%select{struct|interface|class}1 with virtual base "
   "%plural{1:class|:classes}2">;
 def note_non_literal_incomplete : Note<
   "incomplete type %0 is not a literal type">;
 def note_non_literal_virtual_base : Note<"%select{struct|interface|class}0 "
   "with virtual base %plural{1:class|:classes}1 is not a literal type">;
 def note_constexpr_virtual_base_here : Note<"virtual base class declared here">;
 def err_constexpr_non_literal_return : Error<
-  "constexpr function's return type %0 is not a literal type">;
+  "%select{constexpr|consteval}0 function's return type %1 is not a literal type">;
 def err_constexpr_non_literal_param : Error<
-  "constexpr %select{function|constructor}1's %ordinal0 parameter type %2 is "
+  "%select{constexpr|consteval}2 %select{function|constructor}1's %ordinal0 parameter type %3 is "
   "not a literal type">;
 def err_constexpr_body_invalid_stmt : Error<
-  "statement not allowed in constexpr %select{function|constructor}0">;
+  "statement not allowed in %select{constexpr|consteval}1 %select{function|constructor}0">;
 def ext_constexpr_body_invalid_stmt : ExtWarn<
   "use of this statement in a constexpr %select{function|constructor}0 "
   "is a C++14 extension">, InGroup<CXX14>;
 def warn_cxx11_compat_constexpr_body_invalid_stmt : Warning<
   "use of this statement in a constexpr %select{function|constructor}0 "
   "is incompatible with C++ standards before C++14">,
   InGroup<CXXPre14Compat>, DefaultIgnore;
 def ext_constexpr_body_invalid_stmt_cxx2a : ExtWarn<
def ext_constexpr_function_never_constant_expr : ExtWarn<
   "constexpr %select{function|constructor}0 never produces a "
   "constant expression">, InGroup<DiagGroup<"invalid-constexpr">>, DefaultError;
 def err_attr_cond_never_constant_expr : Error<
   "%0 attribute expression never produces a constant expression">;
 def err_diagnose_if_invalid_diagnostic_type : Error<
   "invalid diagnostic type for 'diagnose_if'; use \"error\" or \"warning\" "
   "instead">;
 def err_constexpr_body_no_return : Error<
-  "no return statement in constexpr function">;
+  "no return statement in %select{constexpr|consteval}0 function">;
 def err_constexpr_return_missing_expr : Error<
-  "non-void constexpr function %0 should return a value">;
+  "non-void %select{constexpr|consteval}1 function %0 should return a value">;
 def warn_cxx11_compat_constexpr_body_no_return : Warning<
   "constexpr function with no return statements is incompatible with C++ "
   "standards before C++14">, InGroup<CXXPre14Compat>, DefaultIgnore;
 def ext_constexpr_body_multiple_return : ExtWarn<
   "multiple return statements in constexpr function is a C++14 extension">,
   InGroup<CXX14>;
 def warn_cxx11_compat_constexpr_body_multiple_return : Warning<
   "multiple return statements in constexpr function "
def err_defaulted_special_member_copy_const_param : Error<
   "%select{constructor|assignment operator}0 is const, but a member or base "
   "requires it to be non-const">;
 def err_defaulted_copy_assign_not_ref : Error<
   "the parameter for an explicitly-defaulted copy assignment operator must be an "
   "lvalue reference type">;
 def err_incorrect_defaulted_constexpr : Error<
   "defaulted definition of %sub{select_special_member_kind}0 "
   "is not constexpr">;
+def err_incorrect_defaulted_consteval : Error<
+  "defaulted declaration of %sub{select_special_member_kind}0 "
+  "cannot be consteval because implicit definition is not be constexpr">;
 def warn_defaulted_method_deleted : Warning<
   "explicitly defaulted %sub{select_special_member_kind}0 is implicitly "
   "deleted">, InGroup<DiagGroup<"defaulted-function-deleted">>;
 def err_out_of_line_default_deletes : Error<
   "defaulting this %sub{select_special_member_kind}0 "
   "would delete it after its first declaration">;
 def note_deleted_type_mismatch : Note<
   "function is implicitly deleted because its declared type does not match "

 def err_coroutine_within_handler : Error<
   "'%0' cannot be used in the handler of a try block">;
 def err_coroutine_outside_function : Error<
   "'%0' cannot be used outside a function">;
 def err_coroutine_invalid_func_context : Error<
   "'%1' cannot be used in %select{a constructor|a destructor"
   "|a copy assignment operator|a move assignment operator|the 'main' function"
   "|a constexpr function|a function with a deduced return type"
-  "|a varargs function}0">;
+  "|a varargs function|a consteval function}0">;
 def err_implied_coroutine_type_not_found : Error<
   "%0 type was not found; include <experimental/coroutine> before defining "
   "a coroutine">;
 def err_implicit_coroutine_std_nothrow_type_not_found : Error<
   "std::nothrow was not found; include <new> before defining a coroutine which "
   "uses get_return_object_on_allocation_failure()">;
 def err_malformed_std_nothrow : Error<
   "std::nothrow must be a valid variable declaration">;

 def err_multiversion_diff : Error<
   "multiversioned function declaration has a different %select{calling convention"
   "|return type|constexpr specification|inline specification|storage class|"
   "linkage}0">;
 def err_multiversion_doesnt_support : Error<
   "attribute '%select{target|cpu_specific|cpu_dispatch}0' multiversioned functions do not "
   "yet support %select{function templates|virtual functions|"
   "deduced return types|constructors|destructors|deleted functions|"
-  "defaulted functions|constexpr functions}1">;
+  "defaulted functions|constexpr functions|consteval function}1">;
 def err_multiversion_not_allowed_on_main : Error<
   "'main' cannot be a multiversioned function">;
 def err_multiversion_not_supported : Error<
  "function multiversioning is not supported on the current target">;
 def err_multiversion_types_mixed : Error<
   "multiversioning attributes cannot be combined">;
 def err_cpu_dispatch_mismatch : Error<
  "'cpu_dispatch' function redeclared with different CPUs">;

clang/include/clang/Basic/Specifiers.h

namespace clang {
 
   /// Define the meaning of possible values of the kind in ExplicitSpecifier.
   enum class ExplicitSpecKind : unsigned {
     ResolvedFalse,
     ResolvedTrue,
     Unresolved,
   };
 
+  /// Define the kind of constexpr specifier.
+  enum ConstexprSpecKind {
+    CSK_unspecified,
+    CSK_constexpr,
+    CSK_consteval
+  };
+
   /// Specifies the width of a type, e.g., short, long, or long long.
   enum TypeSpecifierWidth {
     TSW_unspecified,
     TSW_short,
     TSW_long,
     TSW_longlong
   };
 

clang/include/clang/Basic/TokenKinds.def

 COROUTINES_KEYWORD(co_await)
 COROUTINES_KEYWORD(co_return)
 COROUTINES_KEYWORD(co_yield)
 
 // C++ modules TS keywords
 MODULES_KEYWORD(module)
 MODULES_KEYWORD(import)
 
-// C++ char8_t proposal
+// C++20 keywords.
 CXX2A_KEYWORD(char8_t               , CHAR8SUPPORT)
+CXX2A_KEYWORD(consteval             , 0)
 
 // C11 Extension
 KEYWORD(_Float16                    , KEYALL)
 
 // ISO/IEC JTC1 SC22 WG14 N1169 Extension
 KEYWORD(_Accum                      , KEYNOCXX)
 KEYWORD(_Fract                      , KEYNOCXX)
 KEYWORD(_Sat                        , KEYNOCXX)
 
 // GNU Extensions (in impl-reserved namespace)
 KEYWORD(_Decimal32                  , KEYALL)

clang/include/clang/Sema/DeclSpec.h

private:
   unsigned FS_forceinline_specified: 1;
   unsigned FS_virtual_specified : 1;
   unsigned FS_noreturn_specified : 1;
 
   // friend-specifier
   unsigned Friend_specified : 1;
 
   // constexpr-specifier
-  unsigned Constexpr_specified : 1;
+  ConstexprSpecKind ConstexprSpecifier : 2;
 
   union {
     UnionParsedType TypeRep;
     Decl *DeclRep;
     Expr *ExprRep;
   };
 
   /// ExplicitSpecifier - Store information about explicit spicifer.
private:
   /// TSTNameLoc provides source range info for tag types.
   SourceLocation TSTNameLoc;
   SourceRange TypeofParensRange;
   SourceLocation TQ_constLoc, TQ_restrictLoc, TQ_volatileLoc, TQ_atomicLoc,
       TQ_unalignedLoc;
   SourceLocation FS_inlineLoc, FS_virtualLoc, FS_explicitLoc, FS_noreturnLoc;
   SourceLocation FS_explicitCloseParenLoc;
   SourceLocation FS_forceinlineLoc;
   SourceLocation FriendLoc, ModulePrivateLoc, ConstexprLoc;
   SourceLocation TQ_pipeLoc;
 
   WrittenBuiltinSpecs writtenBS;
   void SaveWrittenBuiltinSpecs();
 
   ObjCDeclSpec *ObjCQualifiers;
 
   static bool isTypeRep(TST T) {
DeclSpec(AttributeFactory &attrFactory)
         ThreadStorageClassSpec(TSCS_unspecified),
         SCS_extern_in_linkage_spec(false), TypeSpecWidth(TSW_unspecified),
         TypeSpecComplex(TSC_unspecified), TypeSpecSign(TSS_unspecified),
         TypeSpecType(TST_unspecified), TypeAltiVecVector(false),
         TypeAltiVecPixel(false), TypeAltiVecBool(false), TypeSpecOwned(false),
         TypeSpecPipe(false), TypeSpecSat(false), TypeQualifiers(TQ_unspecified),
         FS_inline_specified(false), FS_forceinline_specified(false),
         FS_virtual_specified(false), FS_noreturn_specified(false),
-        Friend_specified(false), Constexpr_specified(false),
+        Friend_specified(false), ConstexprSpecifier(CSK_unspecified),
         FS_explicit_specifier(), Attrs(attrFactory), writtenBS(),
         ObjCQualifiers(nullptr) {}
 
   // storage-class-specifier
   SCS getStorageClassSpec() const { return (SCS)StorageClassSpec; }
   TSCS getThreadStorageClassSpec() const {
     return (TSCS)ThreadStorageClassSpec;
   }
public:
   static const char *getSpecifierName(DeclSpec::TST T,
                                       const PrintingPolicy &Policy);
   static const char *getSpecifierName(DeclSpec::TQ Q);
   static const char *getSpecifierName(DeclSpec::TSS S);
   static const char *getSpecifierName(DeclSpec::TSC C);
   static const char *getSpecifierName(DeclSpec::TSW W);
   static const char *getSpecifierName(DeclSpec::SCS S);
   static const char *getSpecifierName(DeclSpec::TSCS S);
+  static const char *getSpecifierName(ConstexprSpecKind C);
 
   // type-qualifiers
 
   /// getTypeQualifiers - Return a set of TQs.
   unsigned getTypeQualifiers() const { return TypeQualifiers; }
   SourceLocation getConstSpecLoc() const { return TQ_constLoc; }
   SourceLocation getRestrictSpecLoc() const { return TQ_restrictLoc; }
   SourceLocation getVolatileSpecLoc() const { return TQ_volatileLoc; }
bool setFunctionSpecExplicit(SourceLocation Loc, const char *&PrevSpec,
                                SourceLocation CloseParenLoc);
   bool setFunctionSpecNoreturn(SourceLocation Loc, const char *&PrevSpec,
                                unsigned &DiagID);
 
   bool SetFriendSpec(SourceLocation Loc, const char *&PrevSpec,
                      unsigned &DiagID);
   bool setModulePrivateSpec(SourceLocation Loc, const char *&PrevSpec,
                             unsigned &DiagID);
-  bool SetConstexprSpec(SourceLocation Loc, const char *&PrevSpec,
-                        unsigned &DiagID);
+  bool SetConstexprSpec(ConstexprSpecKind ConstexprKind, SourceLocation Loc,
+                        const char *&PrevSpec, unsigned &DiagID);
 
   bool isFriendSpecified() const { return Friend_specified; }
   SourceLocation getFriendSpecLoc() const { return FriendLoc; }
 
   bool isModulePrivateSpecified() const { return ModulePrivateLoc.isValid(); }
   SourceLocation getModulePrivateSpecLoc() const { return ModulePrivateLoc; }
 
-  bool isConstexprSpecified() const { return Constexpr_specified; }
+  ConstexprSpecKind getConstexprSpecifier() const { return ConstexprSpecifier; }
   SourceLocation getConstexprSpecLoc() const { return ConstexprLoc; }
+  bool hasConstexprSpecifier() const {
+    return ConstexprSpecifier != CSK_unspecified;
+  }
 
   void ClearConstexprSpec() {
-    Constexpr_specified = false;
+    ConstexprSpecifier = CSK_unspecified;
     ConstexprLoc = SourceLocation();
   }
 
   AttributePool &getAttributePool() const {
     return Attrs.getPool();
   }
 
   /// Concatenates two attribute lists.

clang/include/clang/Sema/Sema.h

enum class ExpressionEvaluationContext {
     /// operand that unconditionally permits abstract references to
     /// fields, such as a SIZE operator in MS-style inline assembly.
     UnevaluatedAbstract,
 
     /// 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,
 
     /// 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
     /// declarations referenced inside that expression are only used if
     /// in fact the current expression is used.
     ///
public:
 
   Decl *ActOnExceptionDeclarator(Scope *S, Declarator &D);
 
   StmtResult ActOnCXXCatchBlock(SourceLocation CatchLoc,
                                 Decl *ExDecl, Stmt *HandlerBlock);
   StmtResult ActOnCXXTryBlock(SourceLocation TryLoc, Stmt *TryBlock,
                               ArrayRef<Stmt *> Handlers);
 
   StmtResult ActOnSEHTryBlock(bool IsCXXTry, // try (true) or __try (false) ?
                               SourceLocation TryLoc, Stmt *TryBlock,
                               Stmt *Handler);
   StmtResult ActOnSEHExceptBlock(SourceLocation Loc,
                                  Expr *FilterExpr,
                                  Stmt *Block);
   void ActOnStartSEHFinallyBlock();
   void ActOnAbortSEHFinallyBlock();
   StmtResult ActOnFinishSEHFinallyBlock(SourceLocation Loc, Stmt *Block);
   StmtResult ActOnSEHLeaveStmt(SourceLocation Loc, Scope *CurScope);
 
   void DiagnoseReturnInConstructorExceptionHandler(CXXTryStmt *TryBlock);
 
CXXRecordDecl *createLambdaClosureType(SourceRange IntroducerRange,
                                          bool KnownDependent,
                                          LambdaCaptureDefault CaptureDefault);
 
   /// Start the definition of a lambda expression.
   CXXMethodDecl *
   startLambdaDefinition(CXXRecordDecl *Class, SourceRange IntroducerRange,
                         TypeSourceInfo *MethodType, SourceLocation EndLoc,
                         ArrayRef<ParmVarDecl *> Params,
-                        bool IsConstexprSpecified,
+                        ConstexprSpecKind ConstexprKind,
                         Optional<std::pair<unsigned, Decl *>> Mangling = None);
 
   /// Endow the lambda scope info with the relevant properties.
   void buildLambdaScope(sema::LambdaScopeInfo *LSI,
                         CXXMethodDecl *CallOperator,
                         SourceRange IntroducerRange,
                         LambdaCaptureDefault CaptureDefault,
                         SourceLocation CaptureDefaultLoc,

clang/lib/AST/ASTImporter.cpp

if (FromConstructor->getExplicitSpecifier().getExpr()) {
       std::tie(ExplicitExpr) = *Imp;
     }
     if (GetImportedOrCreateDecl<CXXConstructorDecl>(
             ToFunction, D, Importer.getToContext(), cast<CXXRecordDecl>(DC),
             ToInnerLocStart, NameInfo, T, TInfo,
             ExplicitSpecifier(
                 ExplicitExpr,
                 FromConstructor->getExplicitSpecifier().getKind()),
-            D->isInlineSpecified(), D->isImplicit(), D->isConstexpr()))
+            D->isInlineSpecified(), D->isImplicit(), D->getConstexprKind()))
       return ToFunction;
   } else if (CXXDestructorDecl *FromDtor = dyn_cast<CXXDestructorDecl>(D)) {
 
     auto Imp =
         importSeq(const_cast<FunctionDecl *>(FromDtor->getOperatorDelete()),
                   FromDtor->getOperatorDeleteThisArg());
 
     if (!Imp)
if (FromConversion->getExplicitSpecifier().getExpr()) {
         return Imp.takeError();
       std::tie(ExplicitExpr) = *Imp;
     }
     if (GetImportedOrCreateDecl<CXXConversionDecl>(
             ToFunction, D, Importer.getToContext(), cast<CXXRecordDecl>(DC),
             ToInnerLocStart, NameInfo, T, TInfo, D->isInlineSpecified(),
             ExplicitSpecifier(ExplicitExpr,
                               FromConversion->getExplicitSpecifier().getKind()),
-            D->isConstexpr(), SourceLocation()))
+            D->getConstexprKind(), SourceLocation()))
       return ToFunction;
   } else if (auto *Method = dyn_cast<CXXMethodDecl>(D)) {
     if (GetImportedOrCreateDecl<CXXMethodDecl>(
             ToFunction, D, Importer.getToContext(), cast<CXXRecordDecl>(DC),
             ToInnerLocStart, NameInfo, T, TInfo, Method->getStorageClass(),
-            Method->isInlineSpecified(), D->isConstexpr(), SourceLocation()))
+            Method->isInlineSpecified(), D->getConstexprKind(),
+            SourceLocation()))
       return ToFunction;
   } else {
-    if (GetImportedOrCreateDecl(ToFunction, D, Importer.getToContext(), DC,
-                                ToInnerLocStart, NameInfo, T, TInfo,
-                                D->getStorageClass(), D->isInlineSpecified(),
-                                D->hasWrittenPrototype(), D->isConstexpr()))
+    if (GetImportedOrCreateDecl(
+            ToFunction, D, Importer.getToContext(), DC, ToInnerLocStart,
+            NameInfo, T, TInfo, D->getStorageClass(), D->isInlineSpecified(),
+            D->hasWrittenPrototype(), D->getConstexprKind()))
       return ToFunction;
   }
 
   // Connect the redecl chain.
   if (FoundByLookup) {
     auto *Recent = const_cast<FunctionDecl *>(
           FoundByLookup->getMostRecentDecl());
     ToFunction->setPreviousDecl(Recent);

clang/lib/AST/Decl.cpp

 //===----------------------------------------------------------------------===//
 // FunctionDecl Implementation
 //===----------------------------------------------------------------------===//
 
 FunctionDecl::FunctionDecl(Kind DK, ASTContext &C, DeclContext *DC,
                            SourceLocation StartLoc,
                            const DeclarationNameInfo &NameInfo, QualType T,
                            TypeSourceInfo *TInfo, StorageClass S,
-                           bool isInlineSpecified, bool isConstexprSpecified)
+                           bool isInlineSpecified,
+                           ConstexprSpecKind ConstexprKind)
     : DeclaratorDecl(DK, DC, NameInfo.getLoc(), NameInfo.getName(), T, TInfo,
                      StartLoc),
       DeclContext(DK), redeclarable_base(C), ODRHash(0),
       EndRangeLoc(NameInfo.getEndLoc()), DNLoc(NameInfo.getInfo()) {
   assert(T.isNull() || T->isFunctionType());
   FunctionDeclBits.SClass = S;
   FunctionDeclBits.IsInline = isInlineSpecified;
   FunctionDeclBits.IsInlineSpecified = isInlineSpecified;
   FunctionDeclBits.IsVirtualAsWritten = false;
   FunctionDeclBits.IsPure = false;
   FunctionDeclBits.HasInheritedPrototype = false;
   FunctionDeclBits.HasWrittenPrototype = true;
   FunctionDeclBits.IsDeleted = false;
   FunctionDeclBits.IsTrivial = false;
   FunctionDeclBits.IsTrivialForCall = false;
   FunctionDeclBits.IsDefaulted = false;
   FunctionDeclBits.IsExplicitlyDefaulted = false;
   FunctionDeclBits.HasImplicitReturnZero = false;
   FunctionDeclBits.IsLateTemplateParsed = false;
-  FunctionDeclBits.IsConstexpr = isConstexprSpecified;
+  FunctionDeclBits.ConstexprKind = ConstexprKind;
   FunctionDeclBits.InstantiationIsPending = false;
   FunctionDeclBits.UsesSEHTry = false;
   FunctionDeclBits.HasSkippedBody = false;
   FunctionDeclBits.WillHaveBody = false;
   FunctionDeclBits.IsMultiVersion = false;
   FunctionDeclBits.IsCopyDeductionCandidate = false;
   FunctionDeclBits.HasODRHash = false;
 }
ImplicitParamDecl *ImplicitParamDecl::CreateDeserialized(ASTContext &C,
                                                          unsigned ID) {
   return new (C, ID) ImplicitParamDecl(C, QualType(), ImplicitParamKind::Other);
 }
 
 FunctionDecl *FunctionDecl::Create(ASTContext &C, DeclContext *DC,
                                    SourceLocation StartLoc,
                                    const DeclarationNameInfo &NameInfo,
                                    QualType T, TypeSourceInfo *TInfo,
-                                   StorageClass SC,
-                                   bool isInlineSpecified,
+                                   StorageClass SC, bool isInlineSpecified,
                                    bool hasWrittenPrototype,
-                                   bool isConstexprSpecified) {
+                                   ConstexprSpecKind ConstexprKind) {
   FunctionDecl *New =
       new (C, DC) FunctionDecl(Function, C, DC, StartLoc, NameInfo, T, TInfo,
-                               SC, isInlineSpecified, isConstexprSpecified);
+                               SC, isInlineSpecified, ConstexprKind);
   New->setHasWrittenPrototype(hasWrittenPrototype);
   return New;
 }
 
 FunctionDecl *FunctionDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
   return new (C, ID) FunctionDecl(Function, C, nullptr, SourceLocation(),
                                   DeclarationNameInfo(), QualType(), nullptr,
-                                  SC_None, false, false);
+                                  SC_None, false, CSK_unspecified);
 }
 
 BlockDecl *BlockDecl::Create(ASTContext &C, DeclContext *DC, SourceLocation L) {
   return new (C, DC) BlockDecl(DC, L);
 }
 
 BlockDecl *BlockDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
   return new (C, ID) BlockDecl(nullptr, SourceLocation());

clang/lib/AST/DeclCXX.cpp

void CXXRecordDecl::addedMember(Decl *D) {
   // Handle constructors, including those inherited from base classes.
   if (const auto *Constructor = dyn_cast<CXXConstructorDecl>(DUnderlying)) {
     // Record if we see any constexpr constructors which are neither copy
     // nor move constructors.
     // C++1z [basic.types]p10:
     //   [...] has at least one constexpr constructor or constructor template
     //   (possibly inherited from a base class) that is not a copy or move
     //   constructor [...]
-    if (Constructor->isConstexpr() && !Constructor->isCopyOrMoveConstructor())
+    if ((Constructor->isConstexpr()) && !Constructor->isCopyOrMoveConstructor())
       data().HasConstexprNonCopyMoveConstructor = true;
   }
 
   // Handle destructors.
   if (const auto *DD = dyn_cast<CXXDestructorDecl>(D)) {
     SMKind |= SMF_Destructor;
 
     if (DD->isUserProvided())
for (const auto &I : RD->bases()) {
     CXXMethodDecl *T = this->getCorrespondingMethodInClass(Base);
     if (T)
       return T;
   }
 
   return nullptr;
 }
 
-CXXMethodDecl *
-CXXMethodDecl::Create(ASTContext &C, CXXRecordDecl *RD,
+CXXMethodDecl *CXXMethodDecl::Create(ASTContext &C, CXXRecordDecl *RD,
                                      SourceLocation StartLoc,
                                      const DeclarationNameInfo &NameInfo,
                                      QualType T, TypeSourceInfo *TInfo,
                                      StorageClass SC, bool isInline,
-                      bool isConstexpr, SourceLocation EndLocation) {
-  return new (C, RD) CXXMethodDecl(CXXMethod, C, RD, StartLoc, NameInfo,
-                                   T, TInfo, SC, isInline, isConstexpr,
-                                   EndLocation);
+                                     ConstexprSpecKind ConstexprKind,
+                                     SourceLocation EndLocation) {
+  return new (C, RD)
+      CXXMethodDecl(CXXMethod, C, RD, StartLoc, NameInfo, T, TInfo, SC,
+                    isInline, ConstexprKind, EndLocation);
 }
 
 CXXMethodDecl *CXXMethodDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) CXXMethodDecl(CXXMethod, C, nullptr, SourceLocation(),
-                                   DeclarationNameInfo(), QualType(), nullptr,
-                                   SC_None, false, false, SourceLocation());
+  return new (C, ID) CXXMethodDecl(
+      CXXMethod, C, nullptr, SourceLocation(), DeclarationNameInfo(),
+      QualType(), nullptr, SC_None, false, CSK_unspecified, SourceLocation());
 }
 
 CXXMethodDecl *CXXMethodDecl::getDevirtualizedMethod(const Expr *Base,
                                                      bool IsAppleKext) {
   assert(isVirtual() && "this method is expected to be virtual");
 
   // When building with -fapple-kext, all calls must go through the vtable since
   // the kernel linker can do runtime patching of vtables.
SourceRange CXXCtorInitializer::getSourceRange() const {
 
   return SourceRange(getSourceLocation(), getRParenLoc());
 }
 
 CXXConstructorDecl::CXXConstructorDecl(
     ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
     const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
     ExplicitSpecifier ES, bool isInline, bool isImplicitlyDeclared,
-    bool isConstexpr, InheritedConstructor Inherited)
+    ConstexprSpecKind ConstexprKind, InheritedConstructor Inherited)
     : CXXMethodDecl(CXXConstructor, C, RD, StartLoc, NameInfo, T, TInfo,
-                    SC_None, isInline, isConstexpr, SourceLocation()) {
+                    SC_None, isInline, ConstexprKind, SourceLocation()) {
   setNumCtorInitializers(0);
   setInheritingConstructor(static_cast<bool>(Inherited));
   setImplicit(isImplicitlyDeclared);
   CXXConstructorDeclBits.HasTrailingExplicitSpecifier = ES.getExpr() ? 1 : 0;
   if (Inherited)
     *getTrailingObjects<InheritedConstructor>() = Inherited;
   setExplicitSpecifier(ES);
 }
 
 void CXXConstructorDecl::anchor() {}
 
 CXXConstructorDecl *CXXConstructorDecl::CreateDeserialized(ASTContext &C,
                                                            unsigned ID,
                                                            uint64_t AllocKind) {
   bool hasTraillingExplicit = static_cast<bool>(AllocKind & TAKHasTailExplicit);
   bool isInheritingConstructor =
       static_cast<bool>(AllocKind & TAKInheritsConstructor);
   unsigned Extra =
       additionalSizeToAlloc<InheritedConstructor, ExplicitSpecifier>(
           isInheritingConstructor, hasTraillingExplicit);
-  auto *Result = new (C, ID, Extra) CXXConstructorDecl(
-      C, nullptr, SourceLocation(), DeclarationNameInfo(), QualType(), nullptr,
-      ExplicitSpecifier(), false, false, false, InheritedConstructor());
+  auto *Result = new (C, ID, Extra)
+      CXXConstructorDecl(C, nullptr, SourceLocation(), DeclarationNameInfo(),
+                         QualType(), nullptr, ExplicitSpecifier(), false, false,
+                         CSK_unspecified, InheritedConstructor());
   Result->setInheritingConstructor(isInheritingConstructor);
   Result->CXXConstructorDeclBits.HasTrailingExplicitSpecifier =
       hasTraillingExplicit;
   Result->setExplicitSpecifier(ExplicitSpecifier());
   return Result;
 }
 
 CXXConstructorDecl *CXXConstructorDecl::Create(
     ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
     const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
     ExplicitSpecifier ES, bool isInline, bool isImplicitlyDeclared,
-    bool isConstexpr, InheritedConstructor Inherited) {
+    ConstexprSpecKind ConstexprKind, InheritedConstructor Inherited) {
   assert(NameInfo.getName().getNameKind()
          == DeclarationName::CXXConstructorName &&
          "Name must refer to a constructor");
   unsigned Extra =
       additionalSizeToAlloc<InheritedConstructor, ExplicitSpecifier>(
           Inherited ? 1 : 0, ES.getExpr() ? 1 : 0);
   return new (C, RD, Extra)
       CXXConstructorDecl(C, RD, StartLoc, NameInfo, T, TInfo, ES, isInline,
-                         isImplicitlyDeclared, isConstexpr, Inherited);
+                         isImplicitlyDeclared, ConstexprKind, Inherited);
 }
 
 CXXConstructorDecl::init_const_iterator CXXConstructorDecl::init_begin() const {
   return CtorInitializers.get(getASTContext().getExternalSource());
 }
 
 CXXConstructorDecl *CXXConstructorDecl::getTargetConstructor() const {
   assert(isDelegatingConstructor() && "Not a delegating constructor!");

 }
 
 void CXXConversionDecl::anchor() {}
 
 CXXConversionDecl *
 CXXConversionDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
   return new (C, ID) CXXConversionDecl(
       C, nullptr, SourceLocation(), DeclarationNameInfo(), QualType(), nullptr,
-      false, ExplicitSpecifier(), false, SourceLocation());
+      false, ExplicitSpecifier(), CSK_unspecified, SourceLocation());
 }
 
 CXXConversionDecl *CXXConversionDecl::Create(
     ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
     const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
-    bool isInline, ExplicitSpecifier ES, bool isConstexpr,
+    bool isInline, ExplicitSpecifier ES, ConstexprSpecKind ConstexprKind,
     SourceLocation EndLocation) {
   assert(NameInfo.getName().getNameKind()
          == DeclarationName::CXXConversionFunctionName &&
          "Name must refer to a conversion function");
-  return new (C, RD) CXXConversionDecl(C, RD, StartLoc, NameInfo, T, TInfo,
-                                       isInline, ES, isConstexpr, EndLocation);
+  return new (C, RD)
+      CXXConversionDecl(C, RD, StartLoc, NameInfo, T, TInfo, isInline, ES,
+                        ConstexprKind, EndLocation);
 }
 
 bool CXXConversionDecl::isLambdaToBlockPointerConversion() const {
   return isImplicit() && getParent()->isLambda() &&
          getConversionType()->isBlockPointerType();
 }
 
 LinkageSpecDecl::LinkageSpecDecl(DeclContext *DC, SourceLocation ExternLoc,

clang/lib/AST/DeclPrinter.cpp

if (!Policy.SuppressSpecifiers) {
     case SC_PrivateExtern: Out << "__private_extern__ "; break;
     case SC_Auto: case SC_Register:
       llvm_unreachable("invalid for functions");
     }
 
     if (D->isInlineSpecified())  Out << "inline ";
     if (D->isVirtualAsWritten()) Out << "virtual ";
     if (D->isModulePrivate())    Out << "__module_private__ ";
-    if (D->isConstexpr() && !D->isExplicitlyDefaulted()) Out << "constexpr ";
+    if (D->isConstexprSpecified() && !D->isExplicitlyDefaulted())
+      Out << "constexpr ";
+    if (D->isConsteval())        Out << "consteval ";
     ExplicitSpecifier ExplicitSpec = ExplicitSpecifier::getFromDecl(D);
     if (ExplicitSpec.isSpecified())
       printExplicitSpecifier(ExplicitSpec, Out, Policy, Indentation);
   }
 
   PrintingPolicy SubPolicy(Policy);
   SubPolicy.SuppressSpecifiers = false;
   std::string Proto;

clang/lib/AST/ExprConstant.cpp

while (true) {
       // Increment: ++__begin
       if (!EvaluateIgnoredValue(Info, FS->getInc()))
         return ESR_Failed;
     }
 
     return ESR_Succeeded;
   }
 
   case Stmt::SwitchStmtClass:
     return EvaluateSwitch(Result, Info, cast<SwitchStmt>(S));
 
   case Stmt::ContinueStmtClass:
     return ESR_Continue;
 
   case Stmt::BreakStmtClass:
     return ESR_Break;
 
   case Stmt::LabelStmtClass:
if (!Info.Ctx.getLangOpts().CPlusPlus2a && isa<CXXMethodDecl>(Declaration) &&
     Info.CCEDiag(CallLoc, diag::note_constexpr_virtual_call);
 
   if (Definition && Definition->isInvalidDecl()) {
     Info.FFDiag(CallLoc, diag::note_invalid_subexpr_in_const_expr);
     return false;
   }
 
   // Can we evaluate this function call?
-  if (Definition && Definition->isConstexpr() && Body)
+  if (Definition && Definition->isConstexpr() && !Definition->isInvalidDecl() &&
+      Body)
     return true;
 
   if (Info.getLangOpts().CPlusPlus11) {
     const FunctionDecl *DiagDecl = Definition ? Definition : Declaration;
 
     // If this function is not constexpr because it is an inherited
     // non-constexpr constructor, diagnose that directly.
     auto *CD = dyn_cast<CXXConstructorDecl>(DiagDecl);
     if (CD && CD->isInheritingConstructor()) {
       auto *Inherited = CD->getInheritedConstructor().getConstructor();
       if (!Inherited->isConstexpr())
         DiagDecl = CD = Inherited;
     }
 
     // FIXME: If DiagDecl is an implicitly-declared special member function
     // or an inheriting constructor, we should be much more explicit about why
     // it's not constexpr.
     if (CD && CD->isInheritingConstructor())
       Info.FFDiag(CallLoc, diag::note_constexpr_invalid_inhctor, 1)
         << CD->getInheritedConstructor().getConstructor()->getParent();
     else
       Info.FFDiag(CallLoc, diag::note_constexpr_invalid_function, 1)
           << DiagDecl->isConstexpr() << (bool)CD << DiagDecl;
     Info.Note(DiagDecl->getLocation(), diag::note_declared_at);
   } else {
     Info.FFDiag(CallLoc, diag::note_invalid_subexpr_in_const_expr);
   }
   return false;
 }
 
 namespace {

clang/lib/AST/TextNodeDumper.cpp

void TextNodeDumper::Visit(const Decl *D) {
 
   if (D->isUsed())
     OS << " used";
   else if (D->isThisDeclarationReferenced())
     OS << " referenced";
 
   if (D->isInvalidDecl())
     OS << " invalid";
-  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D))
-    if (FD->isConstexpr())
+  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D)) {
+    if (FD->isConstexprSpecified())
       OS << " constexpr";
+    if (FD->isConsteval())
+      OS << " consteval";
+  }
 
   if (!isa<FunctionDecl>(*D)) {
     const auto *MD = dyn_cast<ObjCMethodDecl>(D);
     if (!MD || !MD->isThisDeclarationADefinition()) {
       const auto *DC = dyn_cast<DeclContext>(D);
       if (DC && DC->hasExternalLexicalStorage()) {
         ColorScope Color(OS, ShowColors, UndeserializedColor);
         OS << " <undeserialized declarations>";

clang/lib/Parse/ParseCXXInlineMethods.cpp

if (SkipFunctionBodies && (!FnD || Actions.canSkipFunctionBody(FnD)) &&
     return FnD;
   }
 
   // In delayed template parsing mode, if we are within a class template
   // or if we are about to parse function member template then consume
   // the tokens and store them for parsing at the end of the translation unit.
   if (getLangOpts().DelayedTemplateParsing &&
       D.getFunctionDefinitionKind() == FDK_Definition &&
-      !D.getDeclSpec().isConstexprSpecified() &&
+      !D.getDeclSpec().hasConstexprSpecifier() &&
       !(FnD && FnD->getAsFunction() &&
         FnD->getAsFunction()->getReturnType()->getContainedAutoType()) &&
       ((Actions.CurContext->isDependentContext() ||
         (TemplateInfo.Kind != ParsedTemplateInfo::NonTemplate &&
          TemplateInfo.Kind != ParsedTemplateInfo::ExplicitSpecialization)) &&
        !Actions.IsInsideALocalClassWithinATemplateFunction())) {
 
     CachedTokens Toks;

clang/lib/Parse/ParseDecl.cpp

if (Specs & DeclSpec::PQ_StorageClassSpecifier) {
     else
       Diag(DS.getThreadStorageClassSpecLoc(),
            diag::err_typename_invalid_storageclass);
     DS.ClearStorageClassSpecs();
   }
 
   // Issue diagnostic and remove function specifier if present.
   if (Specs & DeclSpec::PQ_FunctionSpecifier) {
     if (DS.isInlineSpecified())
       Diag(DS.getInlineSpecLoc(), diag::err_typename_invalid_functionspec);
     if (DS.isVirtualSpecified())
       Diag(DS.getVirtualSpecLoc(), diag::err_typename_invalid_functionspec);
     if (DS.hasExplicitSpecifier())
       Diag(DS.getExplicitSpecLoc(), diag::err_typename_invalid_functionspec);
     DS.ClearFunctionSpecs();
   }
 
   // Issue diagnostic and remove constexpr specifier if present.
-  if (DS.isConstexprSpecified() && DSC != DeclSpecContext::DSC_condition) {
-    Diag(DS.getConstexprSpecLoc(), diag::err_typename_invalid_constexpr);
+  if (DS.hasConstexprSpecifier() && DSC != DeclSpecContext::DSC_condition) {
+    Diag(DS.getConstexprSpecLoc(), diag::err_typename_invalid_constexpr)
+        << (DS.getConstexprSpecifier() == CSK_consteval);
     DS.ClearConstexprSpec();
   }
 }
 
 /// isValidAfterIdentifierInDeclaratorAfterDeclSpec - Return true if the
 /// specified token is valid after the identifier in a declarator which
 /// immediately follows the declspec.  For example, these things are valid:
 ///
while (1) {
 
     // Modules
     case tok::kw___module_private__:
       isInvalid = DS.setModulePrivateSpec(Loc, PrevSpec, DiagID);
       break;
 
     // constexpr
     case tok::kw_constexpr:
-      isInvalid = DS.SetConstexprSpec(Loc, PrevSpec, DiagID);
+      isInvalid = DS.SetConstexprSpec(CSK_constexpr, Loc, PrevSpec, DiagID);
+      break;
+
+    // consteval
+    case tok::kw_consteval:
+      isInvalid = DS.SetConstexprSpec(CSK_consteval, Loc, PrevSpec, DiagID);
       break;
 
     // type-specifier
     case tok::kw_short:
       isInvalid = DS.SetTypeSpecWidth(DeclSpec::TSW_short, Loc, PrevSpec,
                                       DiagID, Policy);
       break;
     case tok::kw_long:
case tok::kw_typeof:
 
     // GNU attributes.
   case tok::kw___attribute:
 
     // C++11 decltype and constexpr.
   case tok::annot_decltype:
   case tok::kw_constexpr:
 
+    // C++20 consteval.
+  case tok::kw_consteval:
+
     // C11 _Atomic
   case tok::kw__Atomic:
     return true;
 
     // GNU ObjC bizarre protocol extension: <proto1,proto2> with implicit 'id'.
   case tok::less:
     return getLangOpts().ObjC;
 
if (getLangOpts().CPlusPlus) {
         D.getDeclSpec().getStorageClassSpec() != DeclSpec::SCS_typedef &&
         (D.getContext() == DeclaratorContext::MemberContext
          ? !D.getDeclSpec().isFriendSpecified()
          : D.getContext() == DeclaratorContext::FileContext &&
            D.getCXXScopeSpec().isValid() &&
            Actions.CurContext->isRecord());
 
       Qualifiers Q = Qualifiers::fromCVRUMask(DS.getTypeQualifiers());
-      if (D.getDeclSpec().isConstexprSpecified() && !getLangOpts().CPlusPlus14)
+      if (D.getDeclSpec().hasConstexprSpecifier() && !getLangOpts().CPlusPlus14)
         Q.addConst();
       // FIXME: Collect C++ address spaces.
       // If there are multiple different address spaces, the source is invalid.
       // Carry on using the first addr space for the qualifiers of 'this'.
       // The diagnostic will be given later while creating the function
       // prototype for the method.
       if (getLangOpts().OpenCLCPlusPlus) {
         for (ParsedAttr &attr : DS.getAttributes()) {

clang/lib/Parse/ParseExprCXX.cpp

if (std::any_of(std::begin(EllipsisLocs), std::end(EllipsisLocs),
           for (SourceLocation &Loc : EllipsisLocs) {
             if (&Loc != ExpectedEllipsisLoc && Loc.isValid())
               D << FixItHint::CreateRemoval(Loc);
           }
         });
       }
     }
 
     // Process the init-capture initializers now rather than delaying until we
     // form the lambda-expression so that they can be handled in the context
     // enclosing the lambda-expression, rather than in the context of the
     // lambda-expression itself.
     ParsedType InitCaptureType;
     if (Init.isUsable())
       Init = Actions.CorrectDelayedTyposInExpr(Init.get());
     if (Init.isUsable()) {
       NonTentativeAction([&] {
         // Get the pointer and store it in an lvalue, so we can use it as an
         // out argument.
Intro.addCapture(Kind, Loc, Id, EllipsisLoc, InitKind, Init,
                      InitCaptureType, SourceRange(LocStart, LocEnd));
   }
 
   T.consumeClose();
   Intro.Range.setEnd(T.getCloseLocation());
   return false;
 }
 
-static void
-tryConsumeMutableOrConstexprToken(Parser &P, SourceLocation &MutableLoc,
+static void tryConsumeLambdaSpecifierToken(Parser &P,
+                                           SourceLocation &MutableLoc,
                                            SourceLocation &ConstexprLoc,
+                                           SourceLocation &ConstevalLoc,
                                            SourceLocation &DeclEndLoc) {
   assert(MutableLoc.isInvalid());
   assert(ConstexprLoc.isInvalid());
   // Consume constexpr-opt mutable-opt in any sequence, and set the DeclEndLoc
   // to the final of those locations. Emit an error if we have multiple
   // copies of those keywords and recover.
 
   while (true) {
     switch (P.getCurToken().getKind()) {
case tok::kw_constexpr:
       if (ConstexprLoc.isValid()) {
         P.Diag(P.getCurToken().getLocation(),
                diag::err_lambda_decl_specifier_repeated)
             << 1 << FixItHint::CreateRemoval(P.getCurToken().getLocation());
       }
       ConstexprLoc = P.ConsumeToken();
       DeclEndLoc = ConstexprLoc;
       break /*switch*/;
+    case tok::kw_consteval:
+      if (ConstevalLoc.isValid()) {
+        P.Diag(P.getCurToken().getLocation(),
+               diag::err_lambda_decl_specifier_repeated)
+            << 2 << FixItHint::CreateRemoval(P.getCurToken().getLocation());
+      }
+      ConstevalLoc = P.ConsumeToken();
+      DeclEndLoc = ConstevalLoc;
+      break /*switch*/;
     default:
       return;
     }
   }
 }
 
 static void
 addConstexprToLambdaDeclSpecifier(Parser &P, SourceLocation ConstexprLoc,
                                   DeclSpec &DS) {
   if (ConstexprLoc.isValid()) {
     P.Diag(ConstexprLoc, !P.getLangOpts().CPlusPlus17
                              ? diag::ext_constexpr_on_lambda_cxx17
                              : diag::warn_cxx14_compat_constexpr_on_lambda);
     const char *PrevSpec = nullptr;
     unsigned DiagID = 0;
-    DS.SetConstexprSpec(ConstexprLoc, PrevSpec, DiagID);
+    DS.SetConstexprSpec(CSK_constexpr, ConstexprLoc, PrevSpec, DiagID);
     assert(PrevSpec == nullptr && DiagID == 0 &&
            "Constexpr cannot have been set previously!");
   }
 }
 
+static void addConstevalToLambdaDeclSpecifier(Parser &P,
+                                              SourceLocation ConstevalLoc,
+                                              DeclSpec &DS) {
+  if (ConstevalLoc.isValid()) {
+    const char *PrevSpec = nullptr;
+    unsigned DiagID = 0;
+    DS.SetConstexprSpec(CSK_consteval, ConstevalLoc, PrevSpec, DiagID);
+    if (DiagID != 0)
+      P.Diag(ConstevalLoc, DiagID) << PrevSpec;
+  }
+}
+
 /// ParseLambdaExpressionAfterIntroducer - Parse the rest of a lambda
 /// expression.
 ExprResult Parser::ParseLambdaExpressionAfterIntroducer(
                      LambdaIntroducer &Intro) {
   SourceLocation LambdaBeginLoc = Intro.Range.getBegin();
   Diag(LambdaBeginLoc, diag::warn_cxx98_compat_lambda);
 
   PrettyStackTraceLoc CrashInfo(PP.getSourceManager(), LambdaBeginLoc,
if (Tok.is(tok::l_paren)) {
     // GNU-style attributes must be parsed before the mutable specifier to be
     // compatible with GCC.
     MaybeParseGNUAttributes(Attr, &DeclEndLoc);
 
     // MSVC-style attributes must be parsed before the mutable specifier to be
     // compatible with MSVC.
     MaybeParseMicrosoftDeclSpecs(Attr, &DeclEndLoc);
 
-    // Parse mutable-opt and/or constexpr-opt, and update the DeclEndLoc.
+    // Parse mutable-opt and/or constexpr-opt or consteval-opt, and update the
+    // DeclEndLoc.
     SourceLocation MutableLoc;
     SourceLocation ConstexprLoc;
-    tryConsumeMutableOrConstexprToken(*this, MutableLoc, ConstexprLoc,
-                                      DeclEndLoc);
+    SourceLocation ConstevalLoc;
+    tryConsumeLambdaSpecifierToken(*this, MutableLoc, ConstexprLoc,
+                                   ConstevalLoc, DeclEndLoc);
 
     addConstexprToLambdaDeclSpecifier(*this, ConstexprLoc, DS);
-
+    addConstevalToLambdaDeclSpecifier(*this, ConstevalLoc, DS);
     // Parse exception-specification[opt].
     ExceptionSpecificationType ESpecType = EST_None;
     SourceRange ESpecRange;
     SmallVector<ParsedType, 2> DynamicExceptions;
     SmallVector<SourceRange, 2> DynamicExceptionRanges;
     ExprResult NoexceptExpr;
     CachedTokens *ExceptionSpecTokens;
     ESpecType = tryParseExceptionSpecification(/*Delayed=*/false,
                                                ESpecRange,
                                                DynamicExceptions,
                                                DynamicExceptionRanges,
                                                NoexceptExpr,
                                                ExceptionSpecTokens);
 
     if (ESpecType != EST_None)
       DeclEndLoc = ESpecRange.getEnd();
D.AddTypeInfo(DeclaratorChunk::getFunction(
                       ESpecRange, DynamicExceptions.data(),
                       DynamicExceptionRanges.data(), DynamicExceptions.size(),
                       NoexceptExpr.isUsable() ? NoexceptExpr.get() : nullptr,
                       /*ExceptionSpecTokens*/ nullptr,
                       /*DeclsInPrototype=*/None, LParenLoc, FunLocalRangeEnd, D,
                       TrailingReturnType),
                   std::move(Attr), DeclEndLoc);
   } else if (Tok.isOneOf(tok::kw_mutable, tok::arrow, tok::kw___attribute,
-                         tok::kw_constexpr) ||
+                         tok::kw_constexpr, tok::kw_consteval) ||
              (Tok.is(tok::l_square) && NextToken().is(tok::l_square))) {
     // It's common to forget that one needs '()' before 'mutable', an attribute
     // specifier, or the result type. Deal with this.
     unsigned TokKind = 0;
     switch (Tok.getKind()) {
     case tok::kw_mutable: TokKind = 0; break;
     case tok::arrow: TokKind = 1; break;
     case tok::kw___attribute:
     case tok::l_square: TokKind = 2; break;
     case tok::kw_constexpr: TokKind = 3; break;
+    case tok::kw_consteval: TokKind = 4; break;
     default: llvm_unreachable("Unknown token kind");
     }
 
     Diag(Tok, diag::err_lambda_missing_parens)
       << TokKind
       << FixItHint::CreateInsertion(Tok.getLocation(), "() ");
     SourceLocation DeclEndLoc = DeclLoc;
 

clang/lib/Parse/ParseTentative.cpp

 ///
 ///         decl-specifier:
 ///           storage-class-specifier
 ///           type-specifier
 ///           function-specifier
 ///           'friend'
 ///           'typedef'
 /// [C++11]   'constexpr'
+/// [C++20]   'consteval'
 /// [GNU]     attributes declaration-specifiers[opt]
 ///
 ///         storage-class-specifier:
 ///           'register'
 ///           'static'
 ///           'extern'
 ///           'mutable'
 ///           'auto'
case tok::kw_decltype:
     //   type-specifier
     //   function-specifier
     //   'friend'
     //   'typedef'
     //   'constexpr'
   case tok::kw_friend:
   case tok::kw_typedef:
   case tok::kw_constexpr:
+  case tok::kw_consteval:
     // storage-class-specifier
   case tok::kw_register:
   case tok::kw_static:
   case tok::kw_extern:
   case tok::kw_mutable:
   case tok::kw_auto:
   case tok::kw___thread:
   case tok::kw_thread_local:

clang/lib/Sema/DeclSpec.cpp

#define GENERIC_IMAGE_TYPE(ImgType, Id) \
   case DeclSpec::TST_##ImgType##_t: \
     return #ImgType "_t";
 #include "clang/Basic/OpenCLImageTypes.def"
   case DeclSpec::TST_error:       return "(error)";
   }
   llvm_unreachable("Unknown typespec!");
 }
 
+const char *DeclSpec::getSpecifierName(ConstexprSpecKind C) {
+  switch (C) {
+  case CSK_unspecified: return "unspecified";
+  case CSK_constexpr:   return "constexpr";
+  case CSK_consteval:   return "consteval";
+  }
+  llvm_unreachable("Unknown ConstexprSpecKind");
+}
+
 const char *DeclSpec::getSpecifierName(TQ T) {
   switch (T) {
   case DeclSpec::TQ_unspecified: return "unspecified";
   case DeclSpec::TQ_const:       return "const";
   case DeclSpec::TQ_restrict:    return "restrict";
   case DeclSpec::TQ_volatile:    return "volatile";
   case DeclSpec::TQ_atomic:      return "_Atomic";
   case DeclSpec::TQ_unaligned:   return "__unaligned";
if (isModulePrivateSpecified()) {
     DiagID = diag::ext_warn_duplicate_declspec;
     return true;
   }
 
   ModulePrivateLoc = Loc;
   return false;
 }
 
-bool DeclSpec::SetConstexprSpec(SourceLocation Loc, const char *&PrevSpec,
+bool DeclSpec::SetConstexprSpec(ConstexprSpecKind ConstexprKind,
+                                SourceLocation Loc, const char *&PrevSpec,
                                 unsigned &DiagID) {
-  // 'constexpr constexpr' is ok, but warn as this is likely not what the user
-  // intended.
-  if (Constexpr_specified) {
+  if (ConstexprSpecifier != CSK_unspecified) {
+    if (ConstexprSpecifier == CSK_consteval || ConstexprKind == CSK_consteval)
+      return BadSpecifier(ConstexprKind, ConstexprSpecifier, PrevSpec, DiagID);
     DiagID = diag::warn_duplicate_declspec;
     PrevSpec = "constexpr";
     return true;
   }
-  Constexpr_specified = true;
+  ConstexprSpecifier = ConstexprKind;
   ConstexprLoc = Loc;
   return false;
 }
 
 void DeclSpec::SaveWrittenBuiltinSpecs() {
   writtenBS.Sign = getTypeSpecSign();
   writtenBS.Width = getTypeSpecWidth();
   writtenBS.Type = getTypeSpecType();
if (S.getLangOpts().CPlusPlus && !S.getLangOpts().CPlusPlus11 &&
       StorageClassSpec == SCS_auto)
     S.Diag(StorageClassSpecLoc, diag::warn_auto_storage_class)
       << FixItHint::CreateRemoval(StorageClassSpecLoc);
   if (TypeSpecType == TST_char8)
     S.Diag(TSTLoc, diag::warn_cxx17_compat_unicode_type);
   else if (TypeSpecType == TST_char16 || TypeSpecType == TST_char32)
     S.Diag(TSTLoc, diag::warn_cxx98_compat_unicode_type)
       << (TypeSpecType == TST_char16 ? "char16_t" : "char32_t");
-  if (Constexpr_specified)
+  if (hasConstexprSpecifier())
     S.Diag(ConstexprLoc, diag::warn_cxx98_compat_constexpr);
 
   // C++ [class.friend]p6:
   //   No storage-class-specifier shall appear in the decl-specifier-seq
   //   of a friend declaration.
   if (isFriendSpecified() &&
       (getStorageClassSpec() || getThreadStorageClassSpec())) {
     SmallString<32> SpecName;
     SourceLocation SCLoc;
if (isFriendSpecified() &&
     S.Diag(SCLoc, diag::err_friend_decl_spec)
       << SpecName << StorageHint << ThreadHint;
 
     ClearStorageClassSpecs();
   }
 
   // C++11 [dcl.fct.spec]p5:
   //   The virtual specifier shall be used only in the initial
   //   declaration of a non-static class member function;
   // C++11 [dcl.fct.spec]p6:
   //   The explicit specifier shall be used only in the declaration of
   //   a constructor or conversion function within its class
   //   definition;
   if (isFriendSpecified() && (isVirtualSpecified() || hasExplicitSpecifier())) {
     StringRef Keyword;
     FixItHint Hint;
     SourceLocation SCLoc;
 
     if (isVirtualSpecified()) {
       Keyword = "virtual";
       SCLoc = getVirtualSpecLoc();
       Hint = FixItHint::CreateRemoval(SCLoc);
     } else {

clang/lib/Sema/SemaCoroutine.cpp

enum InvalidFuncDiag {
     DiagCtor = 0,
     DiagDtor,
     DiagCopyAssign,
     DiagMoveAssign,
     DiagMain,
     DiagConstexpr,
     DiagAutoRet,
     DiagVarargs,
+    DiagConsteval,
   };
   bool Diagnosed = false;
   auto DiagInvalid = [&](InvalidFuncDiag ID) {
     S.Diag(Loc, diag::err_coroutine_invalid_func_context) << ID << Keyword;
     Diagnosed = true;
     return false;
   };
 
static bool isValidCoroutineContext(Sema &S, SourceLocation Loc,
   else if (FD->isMain())
     return DiagInvalid(DiagMain);
 
   // Emit a diagnostics for each of the following conditions which is not met.
   // [expr.const]p2: "An expression e is a core constant expression unless the
   // evaluation of e [...] would evaluate one of the following expressions:
   // [...] an await-expression [...] a yield-expression."
   if (FD->isConstexpr())
-    DiagInvalid(DiagConstexpr);
+    DiagInvalid(FD->isConsteval() ? DiagConsteval : DiagConstexpr);
   // [dcl.spec.auto]p15: "A function declared with a return type that uses a
   // placeholder type shall not be a coroutine."
   if (FD->getReturnType()->isUndeducedType())
     DiagInvalid(DiagAutoRet);
   // [dcl.fct.def.coroutine]p1: "The parameter-declaration-clause of the
   // coroutine shall not terminate with an ellipsis that is not part of a
   // parameter-declaration."
   if (FD->isVariadic())

clang/lib/Sema/SemaDecl.cpp

if (TypeQuals & DeclSpec::TQ_restrict)
            diag::err_typecheck_invalid_restrict_not_pointer_noarg)
            << DS.getSourceRange();
   }
 
   if (DS.isInlineSpecified())
     Diag(DS.getInlineSpecLoc(), diag::err_inline_non_function)
         << getLangOpts().CPlusPlus17;
 
-  if (DS.isConstexprSpecified()) {
+  if (DS.hasConstexprSpecifier()) {
     // C++0x [dcl.constexpr]p1: constexpr can only be applied to declarations
     // and definitions of functions and variables.
+    // C++2a [dcl.constexpr]p1: The consteval specifier shall be applied only to
+    // the declaration of a function or function template
+    bool IsConsteval = DS.getConstexprSpecifier() == CSK_consteval;
     if (Tag)
       Diag(DS.getConstexprSpecLoc(), diag::err_constexpr_tag)
-          << GetDiagnosticTypeSpecifierID(DS.getTypeSpecType());
+          << GetDiagnosticTypeSpecifierID(DS.getTypeSpecType()) << IsConsteval;
     else
-      Diag(DS.getConstexprSpecLoc(), diag::err_constexpr_no_declarators);
+      Diag(DS.getConstexprSpecLoc(), diag::err_constexpr_wrong_decl_kind)
+          << IsConsteval;
     // Don't emit warnings after this error.
     return TagD;
   }
 
   DiagnoseFunctionSpecifiers(DS);
 
   if (DS.isFriendSpecified()) {
     // If we're dealing with a decl but not a TagDecl, assume that
if (D.getCXXScopeSpec().isSet()) {
     Previous.clear();
   }
 
   DiagnoseFunctionSpecifiers(D.getDeclSpec());
 
   if (D.getDeclSpec().isInlineSpecified())
     Diag(D.getDeclSpec().getInlineSpecLoc(), diag::err_inline_non_function)
         << getLangOpts().CPlusPlus17;
-  if (D.getDeclSpec().isConstexprSpecified())
+  if (D.getDeclSpec().hasConstexprSpecifier())
     Diag(D.getDeclSpec().getConstexprSpecLoc(), diag::err_invalid_constexpr)
-      << 1;
+        << 1 << (D.getDeclSpec().getConstexprSpecifier() == CSK_consteval);
 
   if (D.getName().Kind != UnqualifiedIdKind::IK_Identifier) {
     if (D.getName().Kind == UnqualifiedIdKind::IK_DeductionGuideName)
       Diag(D.getName().StartLocation,
            diag::err_deduction_guide_invalid_specifier)
           << "typedef";
     else
       Diag(D.getName().StartLocation, diag::err_typedef_not_identifier)
if (!getLangOpts().CPlusPlus) {
 
     // If we have any template parameter lists that don't directly belong to
     // the variable (matching the scope specifier), store them.
     unsigned VDTemplateParamLists = TemplateParams ? 1 : 0;
     if (TemplateParamLists.size() > VDTemplateParamLists)
       NewVD->setTemplateParameterListsInfo(
           Context, TemplateParamLists.drop_back(VDTemplateParamLists));
 
-    if (D.getDeclSpec().isConstexprSpecified()) {
+    if (D.getDeclSpec().hasConstexprSpecifier()) {
       NewVD->setConstexpr(true);
       // C++1z [dcl.spec.constexpr]p1:
       //   A static data member declared with the constexpr specifier is
       //   implicitly an inline variable.
       if (NewVD->isStaticDataMember() && getLangOpts().CPlusPlus17)
         NewVD->setImplicitlyInline();
+      if (D.getDeclSpec().getConstexprSpecifier() == CSK_consteval)
+        Diag(D.getDeclSpec().getConstexprSpecLoc(),
+             diag::err_constexpr_wrong_decl_kind)
+            << /*consteval*/ 1;
     }
   }
 
   if (D.getDeclSpec().isInlineSpecified()) {
     if (!getLangOpts().CPlusPlus) {
       Diag(D.getDeclSpec().getInlineSpecLoc(), diag::err_inline_non_function)
           << 0;
     } else if (CurContext->isFunctionOrMethod()) {
if (!SemaRef.getLangOpts().CPlusPlus) {
     //   - the type R of the function is some kind of typedef or other non-
     //     attributed reference to a type name (which eventually refers to a
     //     function type).
     bool HasPrototype =
       (D.isFunctionDeclarator() && D.getFunctionTypeInfo().hasPrototype) ||
       (!R->getAsAdjusted<FunctionType>() && R->isFunctionProtoType());
 
     NewFD = FunctionDecl::Create(SemaRef.Context, DC, D.getBeginLoc(), NameInfo,
-                                 R, TInfo, SC, isInline, HasPrototype, false);
+                                 R, TInfo, SC, isInline, HasPrototype,
+                                 CSK_unspecified);
     if (D.isInvalidType())
       NewFD->setInvalidDecl();
 
     return NewFD;
   }
 
   ExplicitSpecifier ExplicitSpecifier = D.getDeclSpec().getExplicitSpecifier();
-  bool isConstexpr = D.getDeclSpec().isConstexprSpecified();
-
+  ConstexprSpecKind ConstexprKind = D.getDeclSpec().getConstexprSpecifier();
   // Check that the return type is not an abstract class type.
   // For record types, this is done by the AbstractClassUsageDiagnoser once
   // the class has been completely parsed.
   if (!DC->isRecord() &&
       SemaRef.RequireNonAbstractType(
           D.getIdentifierLoc(), R->getAs<FunctionType>()->getReturnType(),
           diag::err_abstract_type_in_decl, SemaRef.AbstractReturnType))
     D.setInvalidType();
 
   if (Name.getNameKind() == DeclarationName::CXXConstructorName) {
     // This is a C++ constructor declaration.
     assert(DC->isRecord() &&
            "Constructors can only be declared in a member context");
 
     R = SemaRef.CheckConstructorDeclarator(D, R, SC);
     return CXXConstructorDecl::Create(
         SemaRef.Context, cast<CXXRecordDecl>(DC), D.getBeginLoc(), NameInfo, R,
         TInfo, ExplicitSpecifier, isInline,
-        /*isImplicitlyDeclared=*/false, isConstexpr);
+        /*isImplicitlyDeclared=*/false, ConstexprKind);
 
   } else if (Name.getNameKind() == DeclarationName::CXXDestructorName) {
     // This is a C++ destructor declaration.
     if (DC->isRecord()) {
       R = SemaRef.CheckDestructorDeclarator(D, R, SC);
       CXXRecordDecl *Record = cast<CXXRecordDecl>(DC);
       CXXDestructorDecl *NewDD =
           CXXDestructorDecl::Create(SemaRef.Context, Record, D.getBeginLoc(),
if (DC->isRecord()) {
       SemaRef.Diag(D.getIdentifierLoc(), diag::err_destructor_not_member);
       D.setInvalidType();
 
       // Create a FunctionDecl to satisfy the function definition parsing
       // code path.
       return FunctionDecl::Create(SemaRef.Context, DC, D.getBeginLoc(),
                                   D.getIdentifierLoc(), Name, R, TInfo, SC,
                                   isInline,
-                                  /*hasPrototype=*/true, isConstexpr);
+                                  /*hasPrototype=*/true, ConstexprKind);
     }
 
   } else if (Name.getNameKind() == DeclarationName::CXXConversionFunctionName) {
     if (!DC->isRecord()) {
       SemaRef.Diag(D.getIdentifierLoc(),
            diag::err_conv_function_not_member);
       return nullptr;
     }
 
     SemaRef.CheckConversionDeclarator(D, R, SC);
     IsVirtualOkay = true;
     return CXXConversionDecl::Create(
         SemaRef.Context, cast<CXXRecordDecl>(DC), D.getBeginLoc(), NameInfo, R,
-        TInfo, isInline, ExplicitSpecifier, isConstexpr, SourceLocation());
+        TInfo, isInline, ExplicitSpecifier, ConstexprKind, SourceLocation());
 
   } else if (Name.getNameKind() == DeclarationName::CXXDeductionGuideName) {
     SemaRef.CheckDeductionGuideDeclarator(D, R, SC);
 
     return CXXDeductionGuideDecl::Create(SemaRef.Context, DC, D.getBeginLoc(),
                                          ExplicitSpecifier, NameInfo, R, TInfo,
                                          D.getEndLoc());
   } else if (DC->isRecord()) {
     // If the name of the function is the same as the name of the record,
     // then this must be an invalid constructor that has a return type.
     // (The parser checks for a return type and makes the declarator a
     // constructor if it has no return type).
     if (Name.getAsIdentifierInfo() &&
         Name.getAsIdentifierInfo() == cast<CXXRecordDecl>(DC)->getIdentifier()){
       SemaRef.Diag(D.getIdentifierLoc(), diag::err_constructor_return_type)
         << SourceRange(D.getDeclSpec().getTypeSpecTypeLoc())
         << SourceRange(D.getIdentifierLoc());
       return nullptr;
     }
 
     // This is a C++ method declaration.
     CXXMethodDecl *Ret = CXXMethodDecl::Create(
         SemaRef.Context, cast<CXXRecordDecl>(DC), D.getBeginLoc(), NameInfo, R,
-        TInfo, SC, isInline, isConstexpr, SourceLocation());
+        TInfo, SC, isInline, ConstexprKind, SourceLocation());
     IsVirtualOkay = !Ret->isStatic();
     return Ret;
   } else {
     bool isFriend =
         SemaRef.getLangOpts().CPlusPlus && D.getDeclSpec().isFriendSpecified();
     if (!isFriend && SemaRef.CurContext->isRecord())
       return nullptr;
 
     // Determine whether the function was written with a
     // prototype. This true when:
     //   - we're in C++ (where every function has a prototype),
     return FunctionDecl::Create(SemaRef.Context, DC, D.getBeginLoc(), NameInfo,
                                 R, TInfo, SC, isInline, true /*HasPrototype*/,
-                                isConstexpr);
+                                ConstexprKind);
   }
 }
 
 enum OpenCLParamType {
   ValidKernelParam,
   PtrPtrKernelParam,
   PtrKernelParam,
   InvalidAddrSpacePtrKernelParam,
Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
 
   if (IsLocalExternDecl)
     NewFD->setLocalExternDecl();
 
   if (getLangOpts().CPlusPlus) {
     bool isInline = D.getDeclSpec().isInlineSpecified();
     bool isVirtual = D.getDeclSpec().isVirtualSpecified();
     bool hasExplicit = D.getDeclSpec().hasExplicitSpecifier();
-    bool isConstexpr = D.getDeclSpec().isConstexprSpecified();
+    ConstexprSpecKind ConstexprKind = D.getDeclSpec().getConstexprSpecifier();
     isFriend = D.getDeclSpec().isFriendSpecified();
     if (isFriend && !isInline && D.isFunctionDefinition()) {
       // C++ [class.friend]p5
       //   A function can be defined in a friend declaration of a
       //   class . . . . Such a function is implicitly inline.
       NewFD->setImplicitlyInline();
     }
 
if (hasExplicit && !NewFD->isInvalidDecl() &&
         // 'explicit' was specified on a function that wasn't a constructor
         // or conversion function.
         Diag(D.getDeclSpec().getExplicitSpecLoc(),
              diag::err_explicit_non_ctor_or_conv_function)
             << FixItHint::CreateRemoval(D.getDeclSpec().getExplicitSpecRange());
       }
     }
 
-    if (isConstexpr) {
+    if (ConstexprKind != CSK_unspecified) {
       // C++11 [dcl.constexpr]p2: constexpr functions and constexpr constructors
       // are implicitly inline.
       NewFD->setImplicitlyInline();
 
       // C++11 [dcl.constexpr]p3: functions declared constexpr are required to
       // be either constructors or to return a literal type. Therefore,
       // destructors cannot be declared constexpr.
       if (isa<CXXDestructorDecl>(NewFD))
-        Diag(D.getDeclSpec().getConstexprSpecLoc(), diag::err_constexpr_dtor);
+        Diag(D.getDeclSpec().getConstexprSpecLoc(), diag::err_constexpr_dtor)
+            << (ConstexprKind == CSK_consteval);
     }
 
     // If __module_private__ was specified, mark the function accordingly.
     if (D.getDeclSpec().isModulePrivateSpecified()) {
       if (isFunctionTemplateSpecialization) {
         SourceLocation ModulePrivateLoc
           = D.getDeclSpec().getModulePrivateSpecLoc();
         Diag(ModulePrivateLoc, diag::err_module_private_specialization)
enum DoesntSupport {
     FuncTemplates = 0,
     VirtFuncs = 1,
     DeducedReturn = 2,
     Constructors = 3,
     Destructors = 4,
     DeletedFuncs = 5,
     DefaultedFuncs = 6,
     ConstexprFuncs = 7,
+    ConstevalFuncs = 8,
   };
   enum Different {
     CallingConv = 0,
     ReturnType = 1,
     ConstexprSpec = 2,
     InlineSpec = 3,
     StorageClass = 4,
     Linkage = 5
static bool CheckMultiVersionAdditionalRules(Sema &S, const FunctionDecl *OldFD,
 
   if (NewFD->isDefaulted())
     return S.Diag(NewFD->getLocation(), diag::err_multiversion_doesnt_support)
            << IsCPUSpecificCPUDispatchMVType << DefaultedFuncs;
 
   if (NewFD->isConstexpr() && (MVType == MultiVersionKind::CPUDispatch ||
                                MVType == MultiVersionKind::CPUSpecific))
     return S.Diag(NewFD->getLocation(), diag::err_multiversion_doesnt_support)
-           << IsCPUSpecificCPUDispatchMVType << ConstexprFuncs;
+           << IsCPUSpecificCPUDispatchMVType
+           << (NewFD->isConsteval() ? ConstevalFuncs : ConstexprFuncs);
 
   QualType NewQType = S.getASTContext().getCanonicalType(NewFD->getType());
   const auto *NewType = cast<FunctionType>(NewQType);
   QualType NewReturnType = NewType->getReturnType();
 
   if (NewReturnType->isUndeducedType())
     return S.Diag(NewFD->getLocation(), diag::err_multiversion_doesnt_support)
            << IsCPUSpecificCPUDispatchMVType << DeducedReturn;
if (OldTypeInfo.getCC() != NewTypeInfo.getCC())
              << CallingConv;
 
     QualType OldReturnType = OldType->getReturnType();
 
     if (OldReturnType != NewReturnType)
       return S.Diag(NewFD->getLocation(), diag::err_multiversion_diff)
              << ReturnType;
 
-    if (OldFD->isConstexpr() != NewFD->isConstexpr())
+    if (OldFD->getConstexprKind() != NewFD->getConstexprKind())
       return S.Diag(NewFD->getLocation(), diag::err_multiversion_diff)
              << ConstexprSpec;
 
     if (OldFD->isInlineSpecified() != NewFD->isInlineSpecified())
       return S.Diag(NewFD->getLocation(), diag::err_multiversion_diff)
              << InlineSpec;
 
     if (OldFD->getStorageClass() != NewFD->getStorageClass())
if (DS.isNoreturnSpecified()) {
     SourceLocation NoreturnLoc = DS.getNoreturnSpecLoc();
     SourceRange NoreturnRange(NoreturnLoc, getLocForEndOfToken(NoreturnLoc));
     Diag(NoreturnLoc, diag::ext_noreturn_main);
     Diag(NoreturnLoc, diag::note_main_remove_noreturn)
       << FixItHint::CreateRemoval(NoreturnRange);
   }
   if (FD->isConstexpr()) {
     Diag(DS.getConstexprSpecLoc(), diag::err_constexpr_main)
+        << FD->isConsteval()
         << FixItHint::CreateRemoval(DS.getConstexprSpecLoc());
-    FD->setConstexpr(false);
+    FD->setConstexprKind(CSK_unspecified);
   }
 
   if (getLangOpts().OpenCL) {
     Diag(FD->getLocation(), diag::err_opencl_no_main)
         << FD->hasAttr<OpenCLKernelAttr>();
     FD->setInvalidDecl();
     return;
   }
Decl *Sema::ActOnParamDeclarator(Scope *S, Declarator &D) {
   }
 
   if (DeclSpec::TSCS TSCS = DS.getThreadStorageClassSpec())
     Diag(DS.getThreadStorageClassSpecLoc(), diag::err_invalid_thread)
       << DeclSpec::getSpecifierName(TSCS);
   if (DS.isInlineSpecified())
     Diag(DS.getInlineSpecLoc(), diag::err_inline_non_function)
         << getLangOpts().CPlusPlus17;
-  if (DS.isConstexprSpecified())
+  if (DS.hasConstexprSpecifier())
     Diag(DS.getConstexprSpecLoc(), diag::err_invalid_constexpr)
-      << 0;
+        << 0 << (D.getDeclSpec().getConstexprSpecifier() == CSK_consteval);
 
   DiagnoseFunctionSpecifiers(DS);
 
   TypeSourceInfo *TInfo = GetTypeForDeclarator(D, S);
   QualType parmDeclType = TInfo->getType();
 
   if (getLangOpts().CPlusPlus) {
     // Check that there are no default arguments inside the type of this
if (const VarDecl *NRVOCandidate = Returns[I]->getNRVOCandidate()) {
       if (!NRVOCandidate->isNRVOVariable())
         Returns[I]->setNRVOCandidate(nullptr);
     }
   }
 }
 
 bool Sema::canDelayFunctionBody(const Declarator &D) {
   // We can't delay parsing the body of a constexpr function template (yet).
-  if (D.getDeclSpec().isConstexprSpecified())
+  if (D.getDeclSpec().hasConstexprSpecifier())
     return false;
 
   // We can't delay parsing the body of a function template with a deduced
   // return type (yet).
   if (D.getDeclSpec().hasAutoTypeSpec()) {
     // If the placeholder introduces a non-deduced trailing return type,
     // we can still delay parsing it.
     if (D.getNumTypeObjects()) {

clang/lib/Sema/SemaDeclAttr.cpp

NamedDecl * Sema::DeclClonePragmaWeak(NamedDecl *ND, IdentifierInfo *II,
   assert(isa<FunctionDecl>(ND) || isa<VarDecl>(ND));
   NamedDecl *NewD = nullptr;
   if (auto *FD = dyn_cast<FunctionDecl>(ND)) {
     FunctionDecl *NewFD;
     // FIXME: Missing call to CheckFunctionDeclaration().
     // FIXME: Mangling?
     // FIXME: Is the qualifier info correct?
     // FIXME: Is the DeclContext correct?
-    NewFD = FunctionDecl::Create(FD->getASTContext(), FD->getDeclContext(),
-                                 Loc, Loc, DeclarationName(II),
-                                 FD->getType(), FD->getTypeSourceInfo(),
-                                 SC_None, false/*isInlineSpecified*/,
-                                 FD->hasPrototype(),
-                                 false/*isConstexprSpecified*/);
+    NewFD = FunctionDecl::Create(
+        FD->getASTContext(), FD->getDeclContext(), Loc, Loc,
+        DeclarationName(II), FD->getType(), FD->getTypeSourceInfo(), SC_None,
+        false /*isInlineSpecified*/, FD->hasPrototype(), CSK_unspecified);
     NewD = NewFD;
 
     if (FD->getQualifier())
       NewFD->setQualifierInfo(FD->getQualifierLoc());
 
     // Fake up parameter variables; they are declared as if this were
     // a typedef.
     QualType FDTy = FD->getType();

clang/lib/Sema/SemaDeclCXX.cpp

if (NewSM != OldSM) {
         << NewParam->getDefaultArgRange() << NewSM;
       Diag(Old->getLocation(), diag::note_previous_declaration);
     }
   }
 
   const FunctionDecl *Def;
   // C++11 [dcl.constexpr]p1: If any declaration of a function or function
   // template has a constexpr specifier then all its declarations shall
   // contain the constexpr specifier.
-  if (New->isConstexpr() != Old->isConstexpr()) {
+  if (New->getConstexprKind() != Old->getConstexprKind()) {
     Diag(New->getLocation(), diag::err_constexpr_redecl_mismatch)
-      << New << New->isConstexpr();
+        << New << New->getConstexprKind() << Old->getConstexprKind();
     Diag(Old->getLocation(), diag::note_previous_declaration);
     Invalid = true;
   } else if (!Old->getMostRecentDecl()->isInlined() && New->isInlined() &&
              Old->isDefined(Def) &&
              // If a friend function is inlined but does not have 'inline'
              // specifier, it is a definition. Do not report attribute conflict
              // in this case, redefinition will be diagnosed later.
              (New->isInlineSpecified() ||
               New->getFriendObjectKind() == Decl::FOK_None)) {
     // C++11 [dcl.fcn.spec]p4:
     //   If the definition of a function appears in a translation unit before its
     //   first declaration as inline, the program is ill-formed.
     Diag(New->getLocation(), diag::err_inline_decl_follows_def) << New;
if (auto SCS = DS.getStorageClassSpec()) {
         BadSpecifiers.push_back(DeclSpec::getSpecifierName(SCS));
         BadSpecifierLocs.push_back(DS.getStorageClassSpecLoc());
       }
     }
     if (auto TSCS = DS.getThreadStorageClassSpec()) {
       CPlusPlus20Specifiers.push_back(DeclSpec::getSpecifierName(TSCS));
       CPlusPlus20SpecifierLocs.push_back(DS.getThreadStorageClassSpecLoc());
     }
-    if (DS.isConstexprSpecified()) {
-      BadSpecifiers.push_back("constexpr");
+    if (DS.hasConstexprSpecifier()) {
+      BadSpecifiers.push_back(
+          DeclSpec::getSpecifierName(DS.getConstexprSpecifier()));
       BadSpecifierLocs.push_back(DS.getConstexprSpecLoc());
     }
     if (DS.isInlineSpecified()) {
       BadSpecifiers.push_back("inline");
       BadSpecifierLocs.push_back(DS.getInlineSpecLoc());
     }
     if (!BadSpecifiers.empty()) {
       auto &&Err = Diag(BadSpecifierLocs.front(), diag::err_decomp_decl_spec);
static bool CheckConstexprParameterTypes(Sema &SemaRef,
   unsigned ArgIndex = 0;
   const FunctionProtoType *FT = FD->getType()->getAs<FunctionProtoType>();
   for (FunctionProtoType::param_type_iterator i = FT->param_type_begin(),
                                               e = FT->param_type_end();
        i != e; ++i, ++ArgIndex) {
     const ParmVarDecl *PD = FD->getParamDecl(ArgIndex);
     SourceLocation ParamLoc = PD->getLocation();
     if (!(*i)->isDependentType() &&
-        SemaRef.RequireLiteralType(ParamLoc, *i,
-                                   diag::err_constexpr_non_literal_param,
-                                   ArgIndex+1, PD->getSourceRange(),
-                                   isa<CXXConstructorDecl>(FD)))
+        SemaRef.RequireLiteralType(
+            ParamLoc, *i, diag::err_constexpr_non_literal_param, ArgIndex + 1,
+            PD->getSourceRange(), isa<CXXConstructorDecl>(FD),
+            FD->isConsteval()))
       return false;
   }
   return true;
 }
 
 /// Get diagnostic %select index for tag kind for
 /// record diagnostic message.
 /// WARNING: Indexes apply to particular diagnostics only!
if (Method && Method->isVirtual()) {
         return false;
       }
     }
 
     // - its return type shall be a literal type;
     QualType RT = NewFD->getReturnType();
     if (!RT->isDependentType() &&
         RequireLiteralType(NewFD->getLocation(), RT,
-                           diag::err_constexpr_non_literal_return))
+                           diag::err_constexpr_non_literal_return,
+                           NewFD->isConsteval()))
       return false;
   }
 
   // - each of its parameter types shall be a literal type;
   if (!CheckConstexprParameterTypes(*this, NewFD))
     return false;
 
   return true;
case Decl::Function:
       // These are disallowed in C++11 and permitted in C++1y. Allow them
       // everywhere as an extension.
       if (!Cxx1yLoc.isValid())
         Cxx1yLoc = DS->getBeginLoc();
       continue;
 
     default:
       SemaRef.Diag(DS->getBeginLoc(), diag::err_constexpr_body_invalid_stmt)
-          << isa<CXXConstructorDecl>(Dcl);
+          << isa<CXXConstructorDecl>(Dcl) << Dcl->isConsteval();
       return false;
     }
   }
 
   return true;
 }
 
 /// Check that the given field is initialized within a constexpr constructor.
default:
 
     // C++1y allows expression-statements.
     if (!Cxx1yLoc.isValid())
       Cxx1yLoc = S->getBeginLoc();
     return true;
   }
 
   SemaRef.Diag(S->getBeginLoc(), diag::err_constexpr_body_invalid_stmt)
-      << isa<CXXConstructorDecl>(Dcl);
+      << isa<CXXConstructorDecl>(Dcl) << Dcl->isConsteval();
   return false;
 }
 
 /// Check the body for the given constexpr function declaration only contains
 /// the permitted types of statement. C++11 [dcl.constexpr]p3,p4.
 ///
 /// \return true if the body is OK, false if we have diagnosed a problem.
 bool Sema::CheckConstexprFunctionBody(const FunctionDecl *Dcl, Stmt *Body) {
if (ReturnStmts.empty()) {
       // statement. We still do, unless the return type might be void, because
       // otherwise if there's no return statement, the function cannot
       // be used in a core constant expression.
       bool OK = getLangOpts().CPlusPlus14 &&
                 (Dcl->getReturnType()->isVoidType() ||
                  Dcl->getReturnType()->isDependentType());
       Diag(Dcl->getLocation(),
            OK ? diag::warn_cxx11_compat_constexpr_body_no_return
-              : diag::err_constexpr_body_no_return);
+              : diag::err_constexpr_body_no_return)
+          << Dcl->isConsteval();
       if (!OK)
         return false;
     } else if (ReturnStmts.size() > 1) {
       Diag(ReturnStmts.back(),
            getLangOpts().CPlusPlus14
              ? diag::warn_cxx11_compat_constexpr_body_multiple_return
              : diag::ext_constexpr_body_multiple_return);
       for (unsigned I = 0; I < ReturnStmts.size() - 1; ++I)
default:
     D.getMutableDeclSpec().ClearStorageClassSpecs();
     break;
   }
 
   bool isInstField = ((DS.getStorageClassSpec() == DeclSpec::SCS_unspecified ||
                        DS.getStorageClassSpec() == DeclSpec::SCS_mutable) &&
                       !isFunc);
 
-  if (DS.isConstexprSpecified() && isInstField) {
+  if (DS.hasConstexprSpecifier() && isInstField) {
     SemaDiagnosticBuilder B =
         Diag(DS.getConstexprSpecLoc(), diag::err_invalid_constexpr_member);
     SourceLocation ConstexprLoc = DS.getConstexprSpecLoc();
     if (InitStyle == ICIS_NoInit) {
       B << 0 << 0;
       if (D.getDeclSpec().getTypeQualifiers() & DeclSpec::TQ_const)
         B << FixItHint::CreateRemoval(ConstexprLoc);
       else {
if (ReferentType.isVolatileQualified()) {
         ShouldDeleteForTypeMismatch = true;
       else {
         Diag(MD->getLocation(),
              diag::err_defaulted_special_member_volatile_param) << CSM;
         HadError = true;
       }
     }
 
     if (HasConstParam && !CanHaveConstParam) {
       if (DeleteOnTypeMismatch)
         ShouldDeleteForTypeMismatch = true;
       else if (CSM == CXXCopyConstructor || CSM == CXXCopyAssignment) {
         Diag(MD->getLocation(),
              diag::err_defaulted_special_member_copy_const_param)
           << (CSM == CXXCopyAssignment);
         // FIXME: Explain why this special member can't be const.
         HadError = true;
void Sema::CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD) {
   //
   // FIXME: This should not apply if the member is deleted.
   bool Constexpr = defaultedSpecialMemberIsConstexpr(*this, RD, CSM,
                                                      HasConstParam);
   if ((getLangOpts().CPlusPlus14 ? !isa<CXXDestructorDecl>(MD)
                                  : isa<CXXConstructorDecl>(MD)) &&
       MD->isConstexpr() && !Constexpr &&
       MD->getTemplatedKind() == FunctionDecl::TK_NonTemplate) {
-    Diag(MD->getBeginLoc(), diag::err_incorrect_defaulted_constexpr) << CSM;
+    Diag(MD->getBeginLoc(), MD->isConsteval()
+                                ? diag::err_incorrect_defaulted_consteval
+                                : diag::err_incorrect_defaulted_constexpr)
+        << CSM;
     // FIXME: Explain why the special member can't be constexpr.
     HadError = true;
   }
 
   if (First) {
     // C++2a [dcl.fct.def.default]p3:
     //   If a function is explicitly defaulted on its first declaration, it is
     //   implicitly considered to be constexpr if the implicit declaration
     //   would be.
-    MD->setConstexpr(Constexpr);
+    MD->setConstexprKind(Constexpr ? CSK_constexpr : CSK_unspecified);
 
     if (!Type->hasExceptionSpec()) {
       // C++2a [except.spec]p3:
       //   If a declaration of a function does not have a noexcept-specifier
       //   [and] is defaulted on its first declaration, [...] the exception
       //   specification is as specified below
       FunctionProtoType::ExtProtoInfo EPI = Type->getExtProtoInfo();
       EPI.ExceptionSpec.Type = EST_Unevaluated;
Diag(D.getIdentifierLoc(), diag::err_deduction_guide_wrong_scope)
       << GuidedTemplateDecl;
     Diag(GuidedTemplateDecl->getLocation(), diag::note_template_decl_here);
   }
 
   auto &DS = D.getMutableDeclSpec();
   // We leave 'friend' and 'virtual' to be rejected in the normal way.
   if (DS.hasTypeSpecifier() || DS.getTypeQualifiers() ||
       DS.getStorageClassSpecLoc().isValid() || DS.isInlineSpecified() ||
-      DS.isNoreturnSpecified() || DS.isConstexprSpecified()) {
+      DS.isNoreturnSpecified() || DS.hasConstexprSpecifier()) {
     BadSpecifierDiagnoser Diagnoser(
         *this, D.getIdentifierLoc(),
         diag::err_deduction_guide_invalid_specifier);
 
     Diagnoser.check(DS.getStorageClassSpecLoc(), DS.getStorageClassSpec());
     DS.ClearStorageClassSpecs();
     SC = SC_None;
 
CanQualType ClassType
     = Context.getCanonicalType(Context.getTypeDeclType(ClassDecl));
   SourceLocation ClassLoc = ClassDecl->getLocation();
   DeclarationName Name
     = Context.DeclarationNames.getCXXConstructorName(ClassType);
   DeclarationNameInfo NameInfo(Name, ClassLoc);
   CXXConstructorDecl *DefaultCon = CXXConstructorDecl::Create(
       Context, ClassDecl, ClassLoc, NameInfo, /*Type*/ QualType(),
       /*TInfo=*/nullptr, ExplicitSpecifier(),
-      /*isInline=*/true, /*isImplicitlyDeclared=*/true, Constexpr);
+      /*isInline=*/true, /*isImplicitlyDeclared=*/true,
+      Constexpr ? CSK_constexpr : CSK_unspecified);
   DefaultCon->setAccess(AS_public);
   DefaultCon->setDefaulted();
 
   if (getLangOpts().CUDA) {
     inferCUDATargetForImplicitSpecialMember(ClassDecl, CXXDefaultConstructor,
                                             DefaultCon,
                                             /* ConstRHS */ false,
                                             /* Diagnose */ false);
Sema::findInheritingConstructor(SourceLocation Loc,
   bool Constexpr =
       BaseCtor->isConstexpr() &&
       defaultedSpecialMemberIsConstexpr(*this, Derived, CXXDefaultConstructor,
                                         false, BaseCtor, &ICI);
 
   CXXConstructorDecl *DerivedCtor = CXXConstructorDecl::Create(
       Context, Derived, UsingLoc, NameInfo, TInfo->getType(), TInfo,
       BaseCtor->getExplicitSpecifier(), /*Inline=*/true,
-      /*ImplicitlyDeclared=*/true, Constexpr,
+      /*ImplicitlyDeclared=*/true,
+      Constexpr ? BaseCtor->getConstexprKind() : CSK_unspecified,
       InheritedConstructor(Shadow, BaseCtor));
   if (Shadow->isInvalidDecl())
     DerivedCtor->setInvalidDecl();
 
   // Build an unevaluated exception specification for this fake constructor.
   const FunctionProtoType *FPT = TInfo->getType()->castAs<FunctionProtoType>();
   FunctionProtoType::ExtProtoInfo EPI = FPT->getExtProtoInfo();
   EPI.ExceptionSpec.Type = EST_Unevaluated;
bool Constexpr = defaultedSpecialMemberIsConstexpr(*this, ClassDecl,
                                                      CXXCopyAssignment,
                                                      Const);
 
   //   An implicitly-declared copy assignment operator is an inline public
   //   member of its class.
   DeclarationName Name = Context.DeclarationNames.getCXXOperatorName(OO_Equal);
   SourceLocation ClassLoc = ClassDecl->getLocation();
   DeclarationNameInfo NameInfo(Name, ClassLoc);
-  CXXMethodDecl *CopyAssignment =
-      CXXMethodDecl::Create(Context, ClassDecl, ClassLoc, NameInfo, QualType(),
-                            /*TInfo=*/nullptr, /*StorageClass=*/SC_None,
-                            /*isInline=*/true, Constexpr, SourceLocation());
+  CXXMethodDecl *CopyAssignment = CXXMethodDecl::Create(
+      Context, ClassDecl, ClassLoc, NameInfo, QualType(),
+      /*isInline=*/true, Constexpr ? CSK_constexpr : CSK_unspecified,
+      SourceLocation());
   CopyAssignment->setAccess(AS_public);
   CopyAssignment->setDefaulted();
   CopyAssignment->setImplicit();
 
   if (getLangOpts().CUDA) {
     inferCUDATargetForImplicitSpecialMember(ClassDecl, CXXCopyAssignment,
                                             CopyAssignment,
                                             /* ConstRHS */ Const,
bool Constexpr = defaultedSpecialMemberIsConstexpr(*this, ClassDecl,
                                                      CXXMoveAssignment,
                                                      false);
 
   //   An implicitly-declared move assignment operator is an inline public
   //   member of its class.
   DeclarationName Name = Context.DeclarationNames.getCXXOperatorName(OO_Equal);
   SourceLocation ClassLoc = ClassDecl->getLocation();
   DeclarationNameInfo NameInfo(Name, ClassLoc);
-  CXXMethodDecl *MoveAssignment =
-      CXXMethodDecl::Create(Context, ClassDecl, ClassLoc, NameInfo, QualType(),
-                            /*TInfo=*/nullptr, /*StorageClass=*/SC_None,
-                            /*isInline=*/true, Constexpr, SourceLocation());
+  CXXMethodDecl *MoveAssignment = CXXMethodDecl::Create(
+      Context, ClassDecl, ClassLoc, NameInfo, QualType(),
+      /*isInline=*/true, Constexpr ? CSK_constexpr : CSK_unspecified,
+      SourceLocation());
   MoveAssignment->setAccess(AS_public);
   MoveAssignment->setDefaulted();
   MoveAssignment->setImplicit();
 
   if (getLangOpts().CUDA) {
     inferCUDATargetForImplicitSpecialMember(ClassDecl, CXXMoveAssignment,
                                             MoveAssignment,
                                             /* ConstRHS */ false,
CXXConstructorDecl *Sema::DeclareImplicitCopyConstructor(
   DeclarationNameInfo NameInfo(Name, ClassLoc);
 
   //   An implicitly-declared copy constructor is an inline public
   //   member of its class.
   CXXConstructorDecl *CopyConstructor = CXXConstructorDecl::Create(
       Context, ClassDecl, ClassLoc, NameInfo, QualType(), /*TInfo=*/nullptr,
       ExplicitSpecifier(),
       /*isInline=*/true,
-      /*isImplicitlyDeclared=*/true, Constexpr);
+      /*isImplicitlyDeclared=*/true,
+      Constexpr ? CSK_constexpr : CSK_unspecified);
   CopyConstructor->setAccess(AS_public);
   CopyConstructor->setDefaulted();
 
   if (getLangOpts().CUDA) {
     inferCUDATargetForImplicitSpecialMember(ClassDecl, CXXCopyConstructor,
                                             CopyConstructor,
                                             /* ConstRHS */ Const,
                                             /* Diagnose */ false);
CXXConstructorDecl *Sema::DeclareImplicitMoveConstructor(
 
   // C++11 [class.copy]p11:
   //   An implicitly-declared copy/move constructor is an inline public
   //   member of its class.
   CXXConstructorDecl *MoveConstructor = CXXConstructorDecl::Create(
       Context, ClassDecl, ClassLoc, NameInfo, QualType(), /*TInfo=*/nullptr,
       ExplicitSpecifier(),
       /*isInline=*/true,
-      /*isImplicitlyDeclared=*/true, Constexpr);
+      /*isImplicitlyDeclared=*/true,
+      Constexpr ? CSK_constexpr : CSK_unspecified);
   MoveConstructor->setAccess(AS_public);
   MoveConstructor->setDefaulted();
 
   if (getLangOpts().CUDA) {
     inferCUDATargetForImplicitSpecialMember(ClassDecl, CXXMoveConstructor,
                                             MoveConstructor,
                                             /* ConstRHS */ false,
                                             /* Diagnose */ false);

clang/lib/Sema/SemaExpr.cpp

if (auto *Caller = getCurFunctionDecl())
     }
 
   // Promote the function operand.
   // We special-case function promotion here because we only allow promoting
   // builtin functions to function pointers in the callee of a call.
   ExprResult Result;
   QualType ResultTy;
   if (BuiltinID &&
       Fn->getType()->isSpecificBuiltinType(BuiltinType::BuiltinFn)) {
     // Extract the return type from the (builtin) function pointer type.
     // FIXME Several builtins still have setType in
     // Sema::CheckBuiltinFunctionCall. One should review their definitions in
     // Builtins.def to ensure they are correct before removing setType calls.
     QualType FnPtrTy = Context.getPointerType(FDecl->getType());
     Result = ImpCastExprToType(Fn, FnPtrTy, CK_BuiltinFnToFnPtr).get();
     ResultTy = FDecl->getCallResultType();
   } else {
     Result = CallExprUnaryConversions(Fn);
     ResultTy = Context.BoolTy;
   }
   if (Result.isInvalid())
     return ExprError();
   Fn = Result.get();
 
   // Check for a valid function type, but only if it is not a builtin which
   // requires custom type checking. These will be handled by
   // CheckBuiltinFunctionCall below just after creation of the call expression.
   const FunctionType *FuncT = nullptr;
   if (!BuiltinID || !Context.BuiltinInfo.hasCustomTypechecking(BuiltinID)) {
    retry:
     if (const PointerType *PT = Fn->getType()->getAs<PointerType>()) {
       // C99 6.5.2.2p1 - "The expression that denotes the called function shall
       // have type pointer to function".
       FuncT = PT->getPointeeType()->getAs<FunctionType>();
       if (!FuncT)
         return ExprError(Diag(LParenLoc, diag::err_typecheck_call_not_function)
                            << Fn->getType() << Fn->getSourceRange());
     } else if (const BlockPointerType *BPT =
                  Fn->getType()->getAs<BlockPointerType>()) {
       FuncT = BPT->getPointeeType()->castAs<FunctionType>();
     } else {
       // Handle calls to expressions of unknown-any type.
       if (Fn->getType() == Context.UnknownAnyTy) {
         ExprResult rewrite = rebuildUnknownAnyFunction(*this, Fn);
         if (rewrite.isInvalid()) return ExprError();
         Fn = rewrite.get();
         goto retry;
       }
 
     return ExprError(Diag(LParenLoc, diag::err_typecheck_call_not_function)
       << Fn->getType() << Fn->getSourceRange());
     }
   }
 
   // Get the number of parameters in the function prototype, if any.
   // We will allocate space for max(Args.size(), NumParams) arguments
   // in the call expression.
   const auto *Proto = dyn_cast_or_null<FunctionProtoType>(FuncT);
   unsigned NumParams = Proto ? Proto->getNumParams() : 0;
 
   CallExpr *TheCall;
   if (Config) {
     assert(UsesADL == ADLCallKind::NotADL &&
            "CUDAKernelCallExpr should not use ADL");
     TheCall =
         CUDAKernelCallExpr::Create(Context, Fn, cast<CallExpr>(Config), Args,
ExprResult Sema::ActOnStmtExprResult(ExprResult ER) {
   // result will be +1, and we'll need to balance that out with
   // a bind.
   auto *Cast = dyn_cast<ImplicitCastExpr>(E);
   if (Cast && Cast->getCastKind() == CK_ARCConsumeObject)
     return Cast->getSubExpr();
 
   // FIXME: Provide a better location for the initialization.
   return PerformCopyInitialization(
       InitializedEntity::InitializeStmtExprResult(
           E->getBeginLoc(), E->getType().getUnqualifiedType()),
       SourceLocation(), E);
 }
 
 ExprResult Sema::BuildBuiltinOffsetOf(SourceLocation BuiltinLoc,
                                       TypeSourceInfo *TInfo,
                                       ArrayRef<OffsetOfComponent> Components,
                                       SourceLocation RParenLoc) {
   QualType ArgTy = TInfo->getType();
   bool Dependent = ArgTy->isDependentType();
   SourceRange TypeRange = TInfo->getTypeLoc().getLocalSourceRange();
 
   // We must have at least one component that refers to the type, and the first
   // one is known to be a field designator.  Verify that the ArgTy represents
   // a struct/union/class.
   if (!Dependent && !ArgTy->isRecordType())
     return ExprError(Diag(BuiltinLoc, diag::err_offsetof_record_type)
                        << ArgTy << TypeRange);
 
   // Type must be complete per C99 7.17p3 because a declaring a variable
   // with an incomplete type would be ill-formed.
   if (!Dependent
       && RequireCompleteType(BuiltinLoc, ArgTy,
                              diag::err_offsetof_incomplete_type, TypeRange))
     return ExprError();
 
if (const FunctionProtoType *FT = Type->getAs<FunctionProtoType>()) {
       // RebuildUnknownAnyExpr::VisitCallExpr to vararg functions of unknown
       // type. See the lengthy commentary in that routine.
       QualType FDT = FD->getType();
       const FunctionType *FnType = FDT->castAs<FunctionType>();
       const FunctionProtoType *Proto = dyn_cast_or_null<FunctionProtoType>(FnType);
       DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(E);
       if (DRE && Proto && Proto->getParamTypes().empty() && Proto->isVariadic()) {
         SourceLocation Loc = FD->getLocation();
-        FunctionDecl *NewFD = FunctionDecl::Create(S.Context,
-                                      FD->getDeclContext(),
-                                      Loc, Loc, FD->getNameInfo().getName(),
-                                      DestType, FD->getTypeSourceInfo(),
-                                      SC_None, false/*isInlineSpecified*/,
-                                      FD->hasPrototype(),
-                                      false/*isConstexprSpecified*/);
+        FunctionDecl *NewFD = FunctionDecl::Create(
+            S.Context, FD->getDeclContext(), Loc, Loc,
+            FD->getNameInfo().getName(), DestType, FD->getTypeSourceInfo(),
+            SC_None, false /*isInlineSpecified*/, FD->hasPrototype(),
+            /*ConstexprKind*/ CSK_unspecified);
 
         if (FD->getQualifier())
           NewFD->setQualifierInfo(FD->getQualifierLoc());
 
         SmallVector<ParmVarDecl*, 16> Params;
         for (const auto &AI : FT->param_types()) {
           ParmVarDecl *Param =
             S.BuildParmVarDeclForTypedef(FD, Loc, AI);

clang/lib/Sema/SemaLambda.cpp

Sema::ExpressionEvaluationContextRecord::getMangleNumberingContext(
   if (!MangleNumbering)
     MangleNumbering = Ctx.createMangleNumberingContext();
   return *MangleNumbering;
 }
 
 CXXMethodDecl *Sema::startLambdaDefinition(
     CXXRecordDecl *Class, SourceRange IntroducerRange,
     TypeSourceInfo *MethodTypeInfo, SourceLocation EndLoc,
-    ArrayRef<ParmVarDecl *> Params, const bool IsConstexprSpecified,
+    ArrayRef<ParmVarDecl *> Params, ConstexprSpecKind ConstexprKind,
     Optional<std::pair<unsigned, Decl *>> Mangling) {
   QualType MethodType = MethodTypeInfo->getType();
   TemplateParameterList *TemplateParams =
             getGenericLambdaTemplateParameterList(getCurLambda(), *this);
   // If a lambda appears in a dependent context or is a generic lambda (has
   // template parameters) and has an 'auto' return type, deduce it to a
   // dependent type.
   if (Class->isDependentContext() || TemplateParams) {
CXXMethodDecl *Sema::startLambdaDefinition(
   //   trailing-return-type respectively.
   DeclarationName MethodName
     = Context.DeclarationNames.getCXXOperatorName(OO_Call);
   DeclarationNameLoc MethodNameLoc;
   MethodNameLoc.CXXOperatorName.BeginOpNameLoc
     = IntroducerRange.getBegin().getRawEncoding();
   MethodNameLoc.CXXOperatorName.EndOpNameLoc
     = IntroducerRange.getEnd().getRawEncoding();
-  CXXMethodDecl *Method
-    = CXXMethodDecl::Create(Context, Class, EndLoc,
-                            DeclarationNameInfo(MethodName,
-                                                IntroducerRange.getBegin(),
-                                                MethodNameLoc),
-                            MethodType, MethodTypeInfo,
-                            SC_None,
-                            /*isInline=*/true,
-                            IsConstexprSpecified,
-                            EndLoc);
+  CXXMethodDecl *Method = CXXMethodDecl::Create(
+      Context, Class, EndLoc,
+      DeclarationNameInfo(MethodName, IntroducerRange.getBegin(),
+      MethodType, MethodTypeInfo, SC_None,
+      /*isInline=*/true, ConstexprKind, EndLoc);
   Method->setAccess(AS_public);
 
   // Temporarily set the lexical declaration context to the current
   // context, so that the Scope stack matches the lexical nesting.
   Method->setLexicalDeclContext(CurContext);
   // Create a function template if we have a template parameter list
   FunctionTemplateDecl *const TemplateMethod = TemplateParams ?
             FunctionTemplateDecl::Create(Context, Class,
if (MethodTyInfo->getType()->containsUnexpandedParameterPack())
       ContainsUnexpandedParameterPack = true;
   }
 
   CXXRecordDecl *Class = createLambdaClosureType(Intro.Range, MethodTyInfo,
                                                  KnownDependent, Intro.Default);
 
   CXXMethodDecl *Method =
       startLambdaDefinition(Class, Intro.Range, MethodTyInfo, EndLoc, Params,
-                            ParamInfo.getDeclSpec().isConstexprSpecified());
+                            ParamInfo.getDeclSpec().getConstexprSpecifier());
   if (ExplicitParams)
     CheckCXXDefaultArguments(Method);
 
   // This represents the function body for the lambda function, check if we
   // have to apply optnone due to a pragma.
   AddRangeBasedOptnone(Method);
 
   // code_seg attribute on lambda apply to the method.
for (unsigned I = 0, N = CallOperator->getNumParams(); I != N; ++I) {
     CallOpConvTL.setParam(I, From);
     CallOpConvNameTL.setParam(I, From);
   }
 
   CXXConversionDecl *Conversion = CXXConversionDecl::Create(
       S.Context, Class, Loc,
       DeclarationNameInfo(ConversionName, Loc, ConvNameLoc), ConvTy, ConvTSI,
       /*isInline=*/true, ExplicitSpecifier(),
-      /*isConstexpr=*/S.getLangOpts().CPlusPlus17,
+      S.getLangOpts().CPlusPlus17 ? CSK_constexpr : CSK_unspecified,
       CallOperator->getBody()->getEndLoc());
   Conversion->setAccess(AS_public);
   Conversion->setImplicit(true);
 
   if (Class->isGenericLambda()) {
     // Create a template version of the conversion operator, using the template
     // parameter list of the function call operator.
     FunctionTemplateDecl *TemplateCallOperator =
static void addFunctionPointerConversion(Sema &S,
   // contaminating the type of the invoker, which is currently adjusted
   // in SemaTemplateDeduction.cpp:DeduceTemplateArguments.  Fixing the
   // trailing return type of the invoker would require a visitor to rebuild
   // the trailing return type and adjusting all back DeclRefExpr's to refer
   // to the new static invoker parameters - not the call operator's.
   CXXMethodDecl *Invoke = CXXMethodDecl::Create(
       S.Context, Class, Loc, DeclarationNameInfo(InvokerName, Loc),
       InvokerFunctionTy, CallOperator->getTypeSourceInfo(), SC_Static,
-      /*IsInline=*/true,
-      /*IsConstexpr=*/false, CallOperator->getBody()->getEndLoc());
+      /*IsInline=*/true, CSK_unspecified, CallOperator->getBody()->getEndLoc());
   for (unsigned I = 0, N = CallOperator->getNumParams(); I != N; ++I)
     InvokerParams[I]->setOwningFunction(Invoke);
   Invoke->setParams(InvokerParams);
   Invoke->setAccess(AS_private);
   Invoke->setImplicit(true);
   if (Class->isGenericLambda()) {
     FunctionTemplateDecl *TemplateCallOperator =
             CallOperator->getDescribedFunctionTemplate();
static void addBlockPointerConversion(Sema &S,
   DeclarationName Name
     = S.Context.DeclarationNames.getCXXConversionFunctionName(
         S.Context.getCanonicalType(BlockPtrTy));
   DeclarationNameLoc NameLoc;
   NameLoc.NamedType.TInfo = S.Context.getTrivialTypeSourceInfo(BlockPtrTy, Loc);
   CXXConversionDecl *Conversion = CXXConversionDecl::Create(
       S.Context, Class, Loc, DeclarationNameInfo(Name, Loc, NameLoc), ConvTy,
       S.Context.getTrivialTypeSourceInfo(ConvTy, Loc),
-      /*isInline=*/true, ExplicitSpecifier(),
-      /*isConstexpr=*/false, CallOperator->getBody()->getEndLoc());
+      /*isInline=*/true, ExplicitSpecifier(), CSK_unspecified,
+      CallOperator->getBody()->getEndLoc());
   Conversion->setAccess(AS_public);
   Conversion->setImplicit(true);
   Class->addDecl(Conversion);
 }
 
 ExprResult Sema::BuildCaptureInit(const Capture &Cap,
                                   SourceLocation ImplicitCaptureLoc,
                                   bool IsOpenMPMapping) {
ExprResult Sema::BuildLambdaExpr(SourceLocation StartLoc, SourceLocation EndLoc,
   // If the lambda expression's call operator is not explicitly marked constexpr
   // and we are not in a dependent context, analyze the call operator to infer
   // its constexpr-ness, suppressing diagnostics while doing so.
   if (getLangOpts().CPlusPlus17 && !CallOperator->isInvalidDecl() &&
       !CallOperator->isConstexpr() &&
       !isa<CoroutineBodyStmt>(CallOperator->getBody()) &&
       !Class->getDeclContext()->isDependentContext()) {
     TentativeAnalysisScope DiagnosticScopeGuard(*this);
-    CallOperator->setConstexpr(
-        CheckConstexprFunctionDecl(CallOperator) &&
-        CheckConstexprFunctionBody(CallOperator, CallOperator->getBody()));
+    CallOperator->setConstexprKind(
+        (CheckConstexprFunctionDecl(CallOperator) &&
+         CheckConstexprFunctionBody(CallOperator, CallOperator->getBody()))
+            ? CSK_constexpr
+            : CSK_unspecified);
   }
 
   // Emit delayed shadowing warnings now that the full capture list is known.
   DiagnoseShadowingLambdaDecls(LSI);
 
   if (!CurContext->isDependentContext()) {
     switch (ExprEvalContexts.back().Context) {
     // C++11 [expr.prim.lambda]p2:

clang/lib/Sema/SemaStmt.cpp

if (getLangOpts().CPlusPlus11 && FD && FD->isConstexpr()) {
       // C99 6.8.6.4p1 (ext_ since GCC warns)
       DiagID = diag::ext_return_missing_expr;
     } else {
       // C90 6.6.6.4p4
       DiagID = diag::warn_return_missing_expr;
     }
 
     if (FD)
-      Diag(ReturnLoc, DiagID) << FD->getIdentifier() << 0/*fn*/;
+      Diag(ReturnLoc, DiagID)
+          << FD->getIdentifier() << 0 /*fn*/ << FD->isConsteval();
     else
       Diag(ReturnLoc, DiagID) << getCurMethodDecl()->getDeclName() << 1/*meth*/;
 
     Result = ReturnStmt::Create(Context, ReturnLoc, /* RetExpr=*/nullptr,
                                 /* NRVOCandidate=*/nullptr);
   } else {
     assert(RetValExp || HasDependentReturnType);
     const VarDecl *NRVOCandidate = nullptr;

clang/lib/Sema/SemaTemplate.cpp

auto CheckValidDeclSpecifiers = [this, &D] {
     if (DS.isInlineSpecified())
       EmitDiag(DS.getInlineSpecLoc());
 
     // [dcl.constexpr]p1:
     //   The constexpr specifier shall be applied only to the definition of a
     //   variable or variable template or the declaration of a function or
     //   function template.
 
-    if (DS.isConstexprSpecified())
+    if (DS.hasConstexprSpecifier())
       EmitDiag(DS.getConstexprSpecLoc());
 
     // [dcl.fct.spec]p1:
     //   Function-specifiers can be used only in function declarations.
 
     if (DS.isVirtualSpecified())
       EmitDiag(DS.getVirtualSpecLoc());
 
if (TSK == TSK_Undeclared || TSK == TSK_ImplicitInstantiation) {
     Specialization->setLocation(FD->getLocation());
     Specialization->setLexicalDeclContext(FD->getLexicalDeclContext());
     // C++11 [dcl.constexpr]p1: An explicit specialization of a constexpr
     // function can differ from the template declaration with respect to
     // the constexpr specifier.
     // FIXME: We need an update record for this AST mutation.
     // FIXME: What if there are multiple such prior declarations (for instance,
     // from different modules)?
-    Specialization->setConstexpr(FD->isConstexpr());
+    Specialization->setConstexprKind(FD->getConstexprKind());
   }
 
   // FIXME: Check if the prior specialization has a point of instantiation.
   // If so, we have run afoul of .
 
   // If this is a friend declaration, then we're not really declaring
   // an explicit specialization.
   bool isFriend = (FD->getFriendObjectKind() != Decl::FOK_None);
DeclResult Sema::ActOnExplicitInstantiation(Scope *S,
   // Presumably, this also applies to member functions of class templates as
   // well.
   if (D.getDeclSpec().isInlineSpecified())
     Diag(D.getDeclSpec().getInlineSpecLoc(),
          getLangOpts().CPlusPlus11 ?
            diag::err_explicit_instantiation_inline :
            diag::warn_explicit_instantiation_inline_0x)
       << FixItHint::CreateRemoval(D.getDeclSpec().getInlineSpecLoc());
-  if (D.getDeclSpec().isConstexprSpecified() && R->isFunctionType())
+  if (D.getDeclSpec().hasConstexprSpecifier() && R->isFunctionType())
     // FIXME: Add a fix-it to remove the 'constexpr' and add a 'const' if one is
     // not already specified.
     Diag(D.getDeclSpec().getConstexprSpecLoc(),
          diag::err_explicit_instantiation_constexpr);
 
   // A deduction guide is not on the list of entities that can be explicitly
   // instantiated.
   if (Name.getNameKind() == DeclarationName::CXXDeductionGuideName) {

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

Function = CXXDeductionGuideDecl::Create(
         D->getSourceRange().getEnd());
     if (DGuide->isCopyDeductionCandidate())
       cast<CXXDeductionGuideDecl>(Function)->setIsCopyDeductionCandidate();
     Function->setAccess(D->getAccess());
   } else {
     Function = FunctionDecl::Create(
         SemaRef.Context, DC, D->getInnerLocStart(), NameInfo, T, TInfo,
         D->getCanonicalDecl()->getStorageClass(), D->isInlineSpecified(),
-        D->hasWrittenPrototype(), D->isConstexpr());
+        D->hasWrittenPrototype(), D->getConstexprKind());
     Function->setRangeEnd(D->getSourceRange().getEnd());
   }
 
   if (D->isInlined())
     Function->setImplicitlyInline();
 
   if (QualifierLoc)
     Function->setQualifierInfo(QualifierLoc);
Decl *TemplateDeclInstantiator::VisitCXXMethodDecl(
 
   SourceLocation StartLoc = D->getInnerLocStart();
   DeclarationNameInfo NameInfo
     = SemaRef.SubstDeclarationNameInfo(D->getNameInfo(), TemplateArgs);
   if (CXXConstructorDecl *Constructor = dyn_cast<CXXConstructorDecl>(D)) {
     Method = CXXConstructorDecl::Create(
         SemaRef.Context, Record, StartLoc, NameInfo, T, TInfo,
         InstantiatedExplicitSpecifier, Constructor->isInlineSpecified(), false,
-        Constructor->isConstexpr());
+        Constructor->getConstexprKind());
     Method->setRangeEnd(Constructor->getEndLoc());
   } else if (CXXDestructorDecl *Destructor = dyn_cast<CXXDestructorDecl>(D)) {
     Method = CXXDestructorDecl::Create(SemaRef.Context, Record,
                                        StartLoc, NameInfo, T, TInfo,
                                        Destructor->isInlineSpecified(),
                                        false);
     Method->setRangeEnd(Destructor->getEndLoc());
   } else if (CXXConversionDecl *Conversion = dyn_cast<CXXConversionDecl>(D)) {
     Method = CXXConversionDecl::Create(
         SemaRef.Context, Record, StartLoc, NameInfo, T, TInfo,
         Conversion->isInlineSpecified(), InstantiatedExplicitSpecifier,
-        Conversion->isConstexpr(), Conversion->getEndLoc());
+        Conversion->getConstexprKind(), Conversion->getEndLoc());
   } else {
     StorageClass SC = D->isStatic() ? SC_Static : SC_None;
     Method = CXXMethodDecl::Create(SemaRef.Context, Record, StartLoc, NameInfo,
                                    T, TInfo, SC, D->isInlineSpecified(),
-                                   D->isConstexpr(), D->getEndLoc());
+                                   D->getConstexprKind(), D->getEndLoc());
   }
 
   if (D->isInlined())
     Method->setImplicitlyInline();
 
   if (QualifierLoc)
     Method->setQualifierInfo(QualifierLoc);
 

clang/lib/Sema/SemaType.cpp

   processTypeAttrs(state, T, TAL_DeclName, D.getAttributes());
 
   // Diagnose any ignored type attributes.
   state.diagnoseIgnoredTypeAttrs(T);
 
   // C++0x [dcl.constexpr]p9:
   //  A constexpr specifier used in an object declaration declares the object
   //  as const.
-  if (D.getDeclSpec().isConstexprSpecified() && T->isObjectType()) {
+  if (D.getDeclSpec().hasConstexprSpecifier() && T->isObjectType()) {
     T.addConst();
   }
 
   // If there was an ellipsis in the declarator, the declaration declares a
   // parameter pack whose type may be a pack expansion type.
   if (D.hasEllipsis()) {
     // C++0x [dcl.fct]p13:
     //   A declarator-id or abstract-declarator containing an ellipsis shall

clang/lib/Sema/TreeTransform.h

if (getDerived().ReplacingOriginal())
     Mangling = std::make_pair(OldClass->getLambdaManglingNumber(),
                               OldClass->getLambdaContextDecl());
 
   // Build the call operator.
   CXXMethodDecl *NewCallOperator = getSema().startLambdaDefinition(
       Class, E->getIntroducerRange(), NewCallOpTSI,
       E->getCallOperator()->getEndLoc(),
       NewCallOpTSI->getTypeLoc().castAs<FunctionProtoTypeLoc>().getParams(),
-      E->getCallOperator()->isConstexpr(), Mangling);
+      E->getCallOperator()->getConstexprKind(), Mangling);
 
   LSI->CallOperator = NewCallOperator;
 
   for (unsigned I = 0, NumParams = NewCallOperator->getNumParams();
        I != NumParams; ++I) {
     auto *P = NewCallOperator->getParamDecl(I);
     if (P->hasUninstantiatedDefaultArg()) {
       EnterExpressionEvaluationContext Eval(

clang/lib/Serialization/ASTReaderDecl.cpp

void ASTDeclReader::VisitFunctionDecl(FunctionDecl *FD) {
   FD->setHasInheritedPrototype(Record.readInt());
   FD->setHasWrittenPrototype(Record.readInt());
   FD->setDeletedAsWritten(Record.readInt());
   FD->setTrivial(Record.readInt());
   FD->setTrivialForCall(Record.readInt());
   FD->setDefaulted(Record.readInt());
   FD->setExplicitlyDefaulted(Record.readInt());
   FD->setHasImplicitReturnZero(Record.readInt());
-  FD->setConstexpr(Record.readInt());
+  FD->setConstexprKind(static_cast<ConstexprSpecKind>(Record.readInt()));
   FD->setUsesSEHTry(Record.readInt());
   FD->setHasSkippedBody(Record.readInt());
   FD->setIsMultiVersion(Record.readInt());
   FD->setLateTemplateParsed(Record.readInt());
 
   FD->setCachedLinkage(static_cast<Linkage>(Record.readInt()));
   FD->EndRangeLoc = ReadSourceLocation();
 

clang/test/SemaCXX/cxx2a-compat.cpp

 // expected-error@-8 {{does not refer to a value}}
 // expected-error@-9 {{expected member name or ';'}}
 // expected-error@-10 {{expected ')'}}
 // expected-note@-12 {{declared here}}
 // expected-note@-12 {{to match this '('}}
 #if !defined(__cpp_conditional_explicit) || __cpp_conditional_explicit != 201806L
 #error "the feature test macro __cpp_conditional_explicit isn't correct"
 #endif
+#endif
+
+auto l = []() consteval {};
+int consteval();
+#if __cplusplus <= 201703L
+// expected-warning@-3 {{'consteval' is a keyword in C++2a}}
+// expected-error@-4 {{expected body of lambda expression}}
+#else
+// expected-error@-5 {{expected unqualified-id}}
 #endif
 No newline at end of file

clang/test/SemaCXX/cxx2a-consteval.cpp

+// RUN: %clang_cc1 -std=c++2a -fsyntax-only %s -verify
+
+namespace basic_sema {
+
+consteval int f1(int i) {
+  return i;
+}
+
+consteval constexpr int f2(int i) { 
+  //expected-error@-1 {{cannot combine}}
+  return i;
+}
+
+constexpr auto l_eval = [](int i) consteval {
+
+  return i;
+};
+
+constexpr consteval int f3(int i) {
+  //expected-error@-1 {{cannot combine}}
+  return i;
+}
+
+struct A {
+  consteval int f1(int i) const {
+    return i;
+  }
+  consteval A(int i);
+  consteval A() = default;
+  consteval ~A() = default; // expected-error {{destructor cannot be marked consteval}}
+};
+
+consteval struct B {}; // expected-error {{struct cannot be marked consteval}}
+
+consteval typedef B b; // expected-error {{typedef cannot be consteval}}
+
+consteval int redecl() {return 0;} // expected-note {{previous declaration is here}}
+constexpr int redecl() {return 0;} // expected-error {{constexpr declaration of 'redecl' follows consteval declaration}}
+
+consteval int i = 0; // expected-error {{consteval can only be used in function declarations}}
+
+consteval int; // expected-error {{consteval can only be used in function declarations}}
+
+consteval int f1() {} // expected-error {{no return statement in consteval function}}
+
+struct C {
+  C() {}
+};
+
+struct D {
+  C c;
+  consteval D() = default; // expected-error {{cannot be consteval}}
+};
+}
+
+consteval int main() { // expected-error {{'main' is not allowed to be declared consteval}}
+  return 0;
+}