[Clang] Add support for [[msvc::constexpr]] C++11 attribute

[RIscRIpt]

Summary

I apologize for spam. I meant to submit this patch to https://reviews.llvm.org/D134475

I am looking how to delete this.

[RIscRIpt]

This had to be submitted to https://reviews.llvm.org/D134475

clang/include/clang/Basic/Attr.td

def Pointer : InheritableAttr {
   let Subjects = SubjectList<[Struct]>;
   let Args = [TypeArgument<"DerefType", /*opt=*/1>];
   let Documentation = [LifetimePointerDocs];
 }
 def : MutualExclusions<[Owner, Pointer]>;
 
 // Microsoft-related attributes
 
+def MSConstexpr : InheritableAttr {
+  let LangOpts = [MicrosoftExt];
+  let Spellings = [CXX11<"msvc", "constexpr">];
+  let Subjects = SubjectList<[Function, Stmt], ErrorDiag,
+                             "functions and statements">;
+  let Documentation = [MSConstexprDocs];
+}
+
 def MSNoVTable : InheritableAttr, TargetSpecificAttr<TargetMicrosoftCXXABI> {
   let Spellings = [Declspec<"novtable">];
   let Subjects = SubjectList<[CXXRecord]>;
   let Documentation = [MSNoVTableDocs];
   let SimpleHandler = 1;
 }
 
 def : IgnoredAttr {

clang/include/clang/Basic/AttrDocs.td

 .. code-block:: c++
 
   struct __multiple_inheritance S; // error: inheritance model does not match definition
   int S::*i;
   struct S {};
 }];
 }
 
+def MSConstexprDocs : Documentation {
+  let Category = DocCatStmt;
+  let Content = [{
+The ``[[msvc::constexpr]]`` attribute can be applied only to a function
+definition or a return statement. It has no effect on function declarations.
+This attribute permits constant evaluation of ``[[msvc::constexpr]]`` functions
+in ``[[msvc::constexpr]] return`` statements inside ``constexpr`` or
+``[[msvc::constexpr]]`` functions.
+
+This attribute is available only as Microsoft extension (``-fms-extensions``).
+  }];
+}
+
 def MSNoVTableDocs : Documentation {
   let Category = DocCatDecl;
   let Content = [{
 This attribute can be added to a class declaration or definition to signal to
 the compiler that constructors and destructors will not reference the virtual
 function table. It is only supported when using the Microsoft C++ ABI.
   }];
 }

clang/include/clang/Basic/DiagnosticASTKinds.td

def note_constexpr_delete_base_nonvirt_dtor : Note<
   "base class type %0 with non-virtual destructor">;
 def note_constexpr_memory_leak : Note<
   "allocation performed here was not deallocated"
   "%plural{0:|: (along with %0 other memory leak%s0)}0">;
 def note_constexpr_unsupported_layout : Note<
   "type %0 has unexpected layout">;
 def note_constexpr_unsupported_flexible_array : Note<
   "flexible array initialization is not yet supported">;
+def note_ms_constexpr_invalid_context : Note<
+  "[[msvc::constexpr]] %select{function|constructor}0 %1 is used not in "
+  "'[[msvc::constexpr]] return' statement">;
 def err_experimental_clang_interp_failed : Error<
   "the experimental clang interpreter failed to evaluate an expression">;
 
 def warn_integer_constant_overflow : Warning<
   "overflow in expression; result is %0 with type %1">,
   InGroup<DiagGroup<"integer-overflow">>;
 def warn_fixedpoint_constant_overflow : Warning<
   "overflow in expression; result is %0 with type %1">,

clang/include/clang/Basic/DiagnosticSemaKinds.td

 def ext_constexpr_local_var_no_init : ExtWarn<
   "uninitialized variable in a constexpr %select{function|constructor}0 "
   "is a C++20 extension">, InGroup<CXX20>;
 def warn_cxx17_compat_constexpr_local_var_no_init : Warning<
   "uninitialized variable in a constexpr %select{function|constructor}0 "
   "is incompatible with C++ standards before C++20">,
   InGroup<CXXPre20Compat>, DefaultIgnore;
 def ext_constexpr_function_never_constant_expr : ExtWarn<
-  "%select{constexpr|consteval}1 %select{function|constructor}0 never produces a "
-  "constant expression">, InGroup<DiagGroup<"invalid-constexpr">>, DefaultError;
+  "%select{constexpr|consteval|[[msvc::constexpr]]}1 %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 %select{constexpr|consteval}0 function">;
 def err_constexpr_return_missing_expr : Error<
   "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 "
   "is incompatible with C++ standards before C++14">,
   InGroup<CXXPre14Compat>, DefaultIgnore;
 def note_constexpr_body_previous_return : Note<
   "previous return statement is here">;
+def err_ms_constexpr_not_distinct : Error<
+  "[[msvc::constexpr]] cannot be applied to a %select{constexpr|consteval}0 function %1">;
+def warn_ms_constexpr_no_effect : Warning<
+  "[[msvc::constexpr]] has effect only on function definitions and return statements">,
+  InGroup<IgnoredAttributes>;
 
 // C++20 function try blocks in constexpr
 def ext_constexpr_function_try_block_cxx20 : ExtWarn<
   "function try block in constexpr %select{function|constructor}0 is "
   "a C++20 extension">, InGroup<CXX20>;
 def warn_cxx17_compat_constexpr_function_try_block : Warning<
   "function try block in constexpr %select{function|constructor}0 is "
   "incompatible with C++ standards before C++20">,

clang/lib/AST/ExprConstant.cpp

public:
 
     bool isStdFunction() const {
       for (const DeclContext *DC = Callee; DC; DC = DC->getParent())
         if (DC->isStdNamespace())
           return true;
       return false;
     }
 
+    /// Whether we're in a context where [[msvc::constexpr]] evaluation is
+    /// permitted. See MSConstexprDocs for description of permitted contexts.
+    bool CanEvalMSConstexpr = false;
+
   private:
     APValue &createLocal(APValue::LValueBase Base, const void *Key, QualType T,
                          ScopeKind Scope);
   };
 
   /// Temporarily override 'this'.
   class ThisOverrideRAII {
   public:
public:
     ExprTimeTraceScope(const Expr *E, const ASTContext &Ctx, StringRef Name)
         : TimeScope(Name, [E, &Ctx] {
             return E->getSourceRange().printToString(Ctx.getSourceManager());
           }) {}
 
   private:
     llvm::TimeTraceScope TimeScope;
   };
+
+  /// RAII object used to change the current ability of
+  /// [[msvc::constexpr]] evaulation.
+  struct MSConstexprContextRAII {
+    CallStackFrame &Frame;
+    bool OldValue;
+    explicit MSConstexprContextRAII(CallStackFrame &Frame, bool Value)
+        : Frame(Frame), OldValue(Frame.CanEvalMSConstexpr) {
+      Frame.CanEvalMSConstexpr = Value;
+    }
+
+    ~MSConstexprContextRAII() { Frame.CanEvalMSConstexpr = OldValue; }
+  };
 }
 
 static bool HandleDestruction(EvalInfo &Info, const Expr *E,
                               const LValue &This, QualType ThisType);
 static bool HandleDestruction(EvalInfo &Info, SourceLocation Loc,
                               APValue::LValueBase LVBase, APValue &Value,
                               QualType T);
 
case Stmt::ContinueStmtClass:
     return ESR_Continue;
 
   case Stmt::BreakStmtClass:
     return ESR_Break;
 
   case Stmt::LabelStmtClass:
     return EvaluateStmt(Result, Info, cast<LabelStmt>(S)->getSubStmt(), Case);
 
-  case Stmt::AttributedStmtClass:
-    // As a general principle, C++11 attributes can be ignored without
-    // any semantic impact.
-    return EvaluateStmt(Result, Info, cast<AttributedStmt>(S)->getSubStmt(),
-                        Case);
+  case Stmt::AttributedStmtClass: {
+    const auto *AS = cast<AttributedStmt>(S);
+    const auto *SS = AS->getSubStmt();
+    MSConstexprContextRAII msConstexprContext(
+        *Info.CurrentCall,
+        Info.getLangOpts().MicrosoftExt &&
+            hasSpecificAttr<MSConstexprAttr>(AS->getAttrs()) &&
+            isa<ReturnStmt>(SS));
+    return EvaluateStmt(Result, Info, SS, Case);
+  }
 
   case Stmt::CaseStmtClass:
   case Stmt::DefaultStmtClass:
     return EvaluateStmt(Result, Info, cast<SwitchCase>(S)->getSubStmt(), Case);
   case Stmt::CXXTryStmtClass:
     // Evaluate try blocks by evaluating all sub statements.
     return EvaluateStmt(Result, Info, cast<CXXTryStmt>(S)->getTryBlock(), Case);
   }
if (!Info.Ctx.getLangOpts().CPlusPlus20 && 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 && Body) {
+    if (Definition->isConstexpr())
       return true;
 
+    bool canEvalMSConstexpr = Info.CurrentCall->CanEvalMSConstexpr;
+    bool isMSConstexpr = Definition->hasAttr<MSConstexprAttr>();
+    if (canEvalMSConstexpr && isMSConstexpr)
+      return true;
+
+    if (canEvalMSConstexpr || isMSConstexpr) {
+      // Diagnose invalid usage of [[msvc::constexpr]] function
+      bool isConstructor = isa<CXXConstructorDecl>(Definition);
+      if (canEvalMSConstexpr) { // !isMSConstexpr
+        Info.FFDiag(CallLoc, diag::note_constexpr_invalid_function, 1)
+            << /*IsConstexpr*/ 0 << isConstructor << Definition;
+        Info.Note(Definition->getLocation(), diag::note_declared_at);
+      } else if (isMSConstexpr) { // !canEvalMSConstexpr
+        Info.FFDiag(CallLoc, diag::note_ms_constexpr_invalid_context)
+            << isConstructor << Definition;
+      }
+      return false;
+    }
+  }
+
   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();
bool PointerExprEvaluator::VisitCXXNewExpr(const CXXNewExpr *E) {
   // We cannot speculatively evaluate a delete expression.
   if (Info.SpeculativeEvaluationDepth)
     return false;
 
   FunctionDecl *OperatorNew = E->getOperatorNew();
 
   bool IsNothrow = false;
   bool IsPlacement = false;
+  bool IsMSConstexpr = Info.CurrentCall->CanEvalMSConstexpr &&
+                       OperatorNew->hasAttr<MSConstexprAttr>();
   if (OperatorNew->isReservedGlobalPlacementOperator() &&
-      Info.CurrentCall->isStdFunction() && !E->isArray()) {
+      (Info.CurrentCall->isStdFunction() || IsMSConstexpr) && !E->isArray()) {
     // FIXME Support array placement new.
     assert(E->getNumPlacementArgs() == 1);
     if (!EvaluatePointer(E->getPlacementArg(0), Result, Info))
       return false;
     if (Result.Designator.Invalid)
       return false;
     IsPlacement = true;
   } else if (!OperatorNew->isReplaceableGlobalAllocationFunction()) {

clang/lib/Sema/SemaDecl.cpp

if (Body && (!FD || !FD->isDefaulted())) {
         DiscardCleanupsInEvaluationContext();
       }
       if (!hasUncompilableErrorOccurred() && !isa<FunctionTemplateDecl>(dcl)) {
         // Since the body is valid, issue any analysis-based warnings that are
         // enabled.
         ActivePolicy = &WP;
       }
 
-      if (!IsInstantiation && FD && FD->isConstexpr() && !FD->isInvalidDecl() &&
+      if (!IsInstantiation && FD &&
+          (FD->isConstexpr() || FD->hasAttr<MSConstexprAttr>()) &&
+          !FD->isInvalidDecl() &&
           !CheckConstexprFunctionDefinition(FD, CheckConstexprKind::Diagnose))
         FD->setInvalidDecl();
 
       if (FD && FD->hasAttr<NakedAttr>()) {
         for (const Stmt *S : Body->children()) {
           // Allow local register variables without initializer as they don't
           // require prologue.
           bool RegisterVariables = false;

clang/lib/Sema/SemaDeclAttr.cpp

static void handleDeclspecThreadAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   }
   if (VD->hasLocalStorage()) {
     S.Diag(AL.getLoc(), diag::err_thread_non_global) << "__declspec(thread)";
     return;
   }
   D->addAttr(::new (S.Context) ThreadAttr(S.Context, AL));
 }
 
+static void handleMSConstexprAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+  auto *FD = cast<FunctionDecl>(D);
+  if (FD->isConstexprSpecified() || FD->isConsteval()) {
+    S.Diag(AL.getLoc(), diag::err_ms_constexpr_not_distinct)
+        << FD->isConsteval() << FD;
+    return;
+  }
+  D->addAttr(::new (S.Context) MSConstexprAttr(S.Context, AL));
+}
+
 static void handleAbiTagAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   SmallVector<StringRef, 4> Tags;
   for (unsigned I = 0, E = AL.getNumArgs(); I != E; ++I) {
     StringRef Tag;
     if (!S.checkStringLiteralArgumentAttr(AL, I, Tag))
       return;
     Tags.push_back(Tag);
   }
case ParsedAttr::AT_Uuid:
     handleUuidAttr(S, D, AL);
     break;
   case ParsedAttr::AT_MSInheritance:
     handleMSInheritanceAttr(S, D, AL);
     break;
   case ParsedAttr::AT_Thread:
     handleDeclspecThreadAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_MSConstexpr:
+    handleMSConstexprAttr(S, D, AL);
+    break;
 
   // HLSL attributes:
   case ParsedAttr::AT_HLSLNumThreads:
     handleHLSLNumThreadsAttr(S, D, AL);
     break;
   case ParsedAttr::AT_HLSLSV_GroupIndex:
     handleHLSLSVGroupIndexAttr(S, D, AL);
     break;

clang/lib/Sema/SemaDeclCXX.cpp

static bool CheckConstexprFunctionBody(Sema &SemaRef, const FunctionDecl *Dcl,
   //   - every constructor involved in initializing non-static data members and
   //     base class sub-objects shall be a constexpr constructor.
   //
   // Note that this rule is distinct from the "requirements for a constexpr
   // function", so is not checked in CheckValid mode.
   SmallVector<PartialDiagnosticAt, 8> Diags;
   if (Kind == Sema::CheckConstexprKind::Diagnose &&
       !Expr::isPotentialConstantExpr(Dcl, Diags)) {
+    int ConstKind = Dcl->hasAttr<MSConstexprAttr>() ? 2
+                    : Dcl->isConsteval()            ? 1
+                                                    : 0;
     SemaRef.Diag(Dcl->getLocation(),
                  diag::ext_constexpr_function_never_constant_expr)
-        << isa<CXXConstructorDecl>(Dcl) << Dcl->isConsteval();
+        << isa<CXXConstructorDecl>(Dcl) << ConstKind;
     for (size_t I = 0, N = Diags.size(); I != N; ++I)
       SemaRef.Diag(Diags[I].first, Diags[I].second);
     // Don't return false here: we allow this for compatibility in
     // system headers.
   }
 
   return true;
 }

clang/lib/Sema/SemaStmtAttr.cpp

static Attr *handleUnlikely(Sema &S, Stmt *St, const ParsedAttr &A,
                             SourceRange Range) {
 
   if (!S.getLangOpts().CPlusPlus20 && A.isCXX11Attribute() && !A.getScopeName())
     S.Diag(A.getLoc(), diag::ext_cxx20_attr) << A << Range;
 
   return ::new (S.Context) UnlikelyAttr(S.Context, A);
 }
 
+static Attr *handleMSConstexprAttr(Sema &S, Stmt *St, const ParsedAttr &A,
+                                   SourceRange Range) {
+  if (!isa<ReturnStmt>(St))
+    S.Diag(A.getLoc(), diag::warn_ms_constexpr_no_effect);
+  return ::new (S.Context) MSConstexprAttr(S.Context, A);
+}
+
 #define WANT_STMT_MERGE_LOGIC
 #include "clang/Sema/AttrParsedAttrImpl.inc"
 #undef WANT_STMT_MERGE_LOGIC
 
 static void
 CheckForIncompatibleAttributes(Sema &S,
                                const SmallVectorImpl<const Attr *> &Attrs) {
   // The vast majority of attributed statements will only have one attribute
static Attr *ProcessStmtAttribute(Sema &S, Stmt *St, const ParsedAttr &A,
   case ParsedAttr::AT_NoInline:
     return handleNoInlineAttr(S, St, A, Range);
   case ParsedAttr::AT_MustTail:
     return handleMustTailAttr(S, St, A, Range);
   case ParsedAttr::AT_Likely:
     return handleLikely(S, St, A, Range);
   case ParsedAttr::AT_Unlikely:
     return handleUnlikely(S, St, A, Range);
+  case ParsedAttr::AT_MSConstexpr:
+    return handleMSConstexprAttr(S, St, A, Range);
   default:
     // N.B., ClangAttrEmitter.cpp emits a diagnostic helper that ensures a
     // declaration attribute is not written on a statement, but this code is
     // needed for attributes in Attr.td that do not list any subjects.
     S.Diag(A.getRange().getBegin(), diag::err_decl_attribute_invalid_on_stmt)
         << A << St->getBeginLoc();
     return nullptr;
   }

clang/test/AST/msvc-attrs-invalid.cpp

+// RUN: %clang_cc1 -fms-extensions -std=c++20 -verify %s
+
+// Check explicitly invalid code
+
+void runtime() {} // expected-note {{declared here}}
+
+[[msvc::constexpr]] void f0() { runtime(); } // expected-error {{[[msvc::constexpr]] function never produces a constant expression}} \
+                                             // expected-note {{non-constexpr function 'runtime' cannot be used in a constant expression}}
+[[msvc::constexpr]] constexpr void f1() {} // expected-error {{[[msvc::constexpr]] cannot be applied to a constexpr function 'f1'}}
+[[msvc::constexpr]] consteval void f2() {} // expected-error {{[[msvc::constexpr]] cannot be applied to a consteval function 'f2'}}
+
+// Check invalid code mixed with valid code
+
+[[msvc::constexpr]] bool undefined();
+
+[[msvc::constexpr]] int f4(int x) { return x > 1 ? 1 + f4(x / 2) : 0; } // expected-note {{[[msvc::constexpr]] function 'f4' is used not in '[[msvc::constexpr]] return' statement}}
+constexpr bool f5() { [[msvc::constexpr]] return f4(32) == 5; } // expected-note {{in call to 'f4(32)'}}
+static_assert(f5()); // expected-error {{static assertion expression is not an integral constant expression}} \
+                     // expected-note {{in call to 'f5()'}}
+
+int f6(int x) { [[msvc::constexpr]] return x > 1 ? 1 + f6(x / 2) : 0; } // expected-note {{declared here}} \
+                                                                        // expected-note {{declared here}}
+constexpr bool f7() { [[msvc::constexpr]] return f6(32) == 5; } // expected-error {{constexpr function never produces a constant expression}} \
+                                                                // expected-note {{non-constexpr function 'f6' cannot be used in a constant expression}} \
+                                                                // expected-note {{non-constexpr function 'f6' cannot be used in a constant expression}}
+static_assert(f7()); // expected-error {{static assertion expression is not an integral constant expression}} \
+                     // expected-note {{in call to 'f7()'}}
+
+constexpr bool f8() { // expected-error {{constexpr function never produces a constant expression}}
+    [[msvc::constexpr]] f4(32); // expected-warning {{[[msvc::constexpr]] has effect only on function definitions and return statements}} \
+                                // expected-note {{[[msvc::constexpr]] function 'f4' is used not in '[[msvc::constexpr]] return' statement}} \
+                                // expected-note {{[[msvc::constexpr]] function 'f4' is used not in '[[msvc::constexpr]] return' statement}}
+    [[msvc::constexpr]] int i5 = f4(32); // expected-error {{'constexpr' attribute only applies to functions and statements}}
+    return i5 == 5;
+}
+static_assert(f8()); // expected-error {{static assertion expression is not an integral constant expression}} \
+                     // expected-note {{in call to 'f8()'}}

clang/test/AST/msvc-attrs.cpp

+// RUN: %clang_cc1 -fms-extensions -std=c++20 -ast-dump %s | FileCheck %s
+
+// [[msvc::constexpr]] tests
+
+// MSConstexprDocs (1)
+// CHECK: used f1 'bool ()'
+// CHECK: MSConstexprAttr 0x{{[0-9a-f]+}} <col:3, col:9>
+[[msvc::constexpr]] bool f1() { return true; }
+
+// MSConstexprDocs (2)
+// CHECK: used constexpr f2 'bool ()'
+// CHECK: AttributedStmt 0x{{[0-9a-f]+}} <col:23, col:53>
+// CHECK-NEXT: MSConstexprAttr 0x{{[0-9a-f]+}} <col:25, col:31>
+// CHECK-NEXT: ReturnStmt 0x{{[0-9a-f]+}} <col:43, col:53>
+constexpr bool f2() { [[msvc::constexpr]] return f1(); }
+
+static_assert(f2());
+
+constexpr bool f3() { [[msvc::constexpr]] return f1() || f1() && f1(); }
+static_assert(f3());
+
+[[msvc::constexpr]] int f4(int x) { [[msvc::constexpr]] return x > 1 ? 1 + f4(x / 2) : 0; }
+constexpr bool f5() { [[msvc::constexpr]] return f4(32) == 5; }
+static_assert(f5());
+
+[[msvc::constexpr]] int f6(int x)
+{
+    switch (x)
+    {
+        case 42: return 1337;
+        default:
+                 if (x < 0) [[msvc::constexpr]] return f4(-x);
+                 else return x;
+    }
+}
+
+constexpr bool f7() { [[msvc::constexpr]] return f6(f6(42) - 1337 + f6(-32) - 5 + (f6(1) ? f6(0) : f6(2))) == f6(0); }
+static_assert(f7());

clang/test/AST/msvc-constexpr-new.cpp

+// RUN: %clang_cc1 -fms-extensions -std=c++20 -ast-dump %s | FileCheck %s
+
+// CHECK: used operator new
+// CHECK: MSConstexprAttr 0x{{[0-9a-f]+}} <col:17, col:23>
+[[nodiscard]] [[msvc::constexpr]] inline void* __cdecl operator new(decltype(sizeof(void*)), void* p) noexcept { return p; }
+
+// CHECK: used constexpr construct_at
+// CHECK: AttributedStmt 0x{{[0-9a-f]+}} <col:46, col:88>
+// CHECK-NEXT: MSConstexprAttr 0x{{[0-9a-f]+}} <col:48, col:54>
+// CHECK-NEXT: ReturnStmt 0x{{[0-9a-f]+}} <col:66, col:88>
+constexpr int* construct_at(int* p, int v) { [[msvc::constexpr]] return ::new (p) int(v); }
+
+constexpr bool check_construct_at() { int x; return *construct_at(&x, 42) == 42; }
+
+static_assert(check_construct_at());

clang/test/Misc/pragma-attribute-supported-attributes-list.test

 // CHECK-NEXT: IFunc (SubjectMatchRule_function)
 // CHECK-NEXT: InitPriority (SubjectMatchRule_variable)
 // CHECK-NEXT: InternalLinkage (SubjectMatchRule_variable, SubjectMatchRule_function, SubjectMatchRule_record)
 // CHECK-NEXT: LTOVisibilityPublic (SubjectMatchRule_record)
 // CHECK-NEXT: Leaf (SubjectMatchRule_function)
 // CHECK-NEXT: LoaderUninitialized (SubjectMatchRule_variable_is_global)
 // CHECK-NEXT: Lockable (SubjectMatchRule_record)
 // CHECK-NEXT: MIGServerRoutine (SubjectMatchRule_function, SubjectMatchRule_objc_method, SubjectMatchRule_block)
+// CHECK-NEXT: MSConstexpr (SubjectMatchRule_function)
 // CHECK-NEXT: MSStruct (SubjectMatchRule_record)
 // CHECK-NEXT: MaybeUndef (SubjectMatchRule_variable_is_parameter)
 // CHECK-NEXT: MicroMips (SubjectMatchRule_function)
 // CHECK-NEXT: MinSize (SubjectMatchRule_function, SubjectMatchRule_objc_method)
 // CHECK-NEXT: MinVectorWidth (SubjectMatchRule_function)
 // CHECK-NEXT: Mips16 (SubjectMatchRule_function)
 // CHECK-NEXT: MipsLongCall (SubjectMatchRule_function)
 // CHECK-NEXT: MipsShortCall (SubjectMatchRule_function)