[clang] Consistently handle consteval constructors for variables. (#144970)

443377a handled simple variable
definitions, but it didn't handle uninitialized variables with a
consteval constructor, and it didn't handle template instantiation.

Fixes #135281 .

Author: efriedma-quic

clang/docs/ReleaseNotes.rst

@@ -920,6 +920,9 @@ Bug Fixes to C++ Support
 - Correctly handle allocations in the condition of a ``if constexpr``.(#GH120197) (#GH134820)
 - Fixed a crash when handling invalid member using-declaration in C++20+ mode. (#GH63254)
 - Fix a crash when trying to instantiate an ambiguous specialization. (#GH51866)
+- Improved handling of variables with ``consteval`` constructors, to
+  consistently treat the initializer as manifestly constant-evaluated.
+  (#GH135281)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Sema/Sema.h

@@ -6761,6 +6761,7 @@ class Sema final : public SemaBase {
       EK_Decltype,
       EK_TemplateArgument,
       EK_AttrArgument,
+      EK_VariableInit,
       EK_Other
     } ExprContext;
 

clang/lib/Parse/ParseDecl.cpp

@@ -2710,6 +2710,7 @@ Decl *Parser::ParseDeclarationAfterDeclaratorAndAttributes(
     break;
   }
   case InitKind::Uninitialized: {
+    InitializerScopeRAII InitScope(*this, D, ThisDecl);
     Actions.ActOnUninitializedDecl(ThisDecl);
     break;
   }

clang/lib/Sema/SemaCoroutine.cpp

@@ -783,7 +783,8 @@ static bool checkSuspensionContext(Sema &S, SourceLocation Loc,
   const auto ExprContext = S.currentEvaluationContext().ExprContext;
   const bool BadContext =
       S.isUnevaluatedContext() ||
-      ExprContext != Sema::ExpressionEvaluationContextRecord::EK_Other;
+      (ExprContext != Sema::ExpressionEvaluationContextRecord::EK_Other &&
+       ExprContext != Sema::ExpressionEvaluationContextRecord::EK_VariableInit);
   if (BadContext) {
     S.Diag(Loc, diag::err_coroutine_unevaluated_context) << Keyword;
     return false;

clang/lib/Sema/SemaDeclCXX.cpp

@@ -18892,7 +18892,8 @@ void Sema::ActOnCXXEnterDeclInitializer(Scope *S, Decl *D) {
     EnterDeclaratorContext(S, D->getDeclContext());
 
   PushExpressionEvaluationContext(
-      ExpressionEvaluationContext::PotentiallyEvaluated, D);
+      ExpressionEvaluationContext::PotentiallyEvaluated, D,
+      ExpressionEvaluationContextRecord::EK_VariableInit);
 }
 
 void Sema::ActOnCXXExitDeclInitializer(Scope *S, Decl *D) {
@@ -18901,29 +18902,6 @@ void Sema::ActOnCXXExitDeclInitializer(Scope *S, Decl *D) {
   if (S && D->isOutOfLine())
     ExitDeclaratorContext(S);
 
-  if (getLangOpts().CPlusPlus23) {
-    // An expression or conversion is 'manifestly constant-evaluated' if it is:
-    // [...]
-    // - the initializer of a variable that is usable in constant expressions or
-    //   has constant initialization.
-    if (auto *VD = dyn_cast<VarDecl>(D);
-        VD && (VD->isUsableInConstantExpressions(Context) ||
-               VD->hasConstantInitialization())) {
-      // An expression or conversion is in an 'immediate function context' if it
-      // is potentially evaluated and either:
-      // [...]
-      // - it is a subexpression of a manifestly constant-evaluated expression
-      //   or conversion.
-      ExprEvalContexts.back().InImmediateFunctionContext = true;
-    }
-  }
-
-  // Unless the initializer is in an immediate function context (as determined
-  // above), this will evaluate all contained immediate function calls as
-  // constant expressions. If the initializer IS an immediate function context,
-  // the initializer has been determined to be a constant expression, and all
-  // such evaluations will be elided (i.e., as if we "knew the whole time" that
-  // it was a constant expression).
   PopExpressionEvaluationContext();
 }
 

clang/lib/Sema/SemaExpr.cpp

@@ -17934,6 +17934,25 @@ HandleImmediateInvocations(Sema &SemaRef,
       Rec.isImmediateFunctionContext() || SemaRef.RebuildingImmediateInvocation)
     return;
 
+  // An expression or conversion is 'manifestly constant-evaluated' if it is:
+  // [...]
+  // - the initializer of a variable that is usable in constant expressions or
+  //   has constant initialization.
+  if (SemaRef.getLangOpts().CPlusPlus23 &&
+      Rec.ExprContext ==
+          Sema::ExpressionEvaluationContextRecord::EK_VariableInit) {
+    auto *VD = cast<VarDecl>(Rec.ManglingContextDecl);
+    if (VD->isUsableInConstantExpressions(SemaRef.Context) ||
+        VD->hasConstantInitialization()) {
+      // An expression or conversion is in an 'immediate function context' if it
+      // is potentially evaluated and either:
+      // [...]
+      // - it is a subexpression of a manifestly constant-evaluated expression
+      //   or conversion.
+      return;
+    }
+  }
+
   /// When we have more than 1 ImmediateInvocationCandidates or previously
   /// failed immediate invocations, we need to check for nested
   /// ImmediateInvocationCandidates in order to avoid duplicate diagnostics.

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -6103,7 +6103,8 @@ void Sema::InstantiateVariableInitializer(
   ContextRAII SwitchContext(*this, Var->getDeclContext());
 
   EnterExpressionEvaluationContext Evaluated(
-      *this, Sema::ExpressionEvaluationContext::PotentiallyEvaluated, Var);
+      *this, Sema::ExpressionEvaluationContext::PotentiallyEvaluated, Var,
+      ExpressionEvaluationContextRecord::EK_VariableInit);
   currentEvaluationContext().InLifetimeExtendingContext =
       parentEvaluationContext().InLifetimeExtendingContext;
   currentEvaluationContext().RebuildDefaultArgOrDefaultInit =

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

@@ -576,3 +576,37 @@ int f() {
   //expected-note@-2 {{read of non-const variable 'a' is not allowed in a constant expression}}
 }
 }
+
+#if __cplusplus >= 202302L
+namespace GH135281 {
+  struct B {
+    const void* p;
+    consteval B() : p{this} {}
+  };
+  B b;
+  B b2{};
+  B &&b3{};
+  void f() {
+    static B b4;
+    B b5; // expected-error {{call to consteval function 'GH135281::B::B' is not a constant expression}} \
+          // expected-note {{pointer to temporary is not a constant expression}} \
+          // expected-note {{temporary created here}}
+  }
+  template<typename T> T temp_var_uninit;
+  template<typename T> T temp_var_brace_init{};
+  B* b6 = &temp_var_uninit<B>;
+  B* b7 = &temp_var_brace_init<B>;
+  B* b8 = &temp_var_brace_init<B&&>;
+  template<typename T> void f2() {
+    static T b9;
+    T b10; // expected-error {{call to consteval function 'GH135281::B::B' is not a constant expression}} \
+           // expected-note {{pointer to temporary is not a constant expression}} \
+           // expected-note {{temporary created here}}
+    static B b11;
+    B b12; // expected-error 2 {{call to consteval function 'GH135281::B::B' is not a constant expression}} \
+           // expected-note 2 {{pointer to temporary is not a constant expression}} \
+           // expected-note 2 {{temporary created here}}
+  }
+  void (*ff)() = f2<B>; // expected-note {{instantiation of function template specialization}}
+}
+#endif