[Clang] SubstituteConstraintExpressionWithoutSatisfaction needs an unevaluated context (#123883)

It turns out that the substitution for expression comparing also needs
an unevaluated context, otherwise any reference to immediate functions
might not be properly handled.

As a fallout, this also guards the VLA transformation under unevaluated
context
with `InConditionallyConstantEvaluateContext` to avoid duplicate
diagnostics.

Fixes llvm/llvm-project#123472

---------

Co-authored-by: cor3ntin <[email protected]>

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -968,7 +968,7 @@ Bug Fixes to C++ Support
   constraints are applied. (#GH122134)
 - Fixed canonicalization of pack indexing types - Clang did not always recognized identical pack indexing. (#GH123033)
 - Fixed a nested lambda substitution issue for constraint evaluation. (#GH123441)
-
+- Fixed various false diagnostics related to the use of immediate functions. (#GH123472)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Parse/ParseExpr.cpp

@@ -248,6 +248,25 @@ ExprResult Parser::ParseArrayBoundExpression() {
   // If we parse the bound of a VLA... we parse a non-constant
   // constant-expression!
   Actions.ExprEvalContexts.back().InConditionallyConstantEvaluateContext = true;
+  // For a VLA type inside an unevaluated operator like:
+  //
+  //   sizeof(typeof(*(int (*)[N])array))
+  //
+  // N and array are supposed to be ODR-used.
+  // Initially when encountering `array`, it is deemed unevaluated and non-ODR
+  // used because that occurs before parsing the type cast. Therefore we use
+  // Sema::TransformToPotentiallyEvaluated() to rebuild the expression to ensure
+  // it's actually ODR-used.
+  //
+  // However, in other unevaluated contexts as in constraint substitution, it
+  // would end up rebuilding the type twice which is unnecessary. So we push up
+  // a flag to help distinguish these cases.
+  for (auto Iter = Actions.ExprEvalContexts.rbegin() + 1;
+       Iter != Actions.ExprEvalContexts.rend(); ++Iter) {
+    if (!Iter->isUnevaluated())
+      break;
+    Iter->InConditionallyConstantEvaluateContext = true;
+  }
   return ParseConstantExpressionInExprEvalContext(NotTypeCast);
 }
 

clang/lib/Sema/SemaConcept.cpp

@@ -1027,6 +1027,9 @@ static const Expr *SubstituteConstraintExpressionWithoutSatisfaction(
     ContextScope.emplace(S, const_cast<DeclContext *>(cast<DeclContext>(RD)),
                          /*NewThisContext=*/false);
   }
+  EnterExpressionEvaluationContext UnevaluatedContext(
+      S, Sema::ExpressionEvaluationContext::Unevaluated,
+      Sema::ReuseLambdaContextDecl);
   ExprResult SubstConstr = S.SubstConstraintExprWithoutSatisfaction(
       const_cast<clang::Expr *>(ConstrExpr), MLTAL);
   if (SFINAE.hasErrorOccurred() || !SubstConstr.isUsable())

clang/lib/Sema/SemaExpr.cpp

@@ -4630,8 +4630,9 @@ ExprResult Sema::CreateUnaryExprOrTypeTraitExpr(TypeSourceInfo *TInfo,
 
   // Adds overload of TransformToPotentiallyEvaluated for TypeSourceInfo to
   // properly deal with VLAs in nested calls of sizeof and typeof.
-  if (isUnevaluatedContext() && ExprKind == UETT_SizeOf &&
-      TInfo->getType()->isVariablyModifiedType())
+  if (currentEvaluationContext().isUnevaluated() &&
+      currentEvaluationContext().InConditionallyConstantEvaluateContext &&
+      ExprKind == UETT_SizeOf && TInfo->getType()->isVariablyModifiedType())
     TInfo = TransformToPotentiallyEvaluated(TInfo);
 
   // C99 6.5.3.4p4: the type (an unsigned integer type) is size_t.

clang/test/SemaTemplate/concepts-out-of-line-def.cpp

@@ -737,3 +737,17 @@ ptr<U> make_item(auto &&args)
 ptr<char> p;
 
 } // namespace GH114685
+
+namespace GH123472 {
+
+consteval bool fn() { return true; }
+
+struct S {
+  template <typename T>
+  static consteval void mfn() requires (bool(&fn));
+};
+
+template <typename T>
+consteval void S::mfn() requires (bool(&fn)) {}
+
+}