Fix a VLA diagnostic regression - BuildArrayType() is doubly called under unevaluated context

Author: zyn0217

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))
+  //
+  // in which the expression N is supposed to be ODR-used, as is the `array`.
+  // 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/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.