Use declaresSameEntity, added release notes.

Author: dmpolukhin

clang/docs/ReleaseNotes.rst

@@ -689,6 +689,7 @@ Bug Fixes in This Version
 - Fixed type mismatch error when 'builtin-elementwise-math' arguments have different qualifiers, this should be well-formed. (#GH141397)
 - Constant evaluation now correctly runs the destructor of a variable declared in
   the second clause of a C-style ``for`` loop. (#GH139818)
+- Fixed a bug with constexpr evaluation for structs containing unions in case of C++ modules. (#GH143168)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/ExprConstant.cpp

@@ -6836,15 +6836,16 @@ static bool HandleConstructorCall(const Expr *E, const LValue &This,
       // and make sure we've initialized every step along it.
       auto IndirectFieldChain = IFD->chain();
       for (auto *C : IndirectFieldChain) {
-        FD = cast<FieldDecl>(C)->getCanonicalDecl();
+        FD = cast<FieldDecl>(C);
         CXXRecordDecl *CD = cast<CXXRecordDecl>(FD->getParent());
         // Switch the union field if it differs. This happens if we had
         // preceding zero-initialization, and we're now initializing a union
         // subobject other than the first.
         // FIXME: In this case, the values of the other subobjects are
         // specified, since zero-initialization sets all padding bits to zero.
         if (!Value->hasValue() ||
-            (Value->isUnion() && Value->getUnionField() != FD)) {
+            (Value->isUnion() &&
+             !declaresSameEntity(Value->getUnionField(), FD))) {
           if (CD->isUnion())
             *Value = APValue(FD);
           else