[C++20][Modules] Fix false compilation error with constexpr (#143168)

Use declaresSameEntity when evaluating constexpr to avoid resetting
computed union value due to using different instances of the merged
field decl.

Author: dmpolukhin

clang/docs/ReleaseNotes.rst

@@ -693,6 +693,7 @@ 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

@@ -6844,7 +6844,8 @@ static bool HandleConstructorCall(const Expr *E, const LValue &This,
         // 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

clang/test/Modules/constexpr-initialization-failure.cpp

@@ -0,0 +1,44 @@
+// RUN: rm -fR %t
+// RUN: split-file %s %t
+// RUN: cd %t
+// RUN: %clang_cc1 -verify -w -std=c++20 -fmodule-name=h1.h -emit-header-unit -xc++-user-header h1.h -o h1.pcm
+// RUN: %clang_cc1 -verify -w -std=c++20 -fmodule-map-file=module.modulemap -fmodule-file=h1.h=h1.pcm main.cpp -o main.o
+
+//--- module.modulemap
+module "h1.h" {
+  header "h1.h"
+  export *
+}
+
+//--- h0.h
+// expected-no-diagnostics
+#pragma once
+
+template <typename T> struct A {
+  union {
+    struct {
+      T x, y, z;
+    };
+  };
+  constexpr A(T, T, T) : x(), y(), z() {}
+};
+typedef A<float> packed_vec3;
+
+//--- h1.h
+// expected-no-diagnostics
+#pragma once
+
+#include "h0.h"
+
+constexpr packed_vec3 kMessThingsUp = packed_vec3(5.0f, 5.0f, 5.0f);
+
+//--- main.cpp
+// expected-no-diagnostics
+#include "h0.h"
+
+static_assert(sizeof(packed_vec3) == sizeof(float) * 3);
+static_assert(alignof(packed_vec3) == sizeof(float));
+
+import "h1.h";
+
+constexpr packed_vec3 kDefaultHalfExtents = packed_vec3(5.0f, 5.0f, 5.0f);