[clang] Allow class with anonymous union member to be const-default-constructible even if a union member has a default member initializer (#95854)

Resolves #95854

  Clang incorrectly considers a class with an anonymous union member to not be
  const-default-constructible even if a union member has a default member initializer.

```
struct A {
  union {
    int n = 0;
    int m;
  };
};
const A a;
```

-- As per https://eel.is/c++draft/dcl.init#general-8.3

Author: Rajveer100

clang/docs/ReleaseNotes.rst

@@ -217,6 +217,10 @@ Bug Fixes to C++ Support
 - Clang now preserves the unexpanded flag in a lambda transform used for pack expansion. (#GH56852), (#GH85667),
   (#GH99877).
 - Fixed a bug when diagnosing ambiguous explicit specializations of constrained member functions.
+- Clang incorrectly considers a class with an anonymous union member to not be
+  const-default-constructible even if a union member has a default member initializer.
+  This is valid as per ``8.3`` in `Draft C++ Standard <https://eel.is/c++draft/dcl.init#general-8.3>`_.
+  (#GH95854).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/DeclCXX.cpp

@@ -1057,6 +1057,9 @@ void CXXRecordDecl::addedMember(Decl *D) {
     if (isUnion() && !Field->isAnonymousStructOrUnion())
       data().HasVariantMembers = true;
 
+    if (isUnion() && IsFirstField)
+      data().HasUninitializedFields = true;
+
     // C++0x [class]p9:
     //   A POD struct is a class that is both a trivial class and a
     //   standard-layout class, and has no non-static data members of type
@@ -1125,7 +1128,11 @@ void CXXRecordDecl::addedMember(Decl *D) {
         data().DefaultedCopyConstructorIsDeleted = true;
     }
 
-    if (!Field->hasInClassInitializer() && !Field->isMutable()) {
+    if (isUnion() && !Field->isMutable()) {
+      if (Field->hasInClassInitializer()) {
+        data().HasUninitializedFields = false;
+      }
+    } else if (!Field->hasInClassInitializer() && !Field->isMutable()) {
       if (CXXRecordDecl *FieldType = T->getAsCXXRecordDecl()) {
         if (FieldType->hasDefinition() && !FieldType->allowConstDefaultInit())
           data().HasUninitializedFields = true;

clang/test/SemaCXX/GH95854.cpp

@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify %s
+
+struct A {
+  union {
+    int n = 0;
+    int m;
+  };
+};
+const A a;
+
+struct B {
+  union {
+    struct {
+      int n = 5;
+      int m;
+    };
+  };
+};
+const B b; // expected-error {{default initialization of an object of const type 'const B' without a user-provided default constructor}}