[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.
  This is valid as per ``8.3`` in `Draft C++ Standard <https://eel.is/c++draft/dcl.init#general-8.3>`_.
  The ``allowConstDefaultInit`` member function in ``CXXRecordDecl`` needs
  special-case unions to handle the rule. (#GH95854).

```
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

@@ -624,6 +624,12 @@ Bug Fixes in This Version
 
 - ``__is_array`` and ``__is_bounded_array`` no longer return ``true`` for
   zero-sized arrays. Fixes (#GH54705).
+  
+- 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>`_.
+  The ``allowConstDefaultInit`` member function in ``CXXRecordDecl`` needs 
+  special-case unions to handle the rule. (#GH95854).
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/AST/DeclCXX.h

@@ -1392,7 +1392,9 @@ class CXXRecordDecl : public RecordDecl {
   bool allowConstDefaultInit() const {
     return !data().HasUninitializedFields ||
            !(data().HasDefaultedDefaultConstructor ||
-             needsImplicitDefaultConstructor());
+             needsImplicitDefaultConstructor()) ||
+           (!hasUninitializedReferenceMember() && isUnion() &&
+            hasInClassInitializer());
   }
 
   /// Determine whether this class has a destructor which has no

clang/test/SemaCXX/GH95854.cpp

@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify %s
+//
+// expected-no-diagnostics
+
+struct A {
+  union {
+    int n = 0;
+    int m;
+  };
+};
+const A a;
+
+struct B {
+  union {
+    struct {
+      int n = 5;
+      int m;
+    };
+  };
+};
+const B b;