[clang] [clang] Allow class with anonymous union member to be const-default-constructible even if a union member has a default member initializer (#95854) (PR #96301)

[Rajveer Singh Bharadwaj via cfe-commits]

https://github.com/Rajveer100 updated https://github.com/llvm/llvm-project/pull/96301

>From b964923b9610c9cd53e4d1de8f5d51d8fcebc78c Mon Sep 17 00:00:00 2001
From: Rajveer <rajveer.developer at icloud.com>
Date: Fri, 21 Jun 2024 18:26:36 +0530
Subject: [PATCH] [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
---
 clang/docs/ReleaseNotes.rst       |  6 ++++++
 clang/include/clang/AST/DeclCXX.h |  4 +++-
 clang/test/SemaCXX/GH95854.cpp    | 21 +++++++++++++++++++++
 3 files changed, 30 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/SemaCXX/GH95854.cpp

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 45676a02b760b..c107304ea95f7 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/include/clang/AST/DeclCXX.h b/clang/include/clang/AST/DeclCXX.h
index fb52ac804849d..e4dc50c9b9e00 100644
--- a/clang/include/clang/AST/DeclCXX.h
+++ b/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
diff --git a/clang/test/SemaCXX/GH95854.cpp b/clang/test/SemaCXX/GH95854.cpp
new file mode 100644
index 0000000000000..820cf15e68287
--- /dev/null
+++ b/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;