[Clang] Modify sanity check assert in AggExprEmitter::VisitInitListExpr to cover anonymous struct in a union GNU extension

[shafik]

Summary

AggExprEmitter::VisitInitListExpr sanity checks that an empty union is really empty and not a semantic analysis failure. The assert is missing that we allow anonymous structs as a GNU extension. I have updated the assert to take that into account.

This fixes: https://github.com/llvm/llvm-project/issues/58800

[shafik]

I realize I am modifying codegen but the test is in sema, I am happy to move the test to somewhere more appropriate if someone has a good suggestion.

[aaron.ballman]

LGTM with a formatting nit fixed, but you should add a release note for the fix when landing.

clang/lib/CodeGen/CGExprAgg.cpp

1727

Guessing at the way to solve the 80-col limit issue, but do whatever clang-format says.

[shafik]

• Add Release note

clang/lib/CodeGen/CGExprAgg.cpp

1727

This is what clang-format likes.

clang/docs/ReleaseNotes.rst

- Fix assert that triggers a crash during some types of list initialization that
   `Issue 58302 <https://github.com/llvm/llvm-project/issues/58302>`_
   `Issue 58753 <https://github.com/llvm/llvm-project/issues/58753>`_
   `Issue 59100 <https://github.com/llvm/llvm-project/issues/59100>`_
 - Fix issue using __attribute__((format)) on non-variadic functions that expect
   more than one formatted argument.
 - Fix bug where constant evaluation treated a pointer to member that points to
   a weak member as never being null. Such comparisons are now treated as
   non-constant.
+- Fix sanity check when value initializing an empty union so that it takes into
+  account anonymous structs which is a GNU extension. This fixes
+  `Issue 58800 <https://github.com/llvm/llvm-project/issues/58800>`_
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Clang will now check compile-time determinable string literals as format strings.
   Fixes `Issue 55805: <https://github.com/llvm/llvm-project/issues/55805>`_.
 - ``-Wformat`` now recognizes ``%b`` for the ``printf``/``scanf`` family of
   functions and ``%B`` for the ``printf`` family of functions. Fixes
   `Issue 56885: <https://github.com/llvm/llvm-project/issues/56885>`_.

clang/lib/CodeGen/CGExprAgg.cpp

if (record->isUnion()) {
     // specified by the initializer list.
     if (!InitializedFieldInUnion) {
       // Empty union; we have nothing to do.
 
 #ifndef NDEBUG
       // Make sure that it's really an empty and not a failure of
       // semantic analysis.
       for (const auto *Field : record->fields())
-        assert(Field->isUnnamedBitfield() && "Only unnamed bitfields allowed");
+        assert((Field->isUnnamedBitfield() || Field->isAnonymousStructOrUnion()) && "Only unnamed bitfields or ananymous class allowed");
 #endif
       return;
     }
 
     // FIXME: volatility
     FieldDecl *Field = InitializedFieldInUnion;
 
     LValue FieldLoc = CGF.EmitLValueForFieldInitialization(DestLV, Field);

clang/test/SemaCXX/anonymous-struct.cpp

namespace BuiltinName {
   } A; // expected-note {{given name 'A' for linkage purposes by this typedef}}
 }
 namespace inline_defined_static_member {
 typedef struct { // expected-warning {{anonymous non-C-compatible type}}
   static void f() { // expected-note {{due to this member}}
   }
 } A; // expected-note {{given name 'A' for linkage purposes by this typedef}}
 }
+
+#if __cplusplus > 201103L
+namespace GH58800 {
+struct A {
+  union {
+    struct {
+      float red = 0.0f;
+    };
+  };
+};
+
+A GetA() {
+  A result{};
+  return result;
+}
+}
+#endif