diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -326,6 +326,9 @@
 - 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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/CodeGen/CGExprAgg.cpp b/clang/lib/CodeGen/CGExprAgg.cpp
--- a/clang/lib/CodeGen/CGExprAgg.cpp
+++ b/clang/lib/CodeGen/CGExprAgg.cpp
@@ -1724,7 +1724,7 @@
       // 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;
     }
diff --git a/clang/test/SemaCXX/anonymous-struct.cpp b/clang/test/SemaCXX/anonymous-struct.cpp
--- a/clang/test/SemaCXX/anonymous-struct.cpp
+++ b/clang/test/SemaCXX/anonymous-struct.cpp
@@ -189,3 +189,20 @@
   }
 } 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