[clang] [Clang][Sema] Allow flexible arrays in unions and alone in structs (PR #84428)

[Kees Cook via cfe-commits]

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@@ -271,6 +271,9 @@ Improvements to Clang's diagnostics
 - Clang now correctly diagnoses no arguments to a variadic macro parameter as a C23/C++20 extension.
   Fixes #GH84495.
 
+- ``-Wmicrosoft`` or ``-Wgnu`` is now required to diagnose C99 flexible
+  array members in a union or alone in a struct. Fixes #GB84565.
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kees wrote:

Oh thank you! Nice catch.

https://github.com/llvm/llvm-project/pull/84428