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

[Harald van Dijk via cfe-commits]

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@@ -1,13 +1,58 @@
-// RUN: %clang_cc1 %s -verify=c -fsyntax-only
+// RUN: %clang_cc1 %s -verify -fsyntax-only
 // RUN: %clang_cc1 %s -verify -fsyntax-only -x c++
-// RUN: %clang_cc1 %s -verify -fsyntax-only -fms-compatibility
 // RUN: %clang_cc1 %s -verify -fsyntax-only -fms-compatibility -x c++
+// RUN: %clang_cc1 %s -verify=gnu -fsyntax-only -Wgnu-flexible-array-union-member -Wgnu-empty-struct
+// RUN: %clang_cc1 %s -verify=microsoft -fsyntax-only -fms-compatibility -Wmicrosoft
 
 // The test checks that an attempt to initialize union with flexible array
 // member with an initializer list doesn't crash clang.
 
 
-union { char x[]; } r = {0}; // c-error {{flexible array member 'x' in a union is not allowed}}
+union { char x[]; } r = {0}; /* gnu-warning {{flexible array member 'x' in a union is a GNU extension}}
+                                microsoft-warning {{flexible array member 'x' in a union is a Microsoft extension}}
+                              */
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hvdijk wrote:

I do not see any obvious cases where the code does the wrong thing, sure, I agree with you on that, but as far as I can find, we do not have *any* test for "initialization of flexible array member is not allowed" being correctly emitted for nested unions containing flexible array members, either currently or after this PR. `flexible-array-init.c` contains tests for this for nested structs containing flexible array members, but not for unions. Previously, these would have implicitly been handled by flexible array members being disallowed in unions in the first place, at least for C, but as you are enabling them now, that no longer applies.

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