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

[Eli Friedman via cfe-commits]

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@@ -1,13 +1,158 @@
-// RUN: %clang_cc1 %s -verify=c -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=stock,c -fsyntax-only
+// RUN: %clang_cc1 %s -verify=stock,cpp -fsyntax-only -x c++
+// RUN: %clang_cc1 %s -verify=stock,cpp -fsyntax-only -fms-compatibility -x c++
+// RUN: %clang_cc1 %s -verify=stock,c,gnu -fsyntax-only -Wgnu-flexible-array-union-member -Wgnu-empty-struct
+// RUN: %clang_cc1 %s -verify=stock,c,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}}
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efriedma-quic wrote:

Please add codegen tests to make sure `union { char x[]; } r = {0};` actually works.

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