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

[Kees Cook via cfe-commits]

================
@@ -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}}
+                                microsoft-warning {{flexible array member 'x' in a union is a Microsoft extension}}
+                              */
+struct _name1 {
+  int a;
+  union {
+    int b;
+    char x[]; /* 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}}
+               */
+  };
+} name1 = {
+  10,
+  42,        /* initializes "b" */
+};
 
-// expected-no-diagnostics
+struct _name1i {
+  int a;
+  union {
+    int b;
+    char x[]; /* 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}}
+               */
+  };
+} name1i = {
+  .a = 10,
+  .b = 42,
+};
+
+/* Initialization of flexible array in a union is never allowed. */
+struct _name2 {
+  int a;
+  union {
+    int b;
+    char x[]; /* 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}}
+                 stock-note {{initialized flexible array member 'x' is here}}
+               */
+  };
+} name2 = {
+  12,
+  13,
+  { 'c' },   /* stock-error {{initialization of flexible array member is not allowed}} */
----------------
kees wrote:

This seems to work:

```diff
diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index 011deed7a9a9..79cf2eed46fe 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -2437,6 +2437,7 @@ void InitListChecker::CheckStructUnionTypes(
   }
 
   if (Field == FieldEnd || !Field->getType()->isIncompleteArrayType() ||
+      (RD->isUnion() && Field != RD->field_begin()) ||
       Index >= IList->getNumInits())
     return;
 
```

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