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

[Eli Friedman 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}} */
+};
+
+/* Initialization of flexible array in a union is never allowed. */
+struct _name2i {
+  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}}
+               */
+  };
+} name2i = {
+  .a = 12,
+  .b = 13,      /* stock-note {{previous initialization is here}} */
+  .x = { 'c' }, /* stock-error {{initialization of flexible array member is not allowed}}
+                   c-warning {{initializer overrides prior initialization of this subobject}}
+                   cpp-error {{initializer partially overrides prior initialization of this subobject}}
+                 */
+};
+
+/* Flexible array initialization always allowed when not in a union,
+   and when struct has another member.
+ */
+struct _okay {
+  int a;
+  char x[];
+} okay = {
+  22,
+  { 'x', 'y', 'z' },
+};
+
+struct _okayi {
+  int a;
+  char x[];
+} okayi = {
+  .a = 22,
+  .x = { 'x', 'y', 'z' },
+};
+
+struct _okay0 {
+  int a;
+  char x[];
+} okay0 = { };
+
+struct _flex_extension {
+  char x[]; /* gnu-warning {{flexible array member 'x' in otherwise empty struct is a GNU extension}}
+               microsoft-warning {{flexible array member 'x' in otherwise empty struct is a Microsoft extension}}
+             */
+} flex_extension = {
+  { 'x', 'y', 'z' },
+};
+
+struct _flex_extensioni {
+  char x[]; /* gnu-warning {{flexible array member 'x' in otherwise empty struct is a GNU extension}}
+               microsoft-warning {{flexible array member 'x' in otherwise empty struct is a Microsoft extension}}
+             */
+} flex_extensioni = {
+  .x = { 'x', 'y', 'z' },
+};
 
+struct already_hidden {
+  int a;
+  union {
+    int b;
+    struct {
+      struct { } __empty;  // gnu-warning {{empty struct is a GNU extension}}
+      char x[];
+    };
+  };
+};
+
+struct still_zero_sized {
+  struct { } __unused;  // gnu-warning {{empty struct is a GNU extension}}
+  int x[];
+};
+
+struct warn1 {
+  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}}
+               */
+  };
+};
+
+struct warn2 {
+  int x[];  /* gnu-warning {{flexible array member 'x' in otherwise empty struct is a GNU extension}}
+               microsoft-warning {{flexible array member 'x' in otherwise empty struct is a Microsoft extension}}
+             */
+};
+
+union warn3 {
+  short 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}}
+               */
+};
+
+struct quiet1 {
+  int a;
+  short x[];
+};
+
+// expected-no-diagnostics
----------------
efriedma-quic wrote:

I'd like a test for something like `struct X { union { short x[];}; int y; } = {{}, 3};`.

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