https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119001
--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
So IMHO at least we want something like:
--- gcc/c/c-typeck.cc.jj 2025-02-13 14:10:52.934623189 +0100
+++ gcc/c/c-typeck.cc 2025-02-25 10:47:02.220272763 +0100
@@ -12124,6 +12124,42 @@ retry:
warning (OPT_Wtraditional, "traditional C rejects initialization "
"of unions");
+ /* Error for non-static initialization of a flexible array member.
*/
+ if (fieldcode == ARRAY_TYPE
+ && !require_constant_value
+ && TYPE_SIZE (fieldtype) == NULL_TREE)
+ {
+ error_init (loc, "non-static initialization of a flexible "
+ "array member");
+ break;
+ }
+
+ /* Error for initialization of a flexible array member with
+ a string constant if the structure is in an array. E.g.:
+ union U { int x; char y[]; };
+ union U s[] = { { 1, "foo" } };
+ is invalid. */
+ if (string_flag
+ && fieldcode == ARRAY_TYPE
+ && constructor_depth > 1
+ && TYPE_SIZE (fieldtype) == NULL_TREE)
+ {
+ bool in_array_p = false;
+ for (struct constructor_stack *p = constructor_stack;
+ p && p->type; p = p->next)
+ if (TREE_CODE (p->type) == ARRAY_TYPE)
+ {
+ in_array_p = true;
+ break;
+ }
+ if (in_array_p)
+ {
+ error_init (loc, "initialization of flexible array "
+ "member in a nested context");
+ break;
+ }
+ }
+
/* Accept a string constant to initialize a subarray. */
if (value.value != NULL_TREE
&& fieldcode == ARRAY_TYPE
--- gcc/varasm.cc.jj 2025-01-03 17:59:48.816160159 +0100
+++ gcc/varasm.cc 2025-02-25 10:52:08.043968775 +0100
@@ -5827,10 +5827,13 @@ output_constructor_regular_field (oc_loc
and the FE splits them into dynamic initialization. */
gcc_checking_assert (fieldsize >= fldsize);
/* Given a non-empty initialization, this field had better
- be last. Given a flexible array member, the next field
- on the chain is a TYPE_DECL of the enclosing struct. */
+ be last except in unions. Given a flexible array member, the next
+ field on the chain is a TYPE_DECL of the enclosing struct. */
const_tree next = DECL_CHAIN (local->field);
- gcc_assert (!fieldsize || !next || TREE_CODE (next) != FIELD_DECL);
+ gcc_assert (!fieldsize
+ || !next
+ || TREE_CODE (next) != FIELD_DECL
+ || TREE_CODE (local->type) == UNION_TYPE);
}
else
fieldsize = tree_to_uhwi (DECL_SIZE_UNIT (local->field));
--- gcc/testsuite/gcc.dg/pr119001-1.c.jj 2025-02-25 11:06:07.717169471
+0100
+++ gcc/testsuite/gcc.dg/pr119001-1.c 2025-02-25 11:21:04.947577152 +0100
@@ -0,0 +1,35 @@
+/* PR c/119001 */
+/* { dg-do run } */
+/* { dg-options "" } */
+
+union U { char a[]; int i; };
+union U u = { "12345" };
+union U v = { .a = "6789" };
+union U w = { { 1, 2, 3, 4, 5, 6 } };
+union U x = { .a = { 7, 8, 9 } };
+union V { int i; char a[]; };
+union U y = { .a = "abcdefghijk" };
+union U z = { .a = { 10, 11, 12, 13, 14, 15, 16, 17 } };
+
+int
+main ()
+{
+ for (int i = 0; i < 6; ++i)
+ if (u.a[i] != "12345"[i])
+ __builtin_abort ();
+ for (int i = 0; i < 5; ++i)
+ if (v.a[i] != "6789"[i])
+ __builtin_abort ();
+ for (int i = 0; i < 6; ++i)
+ if (w.a[i] != i + 1)
+ __builtin_abort ();
+ for (int i = 0; i < 3; ++i)
+ if (x.a[i] != i + 7)
+ __builtin_abort ();
+ for (int i = 0; i < 12; ++i)
+ if (y.a[i] != "abcdefghijk"[i])
+ __builtin_abort ();
+ for (int i = 0; i < 8; ++i)
+ if (z.a[i] != i + 10)
+ __builtin_abort ();
+}
--- gcc/testsuite/gcc.dg/pr119001-2.c.jj 2025-02-25 11:06:28.124882967
+0100
+++ gcc/testsuite/gcc.dg/pr119001-2.c 2025-02-25 11:17:56.341223024 +0100
@@ -0,0 +1,12 @@
+/* PR c/119001 */
+/* { dg-do compile } */
+/* { dg-options "" } */
+
+union U { char a[]; int i; };
+union U u[1] = { { "12345" } }; /* { dg-error
"initialization of flexible array member in a nested context" } */
+union U v[1] = { { .a = "6789" } }; /* { dg-error "initialization
of flexible array member in a nested context" } */
+union U w[1] = { { { 1, 2, 3, 4, 5, 6 } } }; /* { dg-error "initialization
of flexible array member in a nested context" } */
+union U x[1] = { { .a = { 7, 8, 9 } } }; /* { dg-error "initialization
of flexible array member in a nested context" } */
+union V { int i; char a[]; };
+union V y[1] = { { .a = "6789" } }; /* { dg-error "initialization
of flexible array member in a nested context" } */
+union V z[1] = { { .a = { 7, 8, 9 } } }; /* { dg-error "initialization
of flexible array member in a nested context" } */
But the pr119001-1.c testcase fails at runtime, for some reason w and x aren't
initialized properly, the initializers are as if they were {}.
