c: pedwarn on flexible array member initialization with {} for C23+ [PR119350]

Even in C23/C2Y any initialization of flexible array member is still
invalid, so we should emit a pedwarn on it.  But we no longer do for
initialization with {}.  The reason is that for C17 and earlier,
we already emitted a pedwarn on the {} initializer and so emitting
another pedwarn on the flexible array member initialization would
be diagnosing the same thing multiple times.

In C23 we no longer pedwarn on {}, it is standard.
The following patch arranges a pedwarning for that for C23+, so that
at least one pedwarning is emitted.

So that we don't "regress" from C17 to C23 on nested flexible array
member initialization with no -pedantic/-pedantic-errors/-Wpedantic,
the patch emits even the
initialization of flexible array member in a nested context
diagnostic as pedwarn in the {} case, after all, it doesn't cause
much trouble, we just ignore it like before, it wouldn't initialize
anything.

2025-03-19  Jakub Jelinek  <jakub@redhat.com>

	PR c/119350
	* c-typeck.cc (pop_init_level): Don't ignore empty brackets for
	flag_isoc23, still set constructor_type to NULL in that case but
	emit a pedwarn_init in that case.

	* gcc.dg/pr119350-1.c: New test.
	* gcc.dg/pr119350-2.c: New test.
	* gcc.dg/pr119350-3.c: New test.

4 files changed, 53 insertions, 6 deletions

diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
index 1a39cbb..71782bc 100644
--- a/gcc/c/c-typeck.cc
+++ b/gcc/c/c-typeck.cc
@@ -10277,24 +10277,29 @@ pop_init_level (location_t loc, int implicit,
       && !TYPE_MAX_VALUE (TYPE_DOMAIN (constructor_type)))
     {
       /* Silently discard empty initializations.  The parser will
-	 already have pedwarned for empty brackets.  */
+	 already have pedwarned for empty brackets for C17 and earlier.  */
       if (integer_zerop (constructor_unfilled_index))
 	constructor_type = NULL_TREE;
-      else
+      if (constructor_type || flag_isoc23)
 	{
-	  gcc_assert (!TYPE_SIZE (constructor_type));
+	  gcc_assert (!constructor_type || !TYPE_SIZE (constructor_type));
 
-	  if (constructor_depth > 2)
+	  if (constructor_depth <= 2)
+	    pedwarn_init (loc, OPT_Wpedantic,
+			  "initialization of a flexible array member");
+	  else if (constructor_type)
 	    error_init (loc, "initialization of flexible array member "
 			     "in a nested context");
 	  else
 	    pedwarn_init (loc, OPT_Wpedantic,
-			  "initialization of a flexible array member");
+			  "initialization of flexible array member "
+			  "in a nested context");
 
 	  /* We have already issued an error message for the existence
 	     of a flexible array member not at the end of the structure.
 	     Discard the initializer so that we do not die later.  */
-	  if (DECL_CHAIN (constructor_fields) != NULL_TREE
+	  if (constructor_type
+	      && DECL_CHAIN (constructor_fields) != NULL_TREE
 	      && (!p->type || TREE_CODE (p->type) != UNION_TYPE))
 	    constructor_type = NULL_TREE;
 	}
diff --git a/gcc/testsuite/gcc.dg/pr119350-1.c b/gcc/testsuite/gcc.dg/pr119350-1.c
new file mode 100644
index 0000000..75ab290
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr119350-1.c
@@ -0,0 +1,14 @@
+/* PR c/119350 */
+/* { dg-do compile } */
+/* { dg-options "-std=c23 -pedantic-errors" } */
+
+struct S { int a; int b[]; };
+struct T { struct S c; };			/* { dg-error "invalid use of structure with flexible array member" } */
+struct S d = { 1, {} };				/* { dg-error "initialization of a flexible array member" } */
+struct S e = { 1, { 2 } };			/* { dg-error "initialization of a flexible array member" } */
+struct S f = { .a = 1, .b = {} };		/* { dg-error "initialization of a flexible array member" } */
+struct S g = { .a = 1, .b = { 2 } };		/* { dg-error "initialization of a flexible array member" } */
+struct T h = { { 1, {} } };			/* { dg-error "initialization of flexible array member in a nested context" } */
+struct T i = { { 1, { 2 } } };			/* { dg-error "initialization of flexible array member in a nested context" } */
+struct T j = { .c = { .a = 1, .b = {} } };	/* { dg-error "initialization of flexible array member in a nested context" } */
+struct T k = { .c = { .a = 1, .b = { 2 } } };	/* { dg-error "initialization of flexible array member in a nested context" } */
diff --git a/gcc/testsuite/gcc.dg/pr119350-2.c b/gcc/testsuite/gcc.dg/pr119350-2.c
new file mode 100644
index 0000000..ddb3502
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr119350-2.c
@@ -0,0 +1,14 @@
+/* PR c/119350 */
+/* { dg-do compile } */
+/* { dg-options "-std=c23 -Wpedantic" } */
+
+struct S { int a; int b[]; };
+struct T { struct S c; };			/* { dg-warning "invalid use of structure with flexible array member" } */
+struct S d = { 1, {} };				/* { dg-warning "initialization of a flexible array member" } */
+struct S e = { 1, { 2 } };			/* { dg-warning "initialization of a flexible array member" } */
+struct S f = { .a = 1, .b = {} };		/* { dg-warning "initialization of a flexible array member" } */
+struct S g = { .a = 1, .b = { 2 } };		/* { dg-warning "initialization of a flexible array member" } */
+struct T h = { { 1, {} } };			/* { dg-warning "initialization of flexible array member in a nested context" } */
+struct T i = { { 1, { 2 } } };			/* { dg-error "initialization of flexible array member in a nested context" } */
+struct T j = { .c = { .a = 1, .b = {} } };	/* { dg-warning "initialization of flexible array member in a nested context" } */
+struct T k = { .c = { .a = 1, .b = { 2 } } };	/* { dg-error "initialization of flexible array member in a nested context" } */
diff --git a/gcc/testsuite/gcc.dg/pr119350-3.c b/gcc/testsuite/gcc.dg/pr119350-3.c
new file mode 100644
index 0000000..20c2b56
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr119350-3.c
@@ -0,0 +1,14 @@
+/* PR c/119350 */
+/* { dg-do compile } */
+/* { dg-options "-std=c23 -Wc11-c23-compat" } */
+
+struct S { int a; int b[]; };
+struct T { struct S c; };
+struct S d = { 1, {} };				/* { dg-warning "ISO C forbids empty initializer braces before C23" } */
+struct S e = { 1, { 2 } };
+struct S f = { .a = 1, .b = {} };		/* { dg-warning "ISO C forbids empty initializer braces before C23" } */
+struct S g = { .a = 1, .b = { 2 } };
+struct T h = { { 1, {} } };			/* { dg-warning "ISO C forbids empty initializer braces before C23" } */
+struct T i = { { 1, { 2 } } };			/* { dg-error "initialization of flexible array member in a nested context" } */
+struct T j = { .c = { .a = 1, .b = {} } };	/* { dg-warning "ISO C forbids empty initializer braces before C23" } */
+struct T k = { .c = { .a = 1, .b = { 2 } } };	/* { dg-error "initialization of flexible array member in a nested context" } */