Evaluate type arguments of sizeof that are structs of variable size [PR101838]

Evaluate type arguments of sizeof for all types of variable size
and not just for VLAs. This fixes PR101838 and some issues related
to PR29970 where statement expressions need to be evaluated so that
the size is well defined.

2021-08-12  Martin Uecker  <muecker@gwdg.de>

gcc/c/
	PR c/101838
	PR c/29970
	* c-typeck.c (c_expr_sizeof_type): Evaluate
	size expressions for structs of variable size.

gcc/testsuite/
	PR c/101838
	* gcc.dg/vla-stexp-2.c: New test.

2 files changed, 40 insertions, 1 deletions

diff --git a/gcc/c/c-typeck.c b/gcc/c/c-typeck.c
index c5bf337..eb5c87d 100644
--- a/gcc/c/c-typeck.c
+++ b/gcc/c/c-typeck.c
@@ -3022,8 +3022,14 @@ c_expr_sizeof_type (location_t loc, struct c_type_name *t)
   c_last_sizeof_loc = loc;
   ret.original_code = SIZEOF_EXPR;
   ret.original_type = NULL;
+  if (type == error_mark_node)
+    {
+      ret.value = error_mark_node;
+      ret.original_code = ERROR_MARK;
+    }
+  else
   if ((type_expr || TREE_CODE (ret.value) == INTEGER_CST)
-      && c_vla_type_p (type))
+      && C_TYPE_VARIABLE_SIZE (type))
     {
       /* If the type is a [*] array, it is a VLA but is represented as
 	 having a size of zero.  In such a case we must ensure that
diff --git a/gcc/testsuite/gcc.dg/vla-stexp-2.c b/gcc/testsuite/gcc.dg/vla-stexp-2.c
new file mode 100644
index 0000000..176f400
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vla-stexp-2.c
@@ -0,0 +1,33 @@
+/* PR101838 */
+/* { dg-do run } */
+/* { dg-options "-Wpedantic -O0" } */
+
+
+int bar0(
+	int (*a)[*],
+	int (*b)[sizeof(*a)]
+);
+
+
+int bar(
+	struct f { 		/* { dg-warning "will not be visible outside of this definition" } */
+		int a[*]; } v,	/* { dg-warning "variably modified type" } */
+	int (*b)[sizeof(struct f)]	// should not warn about zero size
+);
+
+int foo(void)
+{
+	int n = 0;
+	return sizeof(typeof(*({ n = 10; struct foo { 	/* { dg-warning "braced-groups" } */
+					int x[n]; 	/* { dg-warning "variably modified type" } */
+	} x; &x; })));
+}
+
+
+int main()
+{
+	if (sizeof(struct foo { int x[10]; }) != foo())
+		__builtin_abort();
+
+	return 0;
+}