c: Error on va_arg with function type [PR105149]

In the PR Joseph said that the C standard for va_arg talks about
pointers to object type and as a function type is not object type,
it is invalid.

The following patch diagnoses it in the FE, instead of ICEing later on
when optimizations are turned on (and with -O0 doing something weird
at runtime).

2022-04-08  Jakub Jelinek  <jakub@redhat.com>

	PR c/105149
	* c-typeck.cc (c_build_va_arg): Reject function types.

	* gcc.dg/pr105149.c: New test.

2 files changed, 22 insertions, 0 deletions

diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
index 6c4af5e..e130196 100644
--- a/gcc/c/c-typeck.cc
+++ b/gcc/c/c-typeck.cc
@@ -15896,6 +15896,12 @@ c_build_va_arg (location_t loc1, tree expr, location_t loc2, tree type)
 		"type %qT", type);
       return error_mark_node;
     }
+  else if (TREE_CODE (type) == FUNCTION_TYPE)
+    {
+      error_at (loc2, "second argument to %<va_arg%> is a function type %qT",
+		type);
+      return error_mark_node;
+    }
   else if (warn_cxx_compat && TREE_CODE (type) == ENUMERAL_TYPE)
     warning_at (loc2, OPT_Wc___compat,
 		"C++ requires promoted type, not enum type, in %<va_arg%>");
diff --git a/gcc/testsuite/gcc.dg/pr105149.c b/gcc/testsuite/gcc.dg/pr105149.c
new file mode 100644
index 0000000..b748f45
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr105149.c
@@ -0,0 +1,16 @@
+/* PR c/105149 */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+#include <stdarg.h>
+
+void
+foo (int s, ...)
+{
+  int e;
+  va_list ap;
+
+  va_start (ap, s);
+  e = va_arg (ap, int (void)) ();	/* { dg-error "second argument to 'va_arg' is a function type" } */
+  va_end (ap);
+}