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From: "mpolacek at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/65901] no warning or error for va_arg (ap, void)
Date: Mon, 27 Apr 2015 16:04:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65901

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Untested patch:

--- a/gcc/c/c-typeck.c
+++ b/gcc/c/c-typeck.c
@@ -12648,6 +12648,11 @@ c_build_va_arg (location_t loc, tree expr, tree type)
   if (warn_cxx_compat && TREE_CODE (type) == ENUMERAL_TYPE)
     warning_at (loc, OPT_Wc___compat,
                "C++ requires promoted type, not enum type, in %<va_arg%>");
+  if (type == error_mark_node || !COMPLETE_TYPE_P (type))
+    {
+      c_incomplete_type_error (NULL_TREE, type);
+      return error_mark_node;
+    }
   return build_va_arg (loc, expr, type);
 }