C: PR c/79412: Poison decls with error_mark_node after type mismatch

This patch fixes an ICE during error-recovery regression in the C front-end.
The symptom is that the middle-end's sanity checking assertions fail during
gimplification when being asked to increment an array, which is non-sense.
The issue is that the C-front end has detected the type mismatch and
reported an error to the user, but hasn't provided any indication of this
to the middle-end, simply passing bogus trees that the optimizers recognize
as invalid.

This appears to be a frequently reported ICE with 94730, 94731, 101036
and 101365 all marked as duplicates.

I believe the correct (polite) fix is to mark the mismatched types as
problematic/dubious in the front-end, when the error is spotted, so that
the middle-end has a heads-up and can be a little more forgiving.  This
patch to c-decl.c's duplicate_decls sets (both) mismatched types to
error_mark_node if they are significantly different, and we've issued
an error message.  Alas, this is too punitive for FUNCTION_DECLs where
we store return types, parameter lists, parameter types and attributes
in the type, but fortunately the middle-end is already more cautious
about trusting possibly suspect function types.

This fix required one minor change to the testsuite, typedef-var-2.c
where after conflicting type definitions, we now no longer assume that
the (first or) second definition is the correct one.  This change only
affects the behaviour after seen_error(), so should be relatively safe.

2021-09-01  Roger Sayle  <roger@nextmovesoftware.com>
	    Joseph Myers  <joseph@codesourcery.com>

gcc/c/ChangeLog
	PR c/79412
	* c-decl.c (duplicate_decls): On significant mismatches, mark the
	types of both (non-function) decls as error_mark_node, so that the
	middle-end can see the code is malformed.
	(free_attr_access_data): Don't process if the type has been set to
	error_mark_node.

gcc/testsuite/ChangeLog
	PR c/79412
	* gcc.dg/pr79412.c: New test case.
	* gcc.dg/typedef-var-2.c: Update expeted errors.

1 files changed, 12 insertions, 1 deletions

diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
index 221a67f..3482d82 100644
--- a/gcc/c/c-decl.c
+++ b/gcc/c/c-decl.c
@@ -2957,6 +2957,17 @@ duplicate_decls (tree newdecl, tree olddecl)
     {
       /* Avoid `unused variable' and other warnings for OLDDECL.  */
       suppress_warning (olddecl, OPT_Wunused);
+      /* If the types are completely different, poison them both with
+ 	 error_mark_node.  */
+      if (TREE_CODE (TREE_TYPE (newdecl)) != TREE_CODE (TREE_TYPE (olddecl))
+	  && olddecl != error_mark_node
+	  && seen_error ())
+	{
+	  if (TREE_CODE (olddecl) != FUNCTION_DECL)
+	    TREE_TYPE (olddecl) = error_mark_node;
+	  if (TREE_CODE (newdecl) != FUNCTION_DECL)
+	    TREE_TYPE (newdecl) = error_mark_node;
+	}
       return false;
     }
 
@@ -12209,7 +12220,7 @@ free_attr_access_data ()
 	  attr_access::free_lang_data (attrs);
 
       tree fntype = TREE_TYPE (n->decl);
-      if (!fntype)
+      if (!fntype || fntype == error_mark_node)
 	continue;
       tree attrs = TYPE_ATTRIBUTES (fntype);
       if (!attrs)