[gcc r12-3278] C: PR c/79412: Poison decls with error_mark_node after type mismatch

[gcc r12-3278] C: PR c/79412: Poison decls with error_mark_node after type mismatch

[Roger Sayle]

https://gcc.gnu.org/g:823685221de986afb729910a6f2237f07a377f17

commit r12-3278-g823685221de986afb729910a6f2237f07a377f17
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Wed Sep 1 08:38:39 2021 +0100

    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.

Diff:
---
 gcc/c/c-decl.c                       | 13 ++++++++++++-
 gcc/testsuite/gcc.dg/pr79412.c       |  9 +++++++++
 gcc/testsuite/gcc.dg/typedef-var-2.c |  5 +++--
 3 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
index 221a67fe57b..3482d82fae1 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)
diff --git a/gcc/testsuite/gcc.dg/pr79412.c b/gcc/testsuite/gcc.dg/pr79412.c
new file mode 100644
index 00000000000..b60d5e1b2e3
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr79412.c
@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+int a;
+/* { dg-message "note: previous declaration" "previous declaration" { target *-*-* } .-1 } */
+void fn1 ()
+{
+  a++;
+}
+int a[] = {2};  /* { dg-error "conflicting types" } */
diff --git a/gcc/testsuite/gcc.dg/typedef-var-2.c b/gcc/testsuite/gcc.dg/typedef-var-2.c
index 716d29ca323..bc119a0558e 100644
--- a/gcc/testsuite/gcc.dg/typedef-var-2.c
+++ b/gcc/testsuite/gcc.dg/typedef-var-2.c
@@ -4,12 +4,13 @@
 int f (void)
 {
   extern float v;   
-
+/* { dg-message "note: previous declaration" "previous declaration" { target *-*-* } .-1 } */
   return (v > 0.0f);
 }
 
 extern int t;
+/* { dg-message "note: previous declaration" "previous declaration" { target *-*-* } .-1 } */
 
 typedef float t; /* { dg-error "redeclared as different kind of symbol" } */
 
-t v = 4.5f;
+t v = 4.5f;  /* { dg-error "conflicting types" } */