[PATCH][C family] Fix PR86453

[Richard Biener]

This fixes handle_packed_attribute creating a type variant which differs
in TYPE_PACKED.  This cannot be generally allowed since TYPE_PACKED
affects layout and layout is shared between variants.

For the testcase in question the attribute itself is later ignored
but TYPE_PACKED is still applied which eventually leads to an ICE
in type verification (that isn't applied very reliably).

Bootstrapped and tested on x86_64-unknown-linux-gnu, OK?

Thanks,
Richard.

2018-07-11  Richard Biener  <rguent...@suse.de>

        PR c/86453
        * c-attribs.c (handle_packed_attribute): Do not build a variant
        type with TYPE_PACKED, instead ignore the attribute if we may
        not apply to the original type.

        * g++.dg/warn/pr86453.C: New testcase.

diff --git a/gcc/c-family/c-attribs.c b/gcc/c-family/c-attribs.c
index f91add488bb..8cb87eb8154 100644
--- a/gcc/c-family/c-attribs.c
+++ b/gcc/c-family/c-attribs.c
@@ -502,8 +502,13 @@ handle_packed_attribute (tree *node, tree name, tree 
ARG_UNUSED (args),
   if (TYPE_P (*node))
     {
       if (!(flags & (int) ATTR_FLAG_TYPE_IN_PLACE))
-       *node = build_variant_type_copy (*node);
-      TYPE_PACKED (*node) = 1;
+       {
+         warning (OPT_Wattributes,
+                  "%qE attribute ignored for type %qT", name, *node);
+         *no_add_attrs = true;
+       }
+      else
+       TYPE_PACKED (*node) = 1;
     }
   else if (TREE_CODE (*node) == FIELD_DECL)
     {
Index: gcc/testsuite/g++.dg/warn/pr86453.C
===================================================================
--- gcc/testsuite/g++.dg/warn/pr86453.C (nonexistent)
+++ gcc/testsuite/g++.dg/warn/pr86453.C (working copy)
@@ -0,0 +1,5 @@
+// { dg-do compile }
+// { dg-additional-options "-flto" { target lto } }
+struct X {
+  int *__attribute__((aligned(2), packed)) a; // { dg-warning "attribute 
ignored" }
+} b;