C++ PATCH for c++/85033, ICE with enumerator in __builtin_offsetof

[Marek Polacek]

In this test:

struct S {
    enum { X };
};

int
foo (struct S s)
{
  return s.X;
}

unlike in C, lookup_member in C++ is able to find X in S.  So in the following
testcase finish_offsetof happily passes the CONST_DECL X down to fold_offsetof,
which then crashes because fold_offsetof_1 doesn't handle CONST_DECLs.  I think
let's simply disallow 'em in finish_offsetof; we can't take their address 
anyway.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2018-03-23  Marek Polacek  <pola...@redhat.com>

        PR c++/85033
        * semantics.c (finish_offsetof): Don't allow CONST_DECLs.

        * g++.dg/ext/builtin-offsetof2.C: New test.

diff --git gcc/cp/semantics.c gcc/cp/semantics.c
index 97fa57ae94e..035e3951574 100644
--- gcc/cp/semantics.c
+++ gcc/cp/semantics.c
@@ -4072,6 +4072,11 @@ finish_offsetof (tree object_ptr, tree expr, location_t 
loc)
        }
       return error_mark_node;
     }
+  if (TREE_CODE (expr) == CONST_DECL)
+    {
+      error ("cannot apply %<offsetof%> to an enumerator %qD", expr);
+      return error_mark_node;
+    }
   if (REFERENCE_REF_P (expr))
     expr = TREE_OPERAND (expr, 0);
   if (!complete_type_or_else (TREE_TYPE (TREE_TYPE (object_ptr)), object_ptr))
diff --git gcc/testsuite/g++.dg/ext/builtin-offsetof2.C 
gcc/testsuite/g++.dg/ext/builtin-offsetof2.C
index e69de29bb2d..07cc55a65c1 100644
--- gcc/testsuite/g++.dg/ext/builtin-offsetof2.C
+++ gcc/testsuite/g++.dg/ext/builtin-offsetof2.C
@@ -0,0 +1,7 @@
+// PR c++/85033
+
+struct S {
+  enum { E };
+};
+
+int b = __builtin_offsetof(S, E); // { dg-error "cannot apply .offsetof. to an 
enumerator" }

        Marek