tree-cfg: Allow enum types as result of POINTER_DIFF_EXPR [PR98556]

tree-cfg: Allow enum types as result of POINTER_DIFF_EXPR [PR98556]

As conversions between signed integers and signed enums with the same
precision are useless in GIMPLE, it seems strange that we require that
POINTER_DIFF_EXPR result must be INTEGER_TYPE.

If we really wanted to require that, we'd need to change the gimplifier
to ensure that, which it isn't the case on the following testcase.
What is going on during the gimplification is that when we have the
(enum T) (p - q) cast, it is stripped through
      /* Strip away as many useless type conversions as possible
         at the toplevel.  */
      STRIP_USELESS_TYPE_CONVERSION (*expr_p);
and when the MODIFY_EXPR is gimplified, the *to_p has enum T type,
while *from_p has intptr_t type and as there is no conversion in between,
we just create GIMPLE_ASSIGN from that.

2021-01-09  Jakub Jelinek  <jakub@redhat.com>

	PR c++/98556
	* tree-cfg.c (verify_gimple_assign_binary): Allow lhs of
	POINTER_DIFF_EXPR to be any integral type.

	* c-c++-common/pr98556.c: New test.

(cherry picked from commit 991656092f78eeab2a48fdbacf4e1f08567badaf)