[Bug c++/88680] [9 Regression] bogus -Wtype-limits for constant expressions after r267272

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88680

--- Comment #3 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Author: dmalcolm
Date: Sat Feb 16 16:17:17 2019
New Revision: 268961

URL: https://gcc.gnu.org/viewcvs?rev=268961&root=gcc&view=rev
Log:
Fix excess warnings from -Wtype-limits with location wrappers (PR c++/88680)

PR c++/88680 reports excess warnings from -Wtype-limits after the C++
FE's use of location wrappers was extended in r267272 for cases such as:

  const unsigned n = 8;
  static_assert (n >= 0 && n % 2 == 0, "");

t.C:3:18: warning: comparison of unsigned expression >= 0 is always true
  [-Wtype-limits]
    3 | static_assert (n >= 0 && n % 2 == 0, "");
      |                ~~^~~~

The root cause is that the location wrapper around "n" breaks the
suppression of the warning for the "if OP0 is a constant that is >= 0"
case.

This patch fixes it by calling fold_for_warn on OP0, extracting the
constant.

gcc/c-family/ChangeLog:
        PR c++/88680
        * c-common.c (shorten_compare): Call fold_for_warn on op0 when
        implementing -Wtype-limits.

gcc/testsuite/ChangeLog:
        PR c++/88680
        * g++.dg/wrappers/pr88680.C: New test.


Added:
    trunk/gcc/testsuite/g++.dg/wrappers/pr88680.C
Modified:
    trunk/gcc/c-family/ChangeLog
    trunk/gcc/c-family/c-common.c
    trunk/gcc/testsuite/ChangeLog