[gcc(refs/users/ppalka/heads/libstdcxx-constrained-algos)] Add mysteriously failing ranges::mismatch test

[Patrick Palka]

https://gcc.gnu.org/g:268c8651ab4677f09d723db6823d2bad0ba869c1

commit 268c8651ab4677f09d723db6823d2bad0ba869c1
Author: Patrick Palka <ppalka@gcc.gnu.org>
Date:   Fri Jan 17 16:43:48 2020 -0500

    Add mysteriously failing ranges::mismatch test

Diff:
---
 libstdc++-v3/testsuite/25_algorithms/mismatch/constrained.cc | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/libstdc++-v3/testsuite/25_algorithms/mismatch/constrained.cc b/libstdc++-v3/testsuite/25_algorithms/mismatch/constrained.cc
index 06a5a13..27f5bc3 100644
--- a/libstdc++-v3/testsuite/25_algorithms/mismatch/constrained.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/mismatch/constrained.cc
@@ -50,6 +50,8 @@ test01()
   test_range<X, input_iterator_wrapper> rb(xb);
   auto res3 = ranges::mismatch(ra, rb, {}, &X::i, &X::i);
   VERIFY( res3.in1->i == 4 && res3.in2->i == 3 );
+  res3 = ranges::mismatch(ra, ra, {}, &X::i, &X::i);
+  VERIFY( res3.in1 == ranges::end(ra) && res3.in2 == ranges::end(ra) );
 }
 
 struct Y { int i; int j; };