[gcc r11-9122] libstdc++: Fix test that fails for C++20

[gcc r11-9122] libstdc++: Fix test that fails for C++20

[Jonathan Wakely]

https://gcc.gnu.org/g:8104d4fff6202dabcb2eef7406f64d26f97f341d

commit r11-9122-g8104d4fff6202dabcb2eef7406f64d26f97f341d
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Oct 12 15:39:18 2021 +0100

    libstdc++: Fix test that fails for C++20
    
    Restore the test for 'a < a' that was removed by r12-2537 because
    it is ill-formed. We still want to test operator< for tuple, we just
    need to not use std::nullptr_t in that tuple type.
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/20_util/tuple/comparison_operators/overloaded.cc:
            Restore test for operator<.
    
    (cherry picked from commit 727137d6ca6d3d401a0c1b4df6b9aae8b97dacd5)

Diff:
---
 .../testsuite/20_util/tuple/comparison_operators/overloaded.cc     | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/testsuite/20_util/tuple/comparison_operators/overloaded.cc b/libstdc++-v3/testsuite/20_util/tuple/comparison_operators/overloaded.cc
index a25c374a932..a9bc2c7dfb5 100644
--- a/libstdc++-v3/testsuite/20_util/tuple/comparison_operators/overloaded.cc
+++ b/libstdc++-v3/testsuite/20_util/tuple/comparison_operators/overloaded.cc
@@ -48,4 +48,9 @@ TwistedLogic operator<(const Compares&, const Compares&) { return {false}; }
 
 auto a = std::make_tuple(nullptr, Compares{}, 2, 'U');
 auto b = a == a;
-auto c = a < a;
+
+#if ! __cpp_lib_three_way_comparison
+// Not valid in C++20, because TwistedLogic doesn't model boolean-testable.
+auto c = std::make_tuple("", Compares{}, 2, 'U');
+auto d = c < c;
+#endif