[gcc r12-4357] libstdc++: Fix test that fails for C++20

[Jonathan Wakely]

https://gcc.gnu.org/g:727137d6ca6d3d401a0c1b4df6b9aae8b97dacd5

commit r12-4357-g727137d6ca6d3d401a0c1b4df6b9aae8b97dacd5
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Oct 12 15:39:18 2021 +0100

    libstdc++: Fix test that fails for C++20
    
    Also 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<.
            * testsuite/20_util/tuple/comparison_operators/overloaded2.cc:
            Adjust expected errors for C++20.

Diff:
---
 .../testsuite/20_util/tuple/comparison_operators/overloaded.cc      | 6 ++++++
 .../testsuite/20_util/tuple/comparison_operators/overloaded2.cc     | 4 +++-
 2 files changed, 9 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 ef90b6b5b73..a9bc2c7dfb5 100644
--- a/libstdc++-v3/testsuite/20_util/tuple/comparison_operators/overloaded.cc
+++ b/libstdc++-v3/testsuite/20_util/tuple/comparison_operators/overloaded.cc
@@ -48,3 +48,9 @@ TwistedLogic operator<(const Compares&, const Compares&) { return {false}; }
 
 auto a = std::make_tuple(nullptr, Compares{}, 2, 'U');
 auto b = 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
diff --git a/libstdc++-v3/testsuite/20_util/tuple/comparison_operators/overloaded2.cc b/libstdc++-v3/testsuite/20_util/tuple/comparison_operators/overloaded2.cc
index a66a9315902..bac16ffd521 100644
--- a/libstdc++-v3/testsuite/20_util/tuple/comparison_operators/overloaded2.cc
+++ b/libstdc++-v3/testsuite/20_util/tuple/comparison_operators/overloaded2.cc
@@ -49,5 +49,7 @@ TwistedLogic operator<(const Compares&, const Compares&) { return {false}; }
 auto a = std::make_tuple(nullptr, Compares{}, 2, 'U');
 auto b = a < a;
 
-// { dg-error "ordered comparison" "" { target *-*-* } 0 }
+// { dg-error "no match for 'operator<'" "" { target c++20 } 0 }
+// { dg-error "no match for .*_Synth3way|in requirements" "" { target c++20 } 0 }
+// { dg-error "ordered comparison" "" { target c++17_down } 0 }
 // { dg-error "not a return-statement" "" { target c++11_only } 0 }