[gcc r13-2172] libstdc++: Add check for LWG 3741 problem case

[Jonathan Wakely]

https://gcc.gnu.org/g:f0f04e1dffea609cb74ac0b488385401ed7e15a3

commit r13-2172-gf0f04e1dffea609cb74ac0b488385401ed7e15a3
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Aug 24 00:10:59 2022 +0100

    libstdc++: Add check for LWG 3741 problem case
    
    This LWG issue was closed as NAD, as it was just a bug in an
    implementation, not a defect in the standard. Libstdc++ never had that
    bug and always worked for the problem case. Add a test to ensure we
    don't regress.
    
    The problem occurs when abs is implemented using a ternary expression:
    
     return d >= d.zero() ? d : -d;
    
    If decltype(-d) is not the same as decltype(d) then this is ambiguous,
    because each type can be converted to the other, so there is no common
    type.
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/20_util/duration_cast/rounding.cc: Check abs with
            non-reduced duration.

Diff:
---
 libstdc++-v3/testsuite/20_util/duration_cast/rounding.cc | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/libstdc++-v3/testsuite/20_util/duration_cast/rounding.cc b/libstdc++-v3/testsuite/20_util/duration_cast/rounding.cc
index af6e72d9e2e..c5179b6eb6e 100644
--- a/libstdc++-v3/testsuite/20_util/duration_cast/rounding.cc
+++ b/libstdc++-v3/testsuite/20_util/duration_cast/rounding.cc
@@ -58,3 +58,8 @@ static_assert( std::chrono::round<seconds>(2501ms) == 3s );
 
 static_assert( std::chrono::abs(100ms) == 100ms );
 static_assert( std::chrono::abs(-100ms) == 100ms );
+
+// LWG 3741. std::chrono::abs(duration) is ill-formed with non-reduced periods
+using D1000 = std::chrono::duration<int, std::ratio<1000, 1000>>;
+static_assert( std::chrono::abs(D1000(-2)) == D1000(2) );
+static_assert( std::is_same_v<decltype(std::chrono::abs(D1000(-2))), D1000> );