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From: Jonathan Wakely <>
Subject: [gcc r13-2172] libstdc++: Add check for LWG 3741 problem case
Date: Wed, 24 Aug 2022 13:04:31 +0000 (GMT)	[thread overview]
Message-ID: <> (raw)

commit r13-2172-gf0f04e1dffea609cb74ac0b488385401ed7e15a3
Author: Jonathan Wakely <>
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 : -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
            * testsuite/20_util/duration_cast/ Check abs with
            non-reduced duration.

 libstdc++-v3/testsuite/20_util/duration_cast/ | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/libstdc++-v3/testsuite/20_util/duration_cast/ b/libstdc++-v3/testsuite/20_util/duration_cast/
index af6e72d9e2e..c5179b6eb6e 100644
--- a/libstdc++-v3/testsuite/20_util/duration_cast/
+++ b/libstdc++-v3/testsuite/20_util/duration_cast/
@@ -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> );

                 reply	other threads:[~2022-08-24 13:04 UTC|newest]

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