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libstdc++: Add check for LWG 3741 problem case

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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.
This commit is contained in:
Jonathan Wakely 2022-08-24 00:10:59 +01:00
parent 4a907b15b5
commit f0f04e1dff
1 changed files with 5 additions and 0 deletions
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libstdc++-v3/testsuite/20_util/duration_cast/rounding.cc
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@ -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> );