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.

0 files changed, 0 insertions, 0 deletions