From: Jonathan Wakely <jwakely@redhat.com>
Date: Tue, 23 Aug 2022 23:10:59 +0000 (+0100)
Subject: libstdc++: Add check for LWG 3741 problem case
X-Git-Tag: basepoints/gcc-14~5035
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=f0f04e1dffea609cb74ac0b488385401ed7e15a3;p=thirdparty%2Fgcc.git

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 --git a/libstdc++-v3/testsuite/20_util/duration_cast/rounding.cc b/libstdc++-v3/testsuite/20_util/duration_cast/rounding.cc
index af6e72d9e2eb..c5179b6eb6e2 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> );