Glibc extensions no longer supported in std::tm formatter

[euclio]

In short, this assertion (using the make_hour function from the chrono tests) passes in 8.0.1 but fails in 8.1.0.

EXPECT_EQ("8", fmt::format("{:%-I}", make_hour(8)));

According to git bisect, this regressed in 7911d8d.

Seems like this might be expected breakage given the goal of #2544, but the documentation claims that the code uses the format specifiers from the system's strftime, which is no longer the case.

cc Alexays/Waybar#1447

[vitaut]

This is indeed expected, updated the docs in 1f3d44b. Thanks for reporting.