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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/113500] Using std::format with float or double based
 std::chrono::time_point causes error: no match for 'operator<<'
Date: Fri, 19 Jan 2024 11:49:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113500

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
MSVC rejects this the same way, although libc++ from LLVM 17 compiles it.

AFAICT std::format("{}", tp) would be invalid because that formats tp by
writing to a stream, and there is no operator<< for
sys_time<duration<floating-point>>.

Arguably, std::format("{:%S}", tp) could work, because it doesn't need to u=
se
operator<< but the problem is that we can't detect at compile-time whether =
or
not we need to use operator<< for a given format string. So it always fails=
 to
compile.

The reason it works with libc++ is because they have a bug in their operato=
r<<
for sys_time.

I think libstdc++ is correct here, but I've asked the C++ committee whether=
 we
might want to change the standard to support this case.=