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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/102589] spaceship: std::map does not use specialised
 operator< for value
Date: Mon, 04 Oct 2021 10:31:32 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102589

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Gregor Jasny from comment #0)
> I stumbled over this issue when porting a code base from C++17 to C++20.
> Both: GNU libstdc++ as well as MSVC STL show the unexpected behaviour.
> Libc++ seems to do the right thing.

Probably because it doesn't implement <=3D> support in the standard library=
 at
all. Libstdc++ and MSVC do.

> Describe the bug
>=20
> Compiling the test case with C++17 works as expected whereas compiling it
> with C++20 makes the test assumption fail.

The assumption is wrong.

> The reason is that for C++ 20 builds the specialised operator<(const FooP=
tr&
> left, const FooPtr& right) is not picked up. Instead the default operator=
<=3D>
> for std::shared_ptr is used which only compares the raw pointers and not =
the
> content they point to.

This is the correct behaviour.

> Expected behavior
>=20
> I'd expect that with C++17 and C++20 the STL would prefer the specialised
> operator<(const FooPtr& left, const FooPtr& right) over the synthesised <=
=3D>
> one. That would also align with the strong backwards compatibilities C++
> strives for.

No. Comparing the two std::map objects has to use <=3D> because there is no=
 < for
std::map in C++20, so you get a synthesized < from the <=3D> for std::map. =
And
obviously <=3D> for std::map is going to try to use <=3D> for its elements.=
 So it
uses <=3D> for std::pair which uses <=3D> for its members, so it uses <=3D>=
 for
std::shared_ptr. The only < comparison that happens is the one for std::map,
after that everything is a three-way comparison.=