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From: "gjasny at googlemail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/102589] New: spaceship: std::map does not use
 specialised operator< for value
Date: Mon, 04 Oct 2021 09:47:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102589

            Bug ID: 102589
           Summary: spaceship: std::map does not use specialised operator<
                    for value
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gjasny at googlemail dot com
  Target Milestone: ---

Created attachment 51546
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D51546&action=3Dedit
Testcase

Hello,

I stumbled over this issue when porting a code base from C++17 to C++20. Bo=
th:
GNU libstdc++ as well as MSVC STL show the unexpected behaviour. Libc++ see=
ms
to do the right thing.

Describe the bug

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

The reason is that for C++ 20 builds the specialised operator<(const FooPtr&
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.

Expected behavior

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.

GCC version

g++-11 (Debian 11.2.0-8) 11.2.0

Links

Microsoft STL Issue: https://github.com/microsoft/STL/issues/2234
Debian Sid based reproducer: https://github.com/gjasny/map-spaceship-testca=
se
Godbolt: https://godbolt.org/z/objPnjo84


Thanks,
Gregor=