[Bug libstdc++/112796] New: <algorithm> header implicitly includes <concepts>

[Bug libstdc++/112796] New: <algorithm> header implicitly includes <concepts>

[bill.trost at harmonicinc dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112796

            Bug ID: 112796
           Summary: <algorithm> header implicitly includes <concepts>
           Product: gcc
           Version: 12.2.0
               URL: https://godbolt.org/z/Pra17xczj
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bill.trost at harmonicinc dot com
  Target Milestone: ---
            Target: x86-64

Created attachment 56739
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=56739&action=edit
Sample code that incorrectly compiles.

The attached code should not compile (and does not compile if you don't include
<algorithm>). It should have to #include <concepts>.

[Bug libstdc++/112796] <algorithm> header implicitly includes <concepts>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112796

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

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

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is not a bug in GCC, the C++ standard allows any header to include any
other. The bug is in your code, and we can't generally diagnose it.

It's simply not possible to implement <algorithm> without at least some of the
concepts defined in <concepts>, because the algos are specified in terms of
those concepts.

Similarly, nearly every header includes the <type_traits> header.

While it would be nice if we didn't expose all of <concepts> for any program
that doesn't include it explicitly, that's just not practical, sorry.

[Bug libstdc++/112796] <algorithm> header implicitly includes <concepts>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112796

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #1)
> This is not a bug in GCC, the C++ standard allows any header to include any
> other.


See [res.on.headers] in the C++ standard:
"A C++ header may include other C++ headers. A C++ header shall provide the
declarations and definitions that appear in its synopsis. A C++ header shown in
its synopsis as including other C++ headers shall provide the declarations and
definitions that appear in the synopses of those other headers."

N.B. this is different from C, which does not allow e.g. <stdio.h> to include
<stdlib.h>.