[Bug libstdc++/102863] New: Optional monadic ops should not be constrained

[Bug libstdc++/102863] New: Optional monadic ops should not be constrained

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102863

            Bug ID: 102863
           Summary: Optional monadic ops should not be constrained
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com
  Target Milestone: ---

It's important that these are not constrained with invocable; that constraint
can trigger hard errors during overload resolution. For instance:

void f(int&);

int main() {
    std::optional<int> x;
    x.transform([](auto& y) { f(y); return 42; });
}

<source>: In instantiation of 'main()::<lambda(auto:3&)> [with auto:3 = const
int]':
/opt/compiler-explorer/gcc-trunk-20211020/include/c++/12.0.0/type_traits:2565:26:
  required by substitution of 'template<class _Fn, class ... _Args> static
std::__result_of_success<decltype (declval<_Fn>()((declval<_Args>)()...)),
std::__invoke_other> std::__result_of_other_impl::_S_test(int) [with _Fn =
main()::<lambda(auto:3&)>; _Args = {const int&}]'
/opt/compiler-explorer/gcc-trunk-20211020/include/c++/12.0.0/type_traits:2576:55:
  required from 'struct std::__result_of_impl<false, false,
main()::<lambda(auto:3&)>, const int&>'
/opt/compiler-explorer/gcc-trunk-20211020/include/c++/12.0.0/type_traits:3038:12:
  recursively required by substitution of 'template<class _Result, class _Ret>
struct std::__is_invocable_impl<_Result, _Ret, true, std::__void_t<typename
_CTp::type> > [with _Result = std::__invoke_result<main()::<lambda(auto:3&)>,
const int&>; _Ret = void]'
/opt/compiler-explorer/gcc-trunk-20211020/include/c++/12.0.0/type_traits:3038:12:
  required from 'struct std::is_invocable<main()::<lambda(auto:3&)>, const
int&>'
/opt/compiler-explorer/gcc-trunk-20211020/include/c++/12.0.0/type_traits:3286:71:
  required from 'constexpr const bool
std::is_invocable_v<main()::<lambda(auto:3&)>, const int&>'
/opt/compiler-explorer/gcc-trunk-20211020/include/c++/12.0.0/concepts:336:25:  
required by substitution of 'template<class _Fn>  requires  invocable<_Fn,
const _Tp&> constexpr auto std::optional<int>::transform(_Fn&&) const & [with
_Fn = int]'
<source>:7:16:   required from here
<source>:7:32: error: binding reference of type 'int&' to 'const int' discards
qualifiers
    7 |     x.transform([](auto& y) { f(y); return 42; });
      |                               ~^~~
<source>:3:8: note:   initializing argument 1 of 'void f(int&)'
    3 | void f(int&);
      |        ^~~~

[Bug libstdc++/102863] Optional monadic ops should not be constrained

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102863

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Target Milestone|---                         |12.0
     Ever confirmed|0                           |1
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
   Last reconfirmed|                            |2021-10-20

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Oh bother, I forgot to remove the constraints from my P0798R6 implementation
(where they were present).

[Bug libstdc++/102863] Optional monadic ops should not be constrained

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102863

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:0fac85a24f40ef6098b756e8e8655205f4bfbf3e

commit r12-4586-g0fac85a24f40ef6098b756e8e8655205f4bfbf3e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Oct 21 01:19:45 2021 +0100

    libstdc++: Remove constraints from std::optional monadic ops [PR102863]

    The constraints on transform and and_then can cause errors when checking
    satisfaction. The constraints that were present in R6 of the paper were
    moved for he final F8 revision, and so should have been included in the
    implementation.

    libstdc++-v3/ChangeLog:

            PR libstdc++/102863
            * include/std/optional (optional::and_then, optional::transform):
            Remove requires-clause.
            * testsuite/20_util/optional/monadic/and_then.cc: Check
            overload resolution doesn't cause errors.
            * testsuite/20_util/optional/monadic/transform.cc: Likewise.

[Bug libstdc++/102863] Optional monadic ops should not be constrained

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102863

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed, thanks.