[Bug c++/68061] New: Can't use [[deprecated]] with requires clause

[Bug c++/68061] New: Can't use [[deprecated]] with requires clause

[chrisb2244 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68061

            Bug ID: 68061
           Summary: Can't use [[deprecated]] with requires clause
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: chrisb2244 at gmail dot com
  Target Milestone: ---

Either of 

template<ConceptName... parameter> requires RequiresConcept<A, parameter...>
const T x(const parameter... p) const { ... }

or 

template<ConceptName... parameter, std::enable_if<...>
[[deprecated]] const T x(const parameter... p) const { ... }

will compile (using 'g++-trunk -std=c++1z ...')

Using the 'requires' clause along with an attempt to deprecate the function
using [[deprecated]] produces

    "two consecutive '[' shall only introduce an attribute before '[' token"

[Bug c++/68061] Can't use [[deprecated]] with requires clause

[chrisb2244 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68061

--- Comment #1 from Christian Butcher <chrisb2244 at gmail dot com> ---
Created attachment 36565
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36565&action=edit
Short compilable (or not) case showing problem

Commenting out the function definition for 'foo' allows compilation using 
g++ -std=c++1z reqClauseDep.cpp -o Test

[Bug c++/68061] Can't use [[deprecated]] with requires clause

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68061

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This bug is still present on trunk when -fconcepts-ts is used, even in C++20
mode:

$ g++ -c 68061.C -std=c++20 
$ g++ -c 68061.C -std=c++20 -fconcepts-ts
68061.C:3:1: error: two consecutive ‘[’ shall only introduce an attribute
before ‘[’ token
    3 | [[deprecated]] void f(T);
      | ^

It also affects [[nodiscard]] and so caused PR 101782.

[Bug c++/68061] Can't use [[deprecated]] with requires clause

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68061

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This is because with -fconcepts-ts the requires-clause argument is parsed as
logical-or-expression rather than constraint-logical-or-expression and for the
former the [ could be part of postfix-expression in there.  Bet that is the
reason why constraint-logical-or-expression exists and allows only
primary-expressions mixed with ||s and &&s.
Perhaps the FE could have a hack, set some parser flag when parsing
requires-clause and in postfix-expression parsing if that flag is set instead
of the [[ error in there pretend [ is not there and ends the expression.

As a workaround,
template <class T>
  requires true
void f [[deprecated]] (T);
works.