[Bug c++/105110] New: NTTP type deduction fails when dependent of previous NTTPs

[Bug c++/105110] New: NTTP type deduction fails when dependent of previous NTTPs

[j.galecki11 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105110

            Bug ID: 105110
           Summary: NTTP type deduction fails when dependent of previous
                    NTTPs
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: j.galecki11 at gmail dot com
  Target Milestone: ---

Consider the following minimal example:
```
template <int>
struct S {};

template <auto>
struct V {};

template <int I, S<I> s>
void foo(V<s>) {}

void bar() {
    constexpr S<1> s;
    foo(V<s>{});
}
```
CE link for the reader's convenience: https://godbolt.org/z/1cYK9s1Ta

GCC fails to compile this (consistent across versions), complaining about
cv-qualifier mismatch, which seems wholly irrelevant. Clang compiles without
warnings.
The problem is that when instantiating a function parametrized with a NTTP, GCC
fails to deduce the type of the NTTP parameter based on the other function
parameters. This causes problems with passing around constexpr values, as shown
in the following second example: https://godbolt.org/z/3Tnjx1Wxb (again, Clang
compiles and executes as intended).

As I cannot quite figure out whether the standard requires the deduction
described above to be possible, I see 2 possibilities:
1) This is simply a bug in GCC.
2) GCC is correct in refusing to compile the provided examples (Clang is
wrong). In this case, the emitted error should probably better describe the
problem.

[Bug c++/105110] NTTP type deduction fails when dependent of previous NTTPs

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105110

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
           Keywords|                            |rejects-valid
                 CC|                            |ppalka at gcc dot gnu.org
   Last reconfirmed|                            |2022-03-30
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Confirmed, seems like a bug indeed.

[Bug c++/105110] NTTP type deduction fails when dependent of previous NTTPs

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105110

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

https://gcc.gnu.org/g:95533fe4f014c10dd18de649927668aba6117daf

commit r12-7966-g95533fe4f014c10dd18de649927668aba6117daf
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Apr 1 14:56:20 2022 -0400

    c++: deducing from class type of NTTP [PR105110]

    Here when attempting to deduce T in the NTTP type A<T> from the argument
    type 'const A<int>', we give up due to the const:

      types âA<T>â and âconst A<int>â have incompatible cv-qualifiers

    But since the type of an NTTP cannot be cv-qualified, it seems natural
    to ignore cv-qualifiers on the argument type before attempting to unify
    the two types.

            PR c++/105110

    gcc/cp/ChangeLog:

            * pt.cc (unify) <case TEMPLATE_PARM_INDEX>: Drop cv-quals from
            the argument type of an NTTP before deducing from it.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/nontype-class52.C: New test.

[Bug c++/105110] NTTP type deduction fails when dependent of previous NTTPs

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105110

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
   Target Milestone|---                         |12.0
             Status|ASSIGNED                    |RESOLVED

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 12, thanks for the bug report.

[Bug c++/105110] NTTP type deduction fails when dependent of previous NTTPs

[j.galecki11 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105110

--- Comment #4 from Jakub Gałecki <j.galecki11 at gmail dot com> ---
Awesome! Happy to be of use.

[Bug c++/105110] NTTP type deduction fails when dependent of previous NTTPs

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105110

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |davveston at gmail dot com

--- Comment #5 from Patrick Palka <ppalka at gcc dot gnu.org> ---
*** Bug 99699 has been marked as a duplicate of this bug. ***