[Bug c++/98987] New: Concept subsumption doesn't work with by-value vs. by-reference parameters

[Bug c++/98987] New: Concept subsumption doesn't work with by-value vs. by-reference parameters

[david at doublewise dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98987

            Bug ID: 98987
           Summary: Concept subsumption doesn't work with by-value vs.
                    by-reference parameters
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: david at doublewise dot net
  Target Milestone: ---

The following translation unit

```
template<typename T>
concept A = true;

template<typename T>
concept B = A<T> and true;

constexpr bool f(A auto) {
        return false;
}

constexpr bool f(B auto const &) {
        return true;
}

static_assert(f(0));
```

should compile successfully. Instead, gcc reports

```
<source>:15:18: error: call of overloaded 'f(int)' is ambiguous
   15 | static_assert(f(0));
      |                  ^
<source>:7:16: note: candidate: 'constexpr bool f(auto:1) [with auto:1 = int]'
    7 | constexpr bool f(A auto) {
      |                ^
<source>:11:16: note: candidate: 'constexpr bool f(const auto:2&) [with auto:2
= int]'
   11 | constexpr bool f(B auto const &) {
      |                ^
Compiler returned: 1
```

Changing the first `f` overload to accept `A auto const &` or changing the
second to accept `B auto` resolves the ambiguity error.

See it live: https://godbolt.org/z/9aYo64

[Bug c++/98987] Concept subsumption doesn't work with by-value vs. by-reference parameters

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98987

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

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

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
As with PR96333, since the function templates' function parameters aren't
equivalent, neither is considered more constrained than the other, and so we're
correct to reject this testcase from what I understand.

[Bug c++/98987] Concept subsumption doesn't work with by-value vs. by-reference parameters

[david at doublewise dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98987

David Stone <david at doublewise dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |david at doublewise dot net
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #2 from David Stone <david at doublewise dot net> ---
I agree, this is a duplicate of that other issue.

*** This bug has been marked as a duplicate of bug 96333 ***