[Bug c++/109337] New: c++2a test concepts4.C passes when it should fail

[Bug c++/109337] New: c++2a test concepts4.C passes when it should fail

[cjdb.ns at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109337

            Bug ID: 109337
           Summary: c++2a test concepts4.C passes when it should fail
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cjdb.ns at gmail dot com
  Target Milestone: ---

Clang and GCC disagree on the validity of concepts4.C. I think Clang is correct
here, because forming a reference to `void` should be ill-formed.

https://godbolt.org/z/es5GxYhzd

[Bug c++/109337] c++2a test concepts4.C passes when it should fail

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109337

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2023-03-29
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

Changing R42c to be a bool rather than a concept causes GCC to catch the
failure ...

That is:
```
template <class T> struct A { static const int x = 42; };

template <class Ta> concept A42 = A<Ta>::x == 42;
template <class Tv> concept Void = __is_same_as(Tv, void);
template <class Tb, class Ub> concept A42b = Void<Tb> || A42<Ub>;
template <class Tc> bool R42c = A42b<Tc, Tc&>;

static_assert (R42c<void>);
```

[Bug c++/109337] c++2a test concepts4.C passes when it should fail

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109337

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Reduced testcase:
```
template <class Ub> concept A42b = true;
template <class Tc> concept R42c = A42b<Tc&>;

static_assert (R42c<void>);
```


GCC does the right thing for too:
```
template <class Ub> bool A42b = true;
template <class Tc> concept R42c = A42b<Tc&>;

static_assert (!R42c<void>);
```

[Bug c++/109337] c++2a test concepts4.C passes when it should fail

[cjdb.ns at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109337

--- Comment #3 from Christopher Di Bella <cjdb.ns at gmail dot com> ---
This is apparently a Clang bug: the RHS of `R42c` isn't evaluated because of
short-circuiting. Apologies for the noise and thanks for helping me work
through it.

[Bug c++/109337] c++2a test concepts4.C passes when it should fail

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109337

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|NEW                         |RESOLVED

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
invalid as noticed.

[Bug c++/109337] c++2a test concepts4.C passes when it should fail

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109337

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |NEW
         Resolution|INVALID                     |---

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Christopher Di Bella from comment #3)
> This is apparently a Clang bug: the RHS of `R42c` isn't evaluated because of
> short-circuiting. Apologies for the noise and thanks for helping me work
> through it.

No, clang and GCC disagree even on:
```
template <class Ub> concept A42b = true;
template <class Tc> concept R42c = A42b<Tc&>;

static_assert (R42c<void>);
```

There is no short-circuting here.

[Bug c++/109337] c++2a test concepts4.C passes when it should fail

[richard-gccbugzilla at metafoo dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109337

Richard Smith <richard-gccbugzilla at metafoo dot co.uk> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |richard-gccbugzilla@metafoo
                   |                            |.co.uk

--- Comment #6 from Richard Smith <richard-gccbugzilla at metafoo dot co.uk> ---
(In reply to Andrew Pinski from comment #5)
> (In reply to Christopher Di Bella from comment #3)
> > This is apparently a Clang bug: the RHS of `R42c` isn't evaluated because of
> > short-circuiting. Apologies for the noise and thanks for helping me work
> > through it.
> 
> No, clang and GCC disagree even on:
> ```
> template <class Ub> concept A42b = true;
> template <class Tc> concept R42c = A42b<Tc&>;
> 
> static_assert (R42c<void>);
> ```
> 
> There is no short-circuting here.

No substitution is performed into 'Tc&' here, because normalization of R42c
produces the atomic constraint 'true' with an empty parameter mapping. So the
type 'void&' is never formed.

[Bug c++/109337] c++2a test concepts4.C passes when it should fail

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109337

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
                 CC|                            |ppalka at gcc dot gnu.org
             Status|NEW                         |RESOLVED

--- Comment #7 from Patrick Palka <ppalka at gcc dot gnu.org> ---
dup of PR102419 AFAICT

*** This bug has been marked as a duplicate of bug 102419 ***