[Bug c++/96213] New: GCC doesn't complain about ill-formed non-dependent template default argument

[Bug c++/96213] New: GCC doesn't complain about ill-formed non-dependent template default argument

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96213

            Bug ID: 96213
           Summary: GCC doesn't complain about ill-formed non-dependent
                    template default argument
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arthur.j.odwyer at gmail dot com
  Target Milestone: ---

Possibly related (although these seem to complain about the opposite of what
I'm complaining about):
- https://gcc.gnu.org/bugzilla/show_bug.cgi?id=12672
- https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58071


    // https://godbolt.org/z/EYzxx9
    template<class> int g;

    template<int = g<int>(0,1,2)>
    void h() { }

    int main() {
        h<1>();
    }

MSVC and Clang both reject template `h` as ill-formed, because `g<int>(0,1,2)`
is nonsense -- `g<int>` is an `int` and thus cannot be called like a function.

GCC accepts template `h` as well-formed, and in fact will treat this as a
SFINAE situation:

    template<class> int g;
    template<int = g<int>(42)> void h() {} // #1
    template<class = void> void h() {}     // #2

    int main() {
        h<>();  // unambiguously calls #2, because #1 has a deduction failure
    }

My guess is that MSVC and Clang are closer to correct here.

[Bug c++/96213] GCC doesn't complain about ill-formed non-dependent template default argument

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96213

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
                 CC|                            |mpolacek at gcc dot gnu.org
           Keywords|                            |accepts-invalid
   Last reconfirmed|                            |2020-07-16
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Your observation makes sense to me, so confirmed.

[Bug c++/96213] GCC doesn't complain about ill-formed non-dependent template default argument

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96213

--- Comment #2 from Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> ---
Here's a similar situation I just ran into again, somehow!

// https://godbolt.org/z/3TKG1z
struct S {};
template<class = decltype(S(42), void())> void f() {}
template<int I=42, class = decltype(S(I), void())> void g() {}
int main() {
    f();  // correctly errors out
    g();  // incorrectly accepted by GCC
}

GCC is happy to treat `S(I)` as well-formed when `I` is a template parameter.
Obviously `decltype(S(I), void())` can't be anything because `S(I)` is
ill-formed, so I'm not sure what GCC is doing to compile this.

[Bug c++/96213] GCC doesn't complain about ill-formed non-dependent template default argument

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96213

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|NEW                         |RESOLVED
                 CC|                            |ppalka at gcc dot gnu.org
   Target Milestone|---                         |13.0

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Looks like we reject the first testcase since r13-6380, and the second since
r11-434, so we can close this PR.  Adding a new regression test doesn't seem
necessary in either case since these are manifestations of more general issues
(recognition of non-dependent variable template-ids and instantiation of
non-dependent decltype) that are already captured by existing tests.