[Bug c++/111602] New: "Error: symbol is already defined" for variable template dependent on default argument lambda used in sfinae and non-sfinae

[Bug c++/111602] New: "Error: symbol is already defined" for variable template dependent on default argument lambda used in sfinae and non-sfinae

[ed at catmur dot uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111602

            Bug ID: 111602
           Summary: "Error: symbol is already defined" for variable
                    template dependent on default argument lambda used in
                    sfinae and non-sfinae
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ed at catmur dot uk
  Target Milestone: ---

Since gcc 8.1,

template<class> constexpr bool V = true;
struct F { template<class T, bool = V<T>> F(T) {} };
template<class T> struct C { C(F = []{}) {} };
template<class T> decltype(T()) g() { return T(); }
auto f() { struct T {} t; return t; };
int main() { g<C<decltype(f())>>(); }

compiled at -O0 (and -std=c++14 or above) causes assembler error:

/tmp/cccVZwwU.s: Assembler messages:
/tmp/cccVZwwU.s:106: Error: symbol `_Z1VIZN1CIZ1fvE1TEC4E1FEd_UlvE_E' is
already defined

The symbol demangles as 'V<C<f()::T>::C(F)::{default arg#1}::{lambda()#1}>'.

[Bug c++/111602] "Error: symbol is already defined" for variable template dependent on default argument lambda of class dependent on local type used in sfinae and non-sfinae contexts

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111602

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2023-09-26
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Slightly reduced:
```
template<class> constexpr bool V = true;
struct F
{
        template<class T, bool = V<T>>
        F(T) {}
};
template<class T> struct C
{
        C(F = []{}) {}
};
template<class T>
decltype(T()) g()
{
        return T();
}
int main()
{
        g<C<int>>();
}
```

The local type of f() is not needed.

[Bug c++/111602] [11/12/13/14 Regression] "Error: symbol is already defined" for variable template dependent on default argument lambda of class dependent on local type used in sfinae and non-sfinae contexts

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111602

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|"Error: symbol is already   |[11/12/13/14 Regression]
                   |defined" for variable       |"Error: symbol is already
                   |template dependent on       |defined" for variable
                   |default argument lambda of  |template dependent on
                   |class dependent on local    |default argument lambda of
                   |type used in sfinae and     |class dependent on local
                   |non-sfinae contexts         |type used in sfinae and
                   |                            |non-sfinae contexts
   Target Milestone|---                         |11.5
      Known to fail|                            |8.1.0
      Known to work|                            |7.1.0

[Bug c++/111602] [11/12/13/14 Regression] "Error: symbol is already defined" for variable template dependent on default argument lambda of class dependent on local type used in sfinae and non-sfinae contexts

[ed at catmur dot uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111602

--- Comment #2 from Ed Catmur <ed at catmur dot uk> ---
(In reply to Andrew Pinski from comment #1)
> The local type of f() is not needed.

Thanks! I was confused since the local type is required in this similar
example:

template<class> struct P
{
    static constexpr bool value = true;
};
template<class T> constexpr bool P<T>::value;
template<bool> struct E {};
struct F
{
    template<class T, class = E<P<T>::value>>
    F(T) {}
};
template<class> struct C
{
    C(F = []{}) {}
};
auto f()
{
    struct T {} t;
    return t;
}
template<class T>
decltype(T()) g()
{
    return T();
}
int main()
{
    g<C<decltype(f)>>();
}

[Bug c++/111602] [11/12/13/14 Regression] "Error: symbol is already defined" for variable template dependent on default argument lambda of class dependent on local type used in sfinae and non-sfinae contexts

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111602

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

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

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Started with r8-2720-gf44a8dd56f5bfb FWIW