[Bug c++/97819] New: Pack expansion in member initializer lists nested with their parameter list got rejected.

[Bug c++/97819] New: Pack expansion in member initializer lists nested with their parameter list got rejected.

[gnaggnoyil at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97819

            Bug ID: 97819
           Summary: Pack expansion in member initializer lists nested with
                    their parameter list got rejected.
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gnaggnoyil at gmail dot com
  Target Milestone: ---

The following code got accepted by Clang 10.0 and MSVC 19.27.29112, yet fails
to compile under GCC:

template <class ...Ts>
class foo : public Ts...{
public:
    template <typename T, typename ...Us>
    explicit foo(T &&t, Us &&...us)
        //: Ts(Ts(t, us))...{} -> This line is accepted
        //: Ts{t, us...}...{} -> This line is accepted too
        : Ts(t, us...)...{} // This line is rejected
};

class bar{
public:
    explicit bar(int){}
};

class qux{
public:
    explicit qux(int){}
};

class baz{
public:
    explicit baz(int){}
};

int main(){
    foo<bar, baz, qux> x(3);
    (void)x;
    return 0;
}

I tried GCC version 4.9.3 with -std=c++14, 5.5.0, 6.3.0, 7.3.0 with -std=c++17,
and 8.3.0, 9.3.0, 10.1.0 with -std=c++2a, all of them rejects this code.

Error outputs for GCC 4.9.3, 5.5.0, 9.3.0, 10.1.0 ones are:

prog.cc: In instantiation of 'foo<Ts>::foo(T&&, Us&& ...) [with T = int; Us =
{}; Ts = bar, baz, qux]':
prog.cc:27:27:   required from here
prog.cc:8:23: error: invalid use of pack expansion expression
         : Ts(t, us...)...{}
                       ^

Error outputs for GCC 6.3.0, 7.3.0, 8.3.0 ones are:

prog.cc: In instantiation of 'foo<Ts>::foo(T&&, Us&& ...) [with T = int; Us =
{}; Ts = {bar, baz, qux}]':
prog.cc:27:27:   required from here
prog.cc:8:23: error: no matching function for call to 'bar::bar(int&, bool)'
         : Ts(t, us...)...{}
                       ^~~
prog.cc:13:14: note: candidate: bar::bar(int)
     explicit bar(int){}
              ^~~
prog.cc:13:14: note:   candidate expects 1 argument, 2 provided
prog.cc:11:7: note: candidate: constexpr bar::bar(const bar&)
 class bar{
       ^~~
prog.cc:11:7: note:   candidate expects 1 argument, 2 provided
prog.cc:11:7: note: candidate: constexpr bar::bar(bar&&)
prog.cc:11:7: note:   candidate expects 1 argument, 2 provided
prog.cc:8:23: error: no matching function for call to 'baz::baz(int&, bool)'
         : Ts(t, us...)...{}
                       ^~~
prog.cc:23:14: note: candidate: baz::baz(int)
     explicit baz(int){}
              ^~~
prog.cc:23:14: note:   candidate expects 1 argument, 2 provided
prog.cc:21:7: note: candidate: constexpr baz::baz(const baz&)
 class baz{
       ^~~
prog.cc:21:7: note:   candidate expects 1 argument, 2 provided
prog.cc:21:7: note: candidate: constexpr baz::baz(baz&&)
prog.cc:21:7: note:   candidate expects 1 argument, 2 provided
prog.cc:8:23: error: no matching function for call to 'qux::qux(int&, bool)'
         : Ts(t, us...)...{}
                       ^~~
prog.cc:18:14: note: candidate: qux::qux(int)
     explicit qux(int){}
              ^~~
prog.cc:18:14: note:   candidate expects 1 argument, 2 provided
prog.cc:16:7: note: candidate: constexpr qux::qux(const qux&)
 class qux{
       ^~~
prog.cc:16:7: note:   candidate expects 1 argument, 2 provided
prog.cc:16:7: note: candidate: constexpr qux::qux(qux&&)
prog.cc:16:7: note:   candidate expects 1 argument, 2 provided

[Bug c++/97819] Pack expansion in member initializer lists nested with their parameter list got rejected.

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97819

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
                 CC|                            |mpolacek at gcc dot gnu.org
   Last reconfirmed|                            |2020-11-19

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Shorter test:

template <typename... T> struct S : T... {
  template <typename U, typename... V> S(U u, V... v) : T(u, v...)... {}
};
struct R {
  R(int);
};
void h() { S<R>(3); }

[Bug c++/97819] Pack expansion in member initializer lists nested with their parameter list got rejected.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97819

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=65422
      Known to fail|                            |11.1.0

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This looks fixed on the trunk

[Bug c++/97819] Pack expansion in member initializer lists nested with their parameter list got rejected.

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97819

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=88580
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
                 CC|                            |marxin at gcc dot gnu.org

--- Comment #3 from Martin Liška <marxin at gcc dot gnu.org> ---
Fixed in r12-174-g37d2b98100cefcb9.