[Bug c++/98617] New: Failure to recognize pack expansion argument for non-pack parameter of alias template when alias is "exact"

[Bug c++/98617] New: Failure to recognize pack expansion argument for non-pack parameter of alias template when alias is "exact"

[ryan_greenblatt at brown dot edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98617

            Bug ID: 98617
           Summary: Failure to recognize pack expansion argument for
                    non-pack parameter of alias template when alias is
                    "exact"
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ryan_greenblatt at brown dot edu
  Target Milestone: ---

It is invalid to expand a parameter pack into a non pack parameter of an alias
template. However, gcc accepts this when the alias is exactly the same as the
struct it is aliasing.

Consider https://godbolt.org/z/YrqTTx

AliasA doesn't fail, but it should fail with the same error as AliasB and
AliasC.

[Bug c++/98617] Failure to recognize pack expansion argument for non-pack parameter of alias template when alias is "exact"

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98617

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-05-18
             Status|UNCONFIRMED                 |NEW

[Bug c++/98617] Failure to recognize pack expansion argument for non-pack parameter of alias template when alias is "exact"

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98617

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Please provide the actual code, not only a CE link:

template <typename T, typename V> struct Base {};

template <typename T, typename V> using A = Base<T, V>;

template <typename T> using B = Base<T, int>;

template <typename T, typename V, typename> using C = Base<T, V>;

template<typename...T>
using AliasA = A<T...>; // this should fail

template<typename...T>
using AliasB = B<T...>;

template<typename...T>
using AliasC = C<T...>;



Reduced to remove the bits that do fail as expected:


template <typename T, typename V> struct Base {};

template <typename T, typename V> using A = Base<T, V>;

template<typename...T>
using AliasA = A<T...>; // this should fail