[Bug c++/88555] New: [9 Regression] Pack expansion fails

[eric at efcs dot ca]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88555

            Bug ID: 88555
           Summary: [9 Regression] Pack expansion fails
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eric at efcs dot ca
  Target Milestone: ---

GCC fails to expand parameter packs when used in certain contexts. This is a
regression from 8.2.

https://godbolt.org/z/9MrTQQ

Reproducer:

// g++ -std=c++17
template <class ...> struct T {};

template <int ...Indices>
void test() {
    using Test = T<decltype((Indices, char(0)))...>;
}