[Bug c++/90538] New: Redeclaration error when expanding parameter pack multiple times in a lambda

[wtt6 at cornell dot edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90538

            Bug ID: 90538
           Summary: Redeclaration error when expanding parameter pack
                    multiple times in a lambda
           Product: gcc
           Version: 9.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wtt6 at cornell dot edu
  Target Milestone: ---

With gcc-9.1.0, compiling this code
---------------------------------------
template <class... T>
void a(const T&...) {}

template <class... T>
void b(const T&... t) {
  [&]() {
    a(t...);
    a(t...);
  };
}

void c() {
  b(1);
}
---------------------------------------
gives errors
---------------------------------------
test.cpp: In instantiation of ‘void b(const T& ...) [with T = {int}]’:
test.cpp:13:6:   required from here
test.cpp:8:7: error: redeclaration of ‘const int& t#0’
    8 |     a(t...);
      |       ^
test.cpp:7:7: note: ‘const int& t#0’ previously declared here
    7 |     a(t...);
      |       ^
test.cpp:6:3: error: member ‘b(const T& ...) [with T = {int}]::<lambda()>::<t#0
capture>’ is uninitialized reference
    6 |   [&]() {
      |   ^~~~~~~
    7 |     a(t...);
      |     ~~~~~~~~
    8 |     a(t...);
      |     ~~~~~~~~
    9 |   };
      |   ~
test.cpp:6:3: error: designator order for field ‘b(const T& ...) [with T =
{int}]::<lambda()>::<t#0 capture>’ does not match declaration order in ‘b(const
T& ...) [with T = {int}]::<lambda()>’
---------------------------------------

This was accepted by gcc 8.3.