[Bug c++/109017] New: ICE on unexpanded pack from C++20 explicit-template-parameter lambda syntax

["arthur.j.odwyer at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109017

            Bug ID: 109017
           Summary: ICE on unexpanded pack from C++20
                    explicit-template-parameter lambda syntax
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Keywords: ice-on-invalid-code
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arthur.j.odwyer at gmail dot com
  Target Milestone: ---

// https://godbolt.org/z/bdT84P318
template<class...>
struct A {
    static void g() {
        int dummy[] = {
            []<class... Ts>(Ts... ts){
                int i = Ts();
            } ...
        };
    }
};
int main() {
    A<int>::g();
}


<source>: In instantiation of 'static void A< <template-parameter-1-1> >::g()
[with <template-parameter-1-1> = {int}]':
<source>:12:14:   required from here
<source>:4:13: internal compiler error: in tsubst_pack_expansion, at
cp/pt.cc:13388
    4 |         int dummy[] = {
      |             ^~~~~
0x247a53e internal_error(char const*, ...)
        ???:0
0xae95dc fancy_abort(char const*, int, char const*)
        ???:0
0xd09f45 instantiate_decl(tree_node*, bool, bool)
        ???:0
0xd362db instantiate_pending_templates(int)
        ???:0
0xbe4555 c_parse_final_cleanups()
        ???:0
0xe21948 c_common_parse_file()
        ???:0
Please submit a full bug report, with preprocessed source (by using
-freport-bug).
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.
Compiler returned: 1


Apparently GCC fails to recognize the `class... Ts` in the angle brackets (new
syntax in C++20) as pertaining to the lambda itself, and not to the outer
scope. So when GCC sees an unexpanded `Ts` in the inner scope, GCC happily
accepts it (and then ICEs). Notice that if you change `Ts()` to `ts` in the
inner scope, GCC correctly rejects the code with the usual "parameter packs not
expanded" error. The ICE is triggered specifically by misuse of
`Ts`-from-the-new-in-C++20-syntax.