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

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

[arthur.j.odwyer at gmail dot com]

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.

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

[hiraditya at msn dot com]

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

AK <hiraditya at msn dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hiraditya at msn dot com

--- Comment #1 from AK <hiraditya at msn dot com> ---
Example from twitter: https://twitter.com/seanbax/status/1631689332007337985
which had discussion on similar bug.

```
template<int... Is> struct outer1_t {
  void g() {
    // Compiles for mysterious reasons.
    int array[] { []<int... Is2>(){
      int i = Is2;
      return i;
    }.template operator()<Is>() ... };
  }
};

int main() {
  // Compiles OKAY when this is commented out.
  // ICEs when it's compiled.
  outer1_t<1, 5, 10>().g();
}
```

clang issues a compiler error: https://godbolt.org/z/7f6E55svM

```
<source>:6:15: error: initializer contains unexpanded parameter pack 'Is2'
      int i = Is2;
```

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

[rguenth at gcc dot gnu.org]

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

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-03-06
      Known to fail|                            |10.4.1, 12.2.1, 13.0
             Status|UNCONFIRMED                 |NEW

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.

[Bug c++/109017] ICE on unexpanded pack from C++20 explicit-template-parameter lambda syntax since r8-7648-g8859913ea3cbefdc

[marxin at gcc dot gnu.org]

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

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|ICE on unexpanded pack from |ICE on unexpanded pack from
                   |C++20                       |C++20
                   |explicit-template-parameter |explicit-template-parameter
                   |lambda syntax               |lambda syntax since
                   |                            |r8-7648-g8859913ea3cbefdc
                 CC|                            |jason at gcc dot gnu.org,
                   |                            |marxin at gcc dot gnu.org

--- Comment #3 from Martin Liška <marxin at gcc dot gnu.org> ---
Started with r8-7648-g8859913ea3cbefdc.