[Bug c++/109692] New: ICE on concept as default template parameter to iife lambda in fold expression in static member template function

[Bug c++/109692] New: ICE on concept as default template parameter to iife lambda in fold expression in static member template function

[rl.alt.accnt at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109692

            Bug ID: 109692
           Summary: ICE on concept as default template parameter to iife
                    lambda in fold expression in static member template
                    function
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rl.alt.accnt at gmail dot com
  Target Milestone: ---

The following code raises an ICE on GCC (trunk) and all versions later than and
including GCC 11.3. Earlier versions simply fail to instantiate the template.
Both Clang and MSVC accept the code (see also this godbolt link:
https://godbolt.org/z/x5Ys9sPzx):
```c++
template <typename opt> concept always_true = true;

template <typename ...params>
struct variadic {
    template <typename opt> static void callee() { }
    static void bug() {
        ([]<bool x = always_true<params>>() { 
            if constexpr (x) callee<params>(); 
        }(), ...);
    }
};

int main() { variadic<int>::bug(); }
```

GCC (trunk) on godbolt gives me this stack trace:
<source>: In instantiation of 'variadic<int>::bug()::<lambda()> [with bool x =
true]':
<source>:9:10:   required from 'static void variadic<params>::bug() [with
params = {int}]'
<source>:13:32:   required from here
<source>:8:44: error: template argument 1 is invalid
    8 |             if constexpr (x) callee<params>();
      |                              ~~~~~~~~~~~~~~^~
<source>:8:44: error: template argument 1 is invalid
<source>:8:44: internal compiler error: in constructor_name_p, at
cp/name-lookup.cc:4721
0x23605ae internal_error(char const*, ...)
        ???:0
0xa6dd64 fancy_abort(char const*, int, char const*)
        ???:0
0xc81ee5 instantiate_decl(tree_node*, bool, bool)
        ???:0
0xb5c9e1 maybe_instantiate_decl(tree_node*)
        ???:0
0xb5e45e mark_used(tree_node*, int)
        ???:0
0xa9da44 build_op_call(tree_node*, vec<tree_node*, va_gc, vl_embed>**, int)
        ???:0
0xcc7264 finish_call_expr(tree_node*, vec<tree_node*, va_gc, vl_embed>**, bool,
bool, int)
        ???:0
0xc79864 tsubst_pack_expansion(tree_node*, tree_node*, int, tree_node*)
        ???:0
0xc81ee5 instantiate_decl(tree_node*, bool, bool)
        ???:0
0xcad44b instantiate_pending_templates(int)
        ???:0
0xb61635 c_parse_final_cleanups()
        ???:0
0xd91568 c_common_parse_file()
        ???:0

[Bug c++/109692] [12/13/14 Regression] ICE on concept as default template parameter to iife lambda in fold expression in static member template function since r12-8110-ge2c7070ac77405

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109692

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|ICE on concept as default   |[12/13/14 Regression] ICE
                   |template parameter to iife  |on concept as default
                   |lambda in fold expression   |template parameter to iife
                   |in static member template   |lambda in fold expression
                   |function                    |in static member template
                   |                            |function since
                   |                            |r12-8110-ge2c7070ac77405
   Last reconfirmed|                            |2023-05-02
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
                 CC|                            |marxin at gcc dot gnu.org,
                   |                            |ppalka at gcc dot gnu.org

--- Comment #1 from Martin Liška <marxin at gcc dot gnu.org> ---
Started with r12-8110-ge2c7070ac77405.

[Bug c++/109692] [12/13/14 Regression] ICE on concept as default template parameter to iife lambda in fold expression in static member template function since r12-8110-ge2c7070ac77405

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109692

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.4

[Bug c++/109692] [12/13/14 Regression] ICE on concept as default template parameter to iife lambda in fold expression in static member template function since r12-8110-ge2c7070ac77405

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109692

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P4
           Keywords|                            |error-recovery,
                   |                            |ice-on-valid-code,
                   |                            |rejects-valid

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
So indeed GCC 11.2 doesn't ICE but errors like

t.C: In instantiation of 'static void variadic<params>::bug() [with params =
{int}]':
t.C:13:29:   required from here
t.C:9:47: error: no match for call to '(variadic<int>::bug()::<lambda()>) ()'
    7 |                         ([]<bool x = always_true<params>>() {
      |                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    8 |                                      if constexpr (x) callee<params>();
      |                                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    9 |                                              }(), ...);
      |                                              ~^~
t.C:7:26: note: candidate: 'template<bool x> variadic<int>::bug()::<lambda()>'
    7 |                         ([]<bool x = always_true<params>>() {
      |                          ^
t.C:7:26: note:   template argument deduction/substitution failed:

but I regard we should accept this.  Given the ICE is error-recovery the
regression should be P4 and that we reject the code isn't a regression.

Separating these bugs might be nice.  Since it's marked as regression it's P4.