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From: "rl.alt.accnt at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109692] New: ICE on concept as default template parameter
 to iife lambda in fold expression in static member template function
Date: Tue, 02 May 2023 06:17:25 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: rl.alt.accnt at gmail dot com
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109692

            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 templat=
e.
Both Clang and MSVC accept the code (see also this godbolt link:
https://godbolt.org/z/x5Ys9sPzx):
```c++
template <typename opt> concept always_true =3D true;

template <typename ...params>
struct variadic {
    template <typename opt> static void callee() { }
    static void bug() {
        ([]<bool x =3D always_true<params>>() {=20
            if constexpr (x) callee<params>();=20
        }(), ...);
    }
};

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 =3D
true]':
<source>:9:10:   required from 'static void variadic<params>::bug() [with
params =3D {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>**, b=
ool,
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=