[Bug c++/115318] New: decltype(lambda) with some templates causes ICE

[Bug c++/115318] New: decltype(lambda) with some templates causes ICE

[blubban at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115318

            Bug ID: 115318
           Summary: decltype(lambda) with some templates causes ICE
           Product: gcc
           Version: 14.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: blubban at gmail dot com
  Target Milestone: ---

template<typename T> struct S {};

template<int i = 4>
void foo()
{
    using foo_t = decltype([](){});
    []<int k = 6>() { S<foo_t>{}; }();
}

void bar() { foo(); }


-std=c++20


<source>: In instantiation of 'foo<>()::<lambda()> [with int k = 6]':
<source>:7:36:   required from 'void foo() [with int i = 4]'
    7 |     []<int k = 6>() { S<foo_t>{}; }();
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
<source>:10:17:   required from here
   10 | void bar() { foo(); }
      |              ~~~^~
<source>:6:28: internal compiler error: in tsubst, at cp/pt.cc:16401
    6 |     using foo_t = decltype([](){});
      |                            ^~~~~~
0x202ef4c internal_error(char const*, ...)
        ???:0
0x778915 fancy_abort(char const*, int, char const*)
        ???:0
0x938dd0 tsubst(tree_node*, tree_node*, int, tree_node*)
        ???:0
0x943a11 tsubst_lambda_expr(tree_node*, tree_node*, int, tree_node*)
        ???:0
0x939200 tsubst(tree_node*, tree_node*, int, tree_node*)
        ???:0
0x93c437 tsubst_template_args(tree_node*, tree_node*, int, tree_node*)
        ???:0
0x931a0f instantiate_decl(tree_node*, bool, bool)
        ???:0
0x831fb7 maybe_instantiate_decl(tree_node*)
        ???:0
0x8332f7 mark_used(tree_node*, int)
        ???:0
0x7a20a3 build_op_call(tree_node*, vec<tree_node*, va_gc, vl_embed>**, int)
        ???:0
0x9602fe finish_call_expr(tree_node*, vec<tree_node*, va_gc, vl_embed>**, bool,
bool, int)
        ???:0
0x931a0f instantiate_decl(tree_node*, bool, bool)
        ???:0
0x94d2e3 instantiate_pending_templates(int)
        ???:0
0x835608 c_parse_final_cleanups()
        ???:0
0xa0c508 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


If you make the first lambda a template ( decltype([]<int>(){}) ), you'll get a
internal compiler error: Segmentation fault instead. Probably the same root
cause.


Online reproducer: https://godbolt.org/z/EfxPxKWhd


I tried searching for duplicates, but every open issue I could find in bug
#107430 is a mess, I can't determine if it's the same.

[Bug c++/115318] decltype(lambda) from an template function inside a templated lambda causes ICE

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115318

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
            Summary|decltype(lambda) with some  |decltype(lambda) from an
                   |templates causes ICE        |template function inside a
                   |                            |templated lambda causes ICE
   Last reconfirmed|                            |2024-06-01

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug c++/115318] decltype(lambda) from an template function inside a templated lambda causes ICE

[blubban at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115318

--- Comment #2 from Alfred Agrell <blubban at gmail dot com> ---
struct a { int c; };

template<int=1>
struct ddd {
    int f()
    {
        using a2 = decltype([]<int=2>(int a::*)->a{}(&a::c));
        return []<int=4>(int a2::*){ return 2; }(&a::c);
    }
};

int xs() { return ddd().f(); }


-std=c++20 again


-> <source>:8:16: sorry, unimplemented: mangling offset_ref


https://godbolt.org/z/4M3ToqqPW


Adding a <int=2> to the first lambda changes the error to the same ICE as this
issue (https://godbolt.org/z/Pq9T16GvP), so I'm about 98% sure it's the same
root cause.