https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113427
Bug ID: 113427 Summary: ICE: tree check: C++23 `this auto` lambda + multiple (ambiguous) inheritance from closure type Product: gcc Version: unknown Status: UNCONFIRMED 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/fd3zzfrTd auto factory(int x) { return [x=x](this auto self) { return x; }; } using Lambda = decltype(factory(0)); struct LeftCat : Lambda { LeftCat() : Lambda(factory(1)) {} }; struct RightCat : Lambda { RightCat() : Lambda(factory(2)) {} }; struct SiameseCat : LeftCat, RightCat { using Lambda::operator(); }; int main() { return SiameseCat()(); } === <source>: In instantiation of 'factory(int)::<lambda(this auto:1)> [with auto:1 = SiameseCat]': <source>:11:24: required from here 11 | return SiameseCat()(); | ~~~~~~~~~~~~^~ <source>:3:32: error: 'factory(int)::<lambda(this auto:1)>' is an ambiguous base of 'SiameseCat' 3 | return [x=x](this auto self) { return x; }; | ^ <source>:3:32: error: 'factory(int)::<lambda(this auto:1)>' is an ambiguous base of 'SiameseCat' <source>:3:32: internal compiler error: tree check: expected class 'type', have 'exceptional' (error_mark) in tsubst_decl, at cp/pt.cc:15542 0x264113c internal_error(char const*, ...) ???:0 0x9641cf tree_class_check_failed(tree_node const*, tree_code_class, char const*, int, char const*) ???:0 0xa88c8c tree_class_check(tree_node*, tree_code_class, char const*, int, char const*) ???:0 0xc88d63 instantiate_decl(tree_node*, bool, bool) ???:0 0xb4de55 maybe_instantiate_decl(tree_node*) ???:0 0xb4f948 mark_used(tree_node*, int) ???:0 0xa81a04 build_op_call(tree_node*, vec<tree_node*, va_gc, vl_embed>**, int) ???:0 0xcce9e7 finish_call_expr(tree_node*, vec<tree_node*, va_gc, vl_embed>**, bool, bool, int) ???:0 0xc521ea c_parse_file() ???:0 0xda5b99 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. === Found by noodling around with explicit object parameters and corner cases related to https://eel.is/c++draft/expr.prim.lambda#closure-5 . That wording currently demands that the explicit object parameter of a capturing lambda be "a class type derived from the closure type," but it probably ought to say more like "a class type _unambiguously_ derived from the closure type." At least, I don't know what the expected behavior of the above program ought to be, if it's _not_ supposed to be ill-formed. I think GCC's ICE happens because GCC is trying too hard to assign a meaning to an essentially meaningless program.