https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107853
Bug ID: 107853
Summary: Cannot create friend function template with variadic
pack that depends on variadic pack
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: davidfromonline at gmail dot com
Target Milestone: ---
The following valid translation unit
```
template<typename, typename>
concept c = true;
template<typename... Ts>
struct s {
template<typename... Us> requires (... and c<Us, Ts>)
friend void f(Us...) {
}
};
auto x = s<>();
```
is rejected by gcc with the error message
```
<source>: In instantiation of 'struct s<>':
<source>:11:14: required from here
<source>:6:27: error: mismatched argument pack lengths while expanding 'c<Us,
Ts>'
6 | template<c<Ts>... Us>
| ^~
Compiler returned: 1
```
See it live: https://godbolt.org/z/4qGKP9K9a
Note that adding a single template parameter to the instantiated `s`, for
instance, `s<int>`, causes gcc to ICE:
```
template<typename, typename>
concept c = true;
template<typename... Ts>
struct s {
template<typename... Us> requires (... and c<Us, Ts>)
friend void f(Us...) {
}
};
auto x = s<int>();
```
```
<source>: In instantiation of 'struct s<int>':
<source>:11:17: required from here
<source>:6:52: internal compiler error: in use_pack_expansion_extra_args_p, at
cp/pt.cc:12674
6 | template<typename... Us> requires (... and c<Us, Ts>)
| ~~~~~~~~~^~~~~~~~~~
0x22f921e internal_error(char const*, ...)
???:0
0xa4d7bc fancy_abort(char const*, int, char const*)
???:0
0xac1bb0 tsubst_constraint(tree_node*, tree_node*, int, tree_node*)
???:0
0xac1c99 maybe_substitute_reqs_for(tree_node*, tree_node const*)
???:0
0xc7e28c instantiate_class_template(tree_node*)
???:0
0xccdb7f complete_type_or_maybe_complain(tree_node*, tree_node*, int)
???:0
0xcef1a8 build_functional_cast(unsigned int, tree_node*, tree_node*, int)
???:0
0xc26267 c_parse_file()
???:0
0xd61389 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
```
See it live: https://godbolt.org/z/3dfvb5ov3
Using terse syntax (`template<c<Ts>... Us>`) has the same behavior.
