| Issue |
103644
|
| Summary |
Clang 19 regression with mismatched parameter packs in fold expanded constraint
|
| Labels |
clang:frontend,
c++26,
regression:19
|
| Assignees |
|
| Reporter |
CaseyCarter
|
Compiling this well-formed program:
```c++
template <class...>
using void_t = void;
template <class...>
struct tuple {};
template <class Tuple1, class Tuple2>
struct helper {};
template <class... Types1, class... Types2>
requires (requires { typename void_t<Types1, Types2>; } && ...)
struct helper<tuple<Types1...>, tuple<Types2...>> {
using type = void;
};
template <class>
concept False = false;
template <class First, class... Rest>
struct tuple<First, Rest...> : private tuple<Rest...> {
template <False Other>
friend auto f(const tuple&, const Other&) -> helper<tuple, Other>::type;
};
template <class... Types1, class... Types2>
void_t<void_t<Types1, Types2>...> f(const tuple<Types1...>&, const tuple<Types2...>&);
int main() {
using T = tuple<int, int>;
f(T{}, T{});
}
```
with clang 19.1.0-rc2 in `-std=c++26` mode diagnoses:
```
repro.cpp:9:55: error: pack expansion contains parameter packs 'Types1' and 'Types2' that have different lengths (1 vs. 2)
9 | (requires { typename void_t<Types1, Types2>; } && ...)
| ~~~~~~ ~~~~~~ ^
repro.cpp:19:12: note: while checking constraint satisfaction for class template partial specialization 'helper<int, int, int>' required here
19 | friend helper<tuple, Other>::type f(const tuple&, const Other&) {}
| ^~~~~~
repro.cpp:19:12: note: during template argument deduction for class template partial specialization 'helper<tuple<Types1...>, tuple<Types2...>>'
[with Types1 = <int>, Types2 = <int, int>]
repro.cpp:19:12: note: in instantiation of template class 'helper<tuple<int>, tuple<int, int>>' requested here
repro.cpp:28:5: note: while substituting deduced template arguments into function template 'f' [with Other = T]
28 | f(T{}, T{});
| ^
1 error generated.
```
The current release versions of the "big 3" compile this program just fine (https://godbolt.org/z/e71q4Eqhr), as does Clang 19.1.0-rc2 in c++20 and c++23 modes.
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