clang: pack expansion contains parameter pack 'Args' that has a different length (1 vs. 2) from outer parameter packs

[jcelerier]

Hello,

repro:

#include <functional>

struct worker {
    std::function<void(int, int)> request;
    static void work(int, int);
} worker;

template <typename T = int>
void connect_worker() {
    worker.request = []<typename... Args>(Args&&... f) {
        [... ff = std::forward<Args>(f)]() mutable {
            worker::work(std::forward<Args>(ff)...);
        }();
    };
}

int main() { connect_worker(); }

causes:

<source>:14:48: error: pack expansion contains parameter pack 'Args' that has a different length (1 vs. 2) from outer parameter packs
            worker::work(std::forward<Args>(ff)...);
                                      ~~~~     ^

I'm confident this is a compiler bug as if one turns connect_worker into a non-template by removing template <typename T = int>, then the code builds and runs as expected:

https://gcc.godbolt.org/z/YM6ar7WG3

also,

static_assert(sizeof...(Args) == 2);

Just before the offending line passes too.

Not forwarding the arguments works too:

worker::work(ff...);

[shafik]

Maybe duplicate of: #61415

CC @erichkeane

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[zyn0217]

Should have been fixed by #101385