inplace_function doesn't handle arguments with rvalue references well about sg14

[Quuxplusone]

If I understand it correctly, the problem is that operator() can't use a forwarding reference for the function arguments because they are not deduced in the function (they are parameters of the class template).

Not really. The forwarding references are correct (although you are also correct that they're not "forwarding" references in the most common flavor). The bug was that in one place there was a lambda of the form

[](storage_ptr_t storage_ptr, Args&&... args)
        noexcept(noexcept(std::declval<C>()(args...))) -> R
{
    return (*static_cast<C*>(storage_ptr))( std::forward<Args>(args)... );
}

Notice that it takes (pseudo-)forwarding references, and calls std::forward on them in the body of the function, but the noexcept-specification uses args without std::forward! This triggers the hard error you were seeing.

I fixed this by adding the missing std::forward in cb66020, and then on second thought decided to remove the offending noexcept-specification entirely in 76784f8 — it wasn't serving any functional purpose except to gather bug reports. :P I've written before on how nobody gets noexcept-specifications right, including myself.