Implement CWG2369

[gonzalobg]

From http://wg21.link/cwg2369 , https://gcc.godbolt.org/z/nEqhM5dsG
The changes are already in the draft, starting here (https://eel.is/c++draft/temp.deduct.general#5.sentence-8) till the end of that paragraph.

template <class T> struct Z {
  typedef typename T::x xx;
};
template <class T> concept C = requires { typename T::A; };
template <C T> typename Z<T>::xx f(void *, T); // #1
template <class T> void f(int, T);             // #2
struct A {} a;
struct ZZ {
    template <class T, class = typename Z<T>::xx> operator T *();
    operator int();
};
int main() {
    ZZ zz;
    f(1, a);   // OK, deduction fails for #1 because there is no conversion from int to void*
    f(zz, 42); // OK, deduction fails for #1 because C<int> is not satisfied
}

compiles with GCC 11 and newer, but fails with clang-trunk and GCC 10 and older because they don't implement the CWG2369.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[jyknight]

Xref to an issue users ran into with the GCC implementation, in case it's useful to take into account while implementing in Clang: https://gcc.gnu.org/PR99599 -- though I'm not sure what the proper outcome of that is.

[cor3ntin]

I heard of more users who need the implementation of this

[zyn0217]

I remembered I have explored it a few months ago, and for some point (I might be wrong) it somehow ends up relying on @LYP951018's deferred lambda body instantiation. Not quite sure if I misunderstood something, but I'm going to give it another try.

[cor3ntin]

@zyn0217 Hum, the lambda case might indeed be tricky... it would be great to support at least the non-lambda case, and go from there. Thanks!