[clang] [c++20] build failure with libstdc++'s ranges code

[h-2]

#include <ranges>
#include <string_view>
#include <string>
#include <vector>

int main()
{
    auto s = std::vector<std::string>{};
    auto v = s | std::views::transform([] (std::string & s2) { return std::string_view{s2}; })
               | std::views::join;
    auto b = std::ranges::begin(v);      
}

godbolt

This builds with llvm-trunk&libc++, and it builds with gcc12&libstdc++, but it fails to build with llvm-trunk&libstdc++.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[royjacobson]

The error is

error: no viable constructor or deduction guide for deduction of template arguments of '_Optional_func'
            this->_M_payload._M_apply(_Optional_func{__f}, __i);

This is the definition of _Optional_func:

template<typename _Fn> struct _Optional_func { _Fn& _M_f; };

So to me this looks like a case of missing CTAD for aggregates: #54051

[yuanfang-chen]

Fixed with https://reviews.llvm.org/D139837