为什么 `static_pointer_cast` 不能与 ADL 一起使用，但需要显式的 `std::`？

Question

Consider

// https://godbolt.org/z/z5M9b9jzx
#include <memory>
#include <cassert>

struct B {};
struct D : B {};

int main() {
    std::shared_ptr<B> b = std::make_shared<D>();
    auto d = static_pointer_cast<D>(b);
    assert(d);
}

I'd've expected the unqualified call to static_pointer_cast to resolve to std::static_pointer_cast , because b , being a std::shared_ptr , should bring namespace std in using ADL.

Why doesn't it? I need to write std::shared_pointer_cast explicitly to make it work.

Answer 1

https://en.cppreference.com/w/cpp/language/adl

Although a function call can be resolved through ADL even if ordinary lookup finds nothing, a function call to a function template with explicitly-specified template arguments requires that there is a declaration of the template found by ordinary lookup (otherwise, it is a syntax error to encounter an unknown name followed by a less-than character) (until C++20)

In C++20 mode your code compiles fine, demo: https://gcc.godbolt.org/z/b13q4hs68