A parameter pack appears both empty and non-empty at the same time (original) (raw)

Found a really concerning bug:

template <typename ...P> struct A {};

struct B { consteval B() {} // Making this constexpr makes the assert below pass.

template <typename ...P>
consteval operator A<P...>() const
{
    static_assert(sizeof...(P) > 0); // This fails on Clang.
    return {};
}

};

template struct type_identity { using type = T; };

template <typename ...P> void foo(typename type_identity<A<P...>>::type a, P...) {}

int main() { foo(B(), 1, 2); }

The static_assert fires on Clang, meaning it calls operator A<> with no template arguments! But the corresponding function parameter is A<int, int>.

Apparently A<> gets reinterpreted (!!) into A<int, int> at compile-time. E.g. if you add a int x = sizeof...(P) data member to it, it'll be 0 despite the type actually being A<int, int>.

Here's an example of this being used to reinterpret_cast a float into an int: https://gcc.godbolt.org/z/Txbsj8ofM

Tested on Clang 20 and trunk. The only flag needed is -std=c++20 for consteval.