A parameter pack appears both empty and non-empty at the same time #137885
Description
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 <typename T>
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.