Empty struct is wrongly treated as a compile-time constant in C++

[nickhuang99]

Given following code( https://www.godbolt.org/z/v9r5691bG ) , it gives error "non-type template argument is not a constant expression" of "takeInt(n)", complaining "n" has unknown value even though "takeInt" is constexpr returning value of "0".

template<int> struct A { };
constexpr int takeInt(int n){return 1;}
auto f(int n)-> A<takeInt(n)>;

However, similar function with parameter type of struct B passes compilation:

template<int> struct A { };
struct B{};
constexpr int takeB(B b){return 0;}
auto g(B b)-> A<takeB(b)>;

Why does parameter "b" as struct passes while "n" as int fails? These are parameter to function "takeB" and "takeInt" which is not template argument of "A". The template argument of "A" is the constexpr function expression "takeB(b)" and "takeInt(n)" which evaluates to be constexpr value "0".

[Quuxplusone]

This is the standard C++ behavior. A function parameter, being provided by the caller, is never treated as a compile-time constant. It's never okay to write

int f(int n) { static_assert(n); }

because n isn't known at the time the function is compiled. You can't use n in a static_assert, or in an array bound, or in a template argument, because it's not a compile-time constant.

Making the function constexpr doesn't change any of this reasoning.

constexpr int f(int n) { static_assert(n); }  // adding "constexpr" doesn't matter
int main(int argc, char**) { f(argc); }

[llvmbot]

@llvm/issue-subscribers-c-1

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[Quuxplusone]

Actually, I take it back. The Clang bug here is that g is accepted.
https://www.godbolt.org/z/fY4aW61Ka

struct B {};

constexpr int takeB(auto) { return 0; }

void g(B b) {
    static_assert(takeB(b) == 0);  // Clang accepts, but should reject
}

Changing B from an empty struct to using B = int; or struct B {int i=0;}; or pretty much any other type causes Clang correctly to reject. It's only when B is an empty struct that we have the problem.