I do not understand why this compiles

Question

I'm certainly missing something, but I do not understand why this compiles (with both g++ & clang++):

struct A
{
};
struct B
{
};

int main()
{
  A a(B);
}

First of all, B is a type... not a value. How should I interpret this code?

Answer 1

It's interpreted as the declaration of a function named a, which takes one argument of type B and returns A.

Answer 2

It's simply a function declaration declaring a to be a function returning A and taking one unnamed parameter of type B.

It is valid because function declarations as opposed to function definitions are allowed within function definitions.