>> The question I have is: why doesn't (or can't) the second case work? In >> function 'test2' the compiler believes 'a' is a constant expression and the I don't think that's how it works. In test2 the expression test1(a) is just a normal expression, calling a function that returns an int. 'a' is not a constant expression in the body of test2. In the right context it can be substituted with a constant expression, but is not actually a constant expression. In the definition of the constexpr variable 'two' the expression test(2) is a call to a constexpr function, so "function invocation substitution" takes place. That substitutes the constant 2 in place of 'a' in the body of test2, producing test1(2), which is also a call to a constexpr function, so function invocation substitution occurs again, substituting 2 for 'a' in the body of test1, which produces 'return 2' which is a constant expression. Similarly, in test3 'a' is not a constant expression (it's just something which could be substituted for one in the right context) and so it can't be used in a way which *requires* a constant expression, such as a template parameter. Does that help?
