Why Can't I Use a Function Parameter in a `constexpr` Function as a Constant Expression?

Can't Use Function Parameter of a constexpr Function in a Constant Expression

The code snippet provided shows a constexpr function make_const and a function t1 that attempts to use make_const with a non-constant expression. This raises an error because i in t1 is not a constant expression.

A constexpr function, when given constant arguments, can be evaluated at compile time. However, if a non-constexpr parameter is passed to a constexpr function, it does not make that parameter a constant expression.

In the code below, t1 is a constexpr function, but make_const(i) inside t1 is not a constant expression because i is not a constant:

constexpr int t1(const int i)
{
    return make_const(i);
}

The updated code shows that t1 can be declared as constexpr and return the result of make_const:

constexpr int t1(const int i)
{
    return make_const(i);
}

However, the code below will still result in an error because do_something() is not a constant expression:

template<int i>
constexpr bool do_something(){
    return i;
}

constexpr int t1(const int i)
{
    return do_something<make_const(i)>();
}

To summarize, a constexpr function parameter must be a constant expression. If a non-constant parameter is passed, it does not become a constant expression within the constexpr function.

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