Why doesn't an if constexpr make this core constant expression error disappear?

Question

In reference to this question. The core constant expression that is used to initialize the constexpr variable y is ill-formed. So much is a given.<

Answer 1

The standard doesn't say much about the discarded statement of an if constexpr. There are essentially two statements in [stmt.if] about these:

1. In an enclosing template discarded statements are not instantiated.
2. Names referenced from a discarded statement are not required ODR to be defined.

Neither of these applies to your use: the compilers are correct to complain about the constexpr if initialisation. Note that you'll need to make the condition dependent on a template parameter when you want to take advantage of the instantiation to fail: if the value isn't dependent on a template parameter the failure happens when the template is defined. For example, this code still fails:

template <typename T>
void f() {
    constexpr int x = -1;
    if constexpr (x >= 0){
        constexpr int y = 1<<x;
    }
}

However, if you make x dependent on the type T it is OK, even when f is instantiated with int:

template <typename T>
void f() {
    constexpr T x = -1;
    if constexpr (x >= 0){
        constexpr int y = 1<<x;
    }
}
int main() {
    f<int>();
}

Answer 2

I'm not sure why you expect the branch to not be checked. The only time an if branch is "not checked" is when it is part of a template and not instantiated, as per [stmt.if]p2:

During the instantiation of an enclosing templated entity (Clause 17), if the condition is not value-dependent after its instantiation, the discarded substatement (if any) is not instantiated.

Your code doesn't seem to be in a situation where this applies.

Answer 3

Note that for the statement discarded by Constexpr If:

the discarded statement can't be ill-formed for every possible specialization:

To fix the issue you can make the statement depending on the template parameter, e.g.

template<typename T, int X> struct dependent_value { constexpr static int V = X; };

template <typename T>
void foo() {
    constexpr int x = -1;
    if constexpr (x >= 0){
        constexpr int y = 1 << dependent_value<T, x>::V;
    }
}

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