A constant value failed to get evaluated.

Erroneous code example:

enum Enum {
    X = (1 << 500),
    Y = (1 / 0),
}

This error indicates that the compiler was unable to sensibly evaluate a constant expression that had to be evaluated. Attempting to divide by 0 or causing an integer overflow are two ways to induce this error.

Ensure that the expressions given can be evaluated as the desired integer type.

See the Custom Discriminants section of the Reference for more information about setting custom integer types on fieldless enums using the repr attribute.