Cannot Use X as Type Y

Why This Happens

Go is statically typed and does not allow implicit type conversions. Even if two types have the same underlying representation, like type MyInt int and int, they are distinct types. You must explicitly convert between them.

The Problem

type UserID int

func getUser(id int) string {
    return "user"
}

func main() {
    var uid UserID = 42
    getUser(uid) // cannot use uid (variable of type UserID) as type int
}

The Fix

func main() {
    var uid UserID = 42
    getUser(int(uid)) // explicit conversion
}

Step-by-Step Fix

1. 1

   Identify the type mismatch

   Read the compiler error to see which type was provided and which type was expected.

2. 2

   Determine the relationship

   Check if the types share the same underlying type and can be converted, or if they are fundamentally different.

3. 3

   Apply the conversion

   Use an explicit type conversion like int(x) or implement the required interface.