How Can I Use Shared Methods with Union Type Constraints in Go Generics?

Generic Constraints in Go: Using Shared Methods for Union Types

In Go generics (v1.18), a type union constraint allows you to define a set of types that can be assigned to a generic type parameter. However, using a shared method for these types can lead to errors.

Consider the following code:

type A struct {}
type B struct {}

type AB interface {
    *A | *B
}

func (a *A) some() bool {
    return true
}

func (b *B) some() bool {
    return false
}

func some[T AB](x T) bool {
    return x.some()   // <- error
}

The error occurs because the compiler cannot find the some method on the union type AB. To resolve this, you need to add the method to the interface constraint:

type AB interface {
    *A | *B
    some() bool
}

func some[T AB](x T) bool {
    return x.some()   // works
}

This restricts the generic type T to types that implement both *A and *B, and also define the some method.

However, as noted in the Go 1.18 release notes, this is a temporary limitation. Go 1.19 is expected to remove this restriction, allowing the compiler to automatically infer the some method from the union type.

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