Checking Type Parameters for Optional Interface Satisfaction

With Go generics, it is not permitted to check whether a value whose type comes from a type parameter satisfies some optional interface. For example, the following code will not compile (as of Go 1.20):

type Q interface{ Q() }

func G[T any](t T) {
	if q, ok := t.(Q); ok {
		q.Q()
		fmt.Println("Q", q)
	}
	fmt.Println("G", t)
}

Specifically, the compiler complains:

invalid operation: cannot use type assertion on type parameter value t (variable of type T constrained by any)

This follows from the definition at Type Assertions which says (emphasis mine):

For an expression x of interface type, but not a type parameter, and a type T, the primary expression x.(T) asserts that x is not nil and that the value stored in x is of type T. […] If T is an interface type, x.(T) asserts that the dynamic type of x implements the interface T.

The exemption of type parameters permits the compiler to reject x whose type comes from a type parameter.

We can work around this, however, by introducing a temporary variable that is of interface type:

func F[T any](t T) {
	var v any = t  // v is type any, not just a type constrained by any

	if q, ok := v.(Q); ok {  // N.B. type assertion is on v
		q.Q()
		fmt.Println("Q", q)
	}
	fmt.Println("F", t)
}

This example demonstrates that this version correctly handles both a type that implements the optional Q interface, and one that does not.

creachadair/type-parameter-iface.md

Checking Type Parameters for Optional Interface Satisfaction