Many clients will need something like this:

type Type interface {
    ...
    IsAssignableTo(Type) bool
}

This is a generalization of the Implements predicate defined thus:
  Implements(conc Type, iface *Interface) bool
in which you always have a concrete type and an interface.

FWIW, my code only needs the latter.  Specifically,

ssa/interp only need this method:
  func MissingMethod(conc types.Type, iface *types.Interface) *types.Method

ssa/pointer only needs this method:
  func ImplementsInterface(conc types.Type, iface *types.Interface) bool {
    return MissingMethod(conc, iface) == nil
  }