// and corresponding type argument lists. The type argument list may be shorter
// than the type parameter list, and it may contain nil types. Matching type
// parameters and arguments must have the same index.
func newUnifier(tparams []*TypeParam, targs []Type, enableInterfaceInference bool) *unifier {
	assert(len(tparams) >= len(targs))
	handles := make(map[*TypeParam]*Type, len(tparams))