} else {
		unionSets = make(map[*Union]*_TypeSet)
	}

	// Methods of embedded interfaces are collected unchanged; i.e., the identity
	// of a method I.m's Func Object of an interface I is the same as that of
	// the method m in an interface that embeds interface I. On the other hand,
	// if a method is embedded via multiple overlapping embedded interfaces, we
	// don't provide a guarantee which "original m" got chosen for the embedding

				return false
			}
		}

	case *Union:
		for _, t := range t.terms {
			if !check.validType0(pos, t.typ, nest, path) {
				return false
			}
		}

	case *Interface:
		for _, etyp := range t.embeddeds {
			if !check.validType0(pos, etyp, nest, path) {
				return false
			}
		}

	case *Named:
		// TODO(gri) The optimization below is incorrect (see go.dev/issue/65711):

				return false
			}
		}

	case *Union:
		for _, t := range t.terms {
			if !check.validType0(pos, t.typ, nest, path) {
				return false
			}
		}

	case *Interface:
		for _, etyp := range t.embeddeds {
			if !check.validType0(pos, etyp, nest, path) {
				return false
			}
		}

	case *Named:
		// TODO(gri) The optimization below is incorrect (see go.dev/issue/65711):

	} else {
		unionSets = make(map[*Union]*_TypeSet)
	}

	// Methods of embedded interfaces are collected unchanged; i.e., the identity
	// of a method I.m's Func Object of an interface I is the same as that of
	// the method m in an interface that embeds interface I. On the other hand,
	// if a method is embedded via multiple overlapping embedded interfaces, we
	// don't provide a guarantee which "original m" got chosen for the embedding

	}

	return checkComparability()
}

// mentions reports whether type T "mentions" typ in an (embedded) element or term
// of T (whether typ is in the type set of T or not). For better error messages.
func mentions(T, typ Type) bool {
	switch T := T.(type) {
	case *Interface:
		for _, e := range T.embeddeds {
			if mentions(e, typ) {
				return true
			}
		}
	case *Union:
		for _, t := range T.terms {

			// a new *Interface before modifying receivers.
			if iface == orig.underlying {
				old := iface
				iface = check.newInterface()
				iface.embeddeds = old.embeddeds
				assert(old.complete) // otherwise we are copying incomplete data
				iface.complete = old.complete
				iface.implicit = old.implicit // should be false but be conservative
				underlying = iface
			}

			// a new *Interface before modifying receivers.
			if iface == orig.underlying {
				old := iface
				iface = check.newInterface()
				iface.embeddeds = old.embeddeds
				assert(old.complete) // otherwise we are copying incomplete data
				iface.complete = old.complete
				iface.implicit = old.implicit // should be false but be conservative
				underlying = iface
			}

	}

	return checkComparability()
}

// mentions reports whether type T "mentions" typ in an (embedded) element or term
// of T (whether typ is in the type set of T or not). For better error messages.
func mentions(T, typ Type) bool {
	switch T := T.(type) {
	case *Interface:
		for _, e := range T.embeddeds {
			if mentions(e, typ) {
				return true
			}
		}
	case *Union:
		for _, t := range T.terms {

		}

	case *Union:
		for _, t := range t.terms {
			w.typ(t.typ)
		}

	case *Interface:
		for _, m := range t.methods {
			w.typ(m.typ)
		}
		for _, t := range t.embeddeds {
			w.typ(t)
		}

	case *Map:
		w.typ(t.key)
		w.typ(t.elem)

	case *Chan:
		w.typ(t.elem)

	case *Named:
		for _, tpar := range t.TypeArgs().list() {
			w.typ(tpar)
		}

		}

	case *Union:
		for _, t := range t.terms {
			w.typ(t.typ)
		}

	case *Interface:
		for _, m := range t.methods {
			w.typ(m.typ)
		}
		for _, t := range t.embeddeds {
			w.typ(t)
		}

	case *Map:
		w.typ(t.key)
		w.typ(t.elem)

	case *Chan:
		w.typ(t.elem)

	case *Named:
		for _, tpar := range t.TypeArgs().list() {
			w.typ(tpar)
		}