// in normal form.
			return &Union{terms}
		}

	case *Interface:
		methods, mcopied := subst.funcList(t.methods)
		embeddeds, ecopied := subst.typeList(t.embeddeds)
		if mcopied || ecopied {
			iface := subst.check.newInterface()
			iface.embeddeds = embeddeds
			iface.embedPos = t.embedPos
			iface.implicit = t.implicit
			assert(t.complete) // otherwise we are copying incomplete data

	methods := make([]*types.Func, r.Len())
	embeddeds := make([]types.Type, r.Len())
	implicit := len(methods) == 0 && len(embeddeds) == 1 && r.Bool()

	for i := range methods {
		pos := r.pos()
		pkg, name := r.selector()
		mtyp := r.signature(nil, nil, nil)
		methods[i] = types.NewFunc(pos, pkg, name, mtyp)
	}

	for i := range embeddeds {
		embeddeds[i] = r.typ()
	}

				// note for embedded types, type name is field name, and "string" etc are lower case hence unexported.
				pkgAnnotate = true
				w.pkgInfo = false // only tag once
			}

			// This doesn't do the right thing for embedded type
			// aliases where we should print the alias name, not
			// the aliased type (see go.dev/issue/44410).
			if !f.embedded {
				w.string(f.name)
				w.byte(' ')
			}

			// in normal form.
			return &Union{terms}
		}

	case *Interface:
		methods, mcopied := subst.funcList(t.methods)
		embeddeds, ecopied := subst.typeList(t.embeddeds)
		if mcopied || ecopied {
			iface := subst.check.newInterface()
			iface.embeddeds = embeddeds
			iface.embedPos = t.embedPos
			iface.implicit = t.implicit
			assert(t.complete) // otherwise we are copying incomplete data

				// note for embedded types, type name is field name, and "string" etc are lower case hence unexported.
				pkgAnnotate = true
				w.pkgInfo = false // only tag once
			}

			// This doesn't do the right thing for embedded type
			// aliases where we should print the alias name, not
			// the aliased type (see go.dev/issue/44410).
			if !f.embedded {
				w.string(f.name)
				w.byte(' ')
			}

			tags[i] = tag
		}
		return types.NewStruct(fields, tags)

	case interfaceType:
		r.currPkg = r.pkg()

		embeddeds := make([]types.Type, r.uint64())
		for i := range embeddeds {
			_ = r.pos()
			embeddeds[i] = r.typ()
		}

		methods := make([]*types.Func, r.uint64())
		for i := range methods {
			mpos := r.pos()
			mname := r.ident()

	} 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

	} 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
			}