// term has a tilde. In all other cases coreTerm returns (nil, false).
func coreTerm(tpar *TypeParam) (*term, bool) {
	n := 0
	var single *term // valid if n == 1
	var tilde bool
	tpar.is(func(t *term) bool {
		if t == nil {
			assert(n == 0)
			return false // no terms
		}
		n++
		single = t
		if t.tilde {
			tilde = true
		}
		return true

// term has a tilde. In all other cases coreTerm returns (nil, false).
func coreTerm(tpar *TypeParam) (*term, bool) {
	n := 0
	var single *term // valid if n == 1
	var tilde bool
	tpar.is(func(t *term) bool {
		if t == nil {
			assert(n == 0)
			return false // no terms
		}
		n++
		single = t
		if t.tilde {
			tilde = true
		}
		return true

		if err != nil {
			return nil // invalid type
		}
		for _, term := range terms {
			subpath := lockPath(tpkg, term.Type(), seen)
			if len(subpath) > 0 {
				if term.Tilde() {
					// Prepend a tilde to our lock path entry to clarify the resulting
					// diagnostic message. Consider the following example:
					//
					//  func _[Mutex interface{ ~sync.Mutex; M() }](m Mutex) {}
					//

	// Those usually end in a tilde followed by one or more ASCII digits.
	if tilde := strings.LastIndexByte(short, '~'); tilde >= 0 && tilde < len(short)-1 {
		suffix := short[tilde+1:]
		suffixIsDigits := true
		for _, r := range suffix {
			if r < '0' || r > '9' {
				suffixIsDigits = false
				break
			}
		}
		if suffixIsDigits {
			return fmt.Errorf("trailing tilde and digits in path element")
		}
	}

pkg go/ast, type IndexListExpr struct, X Expr
pkg go/ast, type TypeSpec struct, TypeParams *FieldList
pkg go/constant, method (Kind) String() string
pkg go/token, const TILDE = 88
pkg go/token, const TILDE Token
pkg go/types, func Instantiate(*Context, Type, []Type, bool) (Type, error)
pkg go/types, func NewContext() *Context
pkg go/types, func NewSignature //deprecated

						case 0x81: // U+0301 COMBINING ACUTE ACCENT
							s = "\u00cd" // U+00CD LATIN CAPITAL LETTER I WITH ACUTE
						case 0x83: // U+0303 COMBINING TILDE
							s = "\u0128" // U+0128 LATIN CAPITAL LETTER I WITH TILDE
						case 0x88: // U+0308 COMBINING DIAERESIS
							s = "\u00cf" // U+00CF LATIN CAPITAL LETTER I WITH DIAERESIS
						default:
						}
						if s != "" {
							c.pDst = oldPDst

		// in interfaces and syntactically cannot be empty.
		if t.Len() == 0 {
			w.error("empty union")
			break
		}
		for i, t := range t.terms {
			if i > 0 {
				w.string(termSep)
			}
			if t.tilde {
				w.byte('~')
			}
			w.typ(t.typ)
		}

	case *Interface:
		if w.ctxt == nil {
			if t == universeAnyAlias.Type().Underlying() {

		// in interfaces and syntactically cannot be empty.
		if t.Len() == 0 {
			w.error("empty union")
			break
		}
		for i, t := range t.terms {
			if i > 0 {
				w.string(termSep)
			}
			if t.tilde {
				w.byte('~')
			}
			w.typ(t.typ)
		}

	case *Interface:
		if w.ctxt == nil {
			if t == universeAnyAlias.Type().Underlying() {

		if !t.includes(V) {
			// If V ∉ t.typ but V ∈ ~t.typ then remember this type
			// so we can suggest it as an alternative in the error
			// message.
			if alt == nil && !t.tilde && Identical(t.typ, under(t.typ)) {
				tt := *t
				tt.tilde = true
				if tt.includes(V) {
					alt = t.typ
				}
			}
			return true
		}
		return false
	}) {
		if cause != nil {
			var detail string
			switch {

		if !t.includes(V) {
			// If V ∉ t.typ but V ∈ ~t.typ then remember this type
			// so we can suggest it as an alternative in the error
			// message.
			if alt == nil && !t.tilde && Identical(t.typ, under(t.typ)) {
				tt := *t
				tt.tilde = true
				if tt.includes(V) {
					alt = t.typ
				}
			}
			return true
		}
		return false
	}) {
		if cause != nil {
			var detail string
			switch {