// applyTypeFunc returns nil.
// If x is not a type parameter, the result is f(x).
func (check *Checker) applyTypeFunc(f func(Type) Type, x *operand, id builtinId) Type {
	if tp, _ := Unalias(x.typ).(*TypeParam); tp != nil {
		// Test if t satisfies the requirements for the argument
		// type and collect possible result types at the same time.
		var terms []*Term
		if !tp.is(func(t *term) bool {
			if t == nil {

		err.addf(nopos, "want %s", check.typesSummary(varTypes(params), sig.variadic))
		err.report()
		return
	}

	// collect type parameters of callee and generic function arguments
	var tparams []*TypeParam

	// collect type parameters of callee
	n := sig.TypeParams().Len()
	if n > 0 {
		if !check.allowVersion(call.Pos(), go1_18) {
			if iexpr, _ := call.Fun.(*syntax.IndexExpr); iexpr != nil {

		err.addf(noposn, "want %s", check.typesSummary(varTypes(params), sig.variadic))
		err.report()
		return
	}

	// collect type parameters of callee and generic function arguments
	var tparams []*TypeParam

	// collect type parameters of callee
	n := sig.TypeParams().Len()
	if n > 0 {
		if !check.allowVersion(call, go1_18) {
			switch call.Fun.(type) {
			case *ast.IndexExpr, *ast.IndexListExpr:

// If typ is a type parameter, underIs returns the result of typ.underIs(f).
// Otherwise, underIs returns the result of f(under(typ)).
func underIs(typ Type, f func(Type) bool) bool {
	typ = Unalias(typ)
	if tpar, _ := typ.(*TypeParam); tpar != nil {
		return tpar.underIs(f)
	}
	return f(under(typ))
}

// The unary expression e may be nil. It's passed in for better error messages only.

// If typ is a type parameter, underIs returns the result of typ.underIs(f).
// Otherwise, underIs returns the result of f(under(typ)).
func underIs(typ Type, f func(Type) bool) bool {
	typ = Unalias(typ)
	if tpar, _ := typ.(*TypeParam); tpar != nil {
		return tpar.underIs(f)
	}
	return f(under(typ))
}

func (check *Checker) unary(x *operand, e *syntax.Operation) {
	check.expr(nil, x, e.X)
	if x.mode == invalid {
		return
	}