// 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: