switch e := n.(type) {
	case *ast.IndexExpr:
		return e.X, e.Lbrack, []ast.Expr{e.Index}, e.Rbrack
	case *ast.IndexListExpr:
		return e.X, e.Lbrack, e.Indices, e.Rbrack
	}
	return nil, token.NoPos, nil, token.NoPos
}

// PackIndexExpr returns an *ast.IndexExpr or *ast.IndexListExpr, depending on
// the cardinality of indices. Calling PackIndexExpr with len(indices) == 0

	return inst
}

func (check *Checker) callExpr(x *operand, call *syntax.CallExpr) exprKind {
	var inst *syntax.IndexExpr // function instantiation, if any
	if iexpr, _ := call.Fun.(*syntax.IndexExpr); iexpr != nil {
		if check.indexExpr(x, iexpr) {
			// Delay function instantiation to argument checking,
			// where we combine type and value arguments for type
			// inference.

			if n.Type != nil {
				m = n.Type
				continue
			}
			return n.Pos()
		case *KeyValueExpr:
			m = n.Key
		// case *FuncLit:
		// case *ParenExpr:
		case *SelectorExpr:
			m = n.X
		case *IndexExpr:
			m = n.X
		// case *SliceExpr:
		case *AssertExpr:
			m = n.X
		case *TypeSwitchGuard:
			if n.Lhs != nil {
				m = n.Lhs
				continue
			}
			m = n.X
		case *Operation:
			if n.Y != nil {

		left, right = n.X, n.Y
	}

	// Recognize m[k] = append(m[k], ...) so we can reuse
	// the mapassign call.
	var mapAppend *ir.CallExpr
	if left.Op() == ir.OINDEXMAP && right.Op() == ir.OAPPEND {
		left := left.(*ir.IndexExpr)
		mapAppend = right.(*ir.CallExpr)
		if !ir.SameSafeExpr(left, mapAppend.Args[0]) {
			base.Fatalf("not same expressions: %v != %v", left, mapAppend.Args[0])
		}
	}

	left = walkExpr(left, init)

//     unchanged.
//
// If an error (other than a version error) occurs in any case, it is reported
// and x.mode is set to invalid.
func (check *Checker) funcInst(T *target, pos token.Pos, x *operand, ix *typeparams.IndexExpr, infer bool) ([]Type, []ast.Expr) {
	assert(T != nil || ix != nil)

	var instErrPos positioner
	if ix != nil {
		instErrPos = inNode(ix.Orig, ix.Lbrack)

					i = p.badExpr()
				} else {
					i, comma = p.typeList(false)
				}
				if comma || p.tok == _Rbrack {
					p.want(_Rbrack)
					// x[], x[i,] or x[i, j, ...]
					t := new(IndexExpr)
					t.pos = pos
					t.X = x
					t.Index = i
					x = t
					break
				}
			}

			// x[i:...

	case *CompositeLit:
		if n.Type != nil {
			Walk(v, n.Type)
		}
		walkList(v, n.Elts)

	case *ParenExpr:
		Walk(v, n.X)

	case *SelectorExpr:
		Walk(v, n.X)
		Walk(v, n.Sel)

	case *IndexExpr:
		Walk(v, n.X)
		Walk(v, n.Index)

	case *IndexListExpr:
		Walk(v, n.X)
		walkList(v, n.Indices)

	case *SliceExpr:
		Walk(v, n.X)
		if n.Low != nil {
			Walk(v, n.Low)
		}

		case invalid:
			// ignore - error reported before
		case novalue:
			check.errorf(&x, NotAType, "%s used as type", &x)
		default:
			check.errorf(&x, NotAType, "%s is not a type", &x)
		}

	case *ast.IndexExpr, *ast.IndexListExpr:
		ix := typeparams.UnpackIndexExpr(e)
		check.verifyVersionf(inNode(e, ix.Lbrack), go1_18, "type instantiation")
		return check.instantiatedType(ix, def)

	case *ast.ParenExpr:

func recvString(recv ast.Expr) string {
	switch t := recv.(type) {
	case *ast.Ident:
		return t.Name
	case *ast.StarExpr:
		return "*" + recvString(t.X)
	case *ast.IndexExpr:
		// Generic type with one parameter.
		return fmt.Sprintf("%s[%s]", recvString(t.X), recvParam(t.Index))
	case *ast.IndexListExpr:
		// Generic type with multiple parameters.
		if len(t.Indices) > 0 {

		kind := check.rawExpr(nil, x, e.X, nil, false)
		x.expr = e
		return kind

	case *ast.SelectorExpr:
		check.selector(x, e, nil, false)

	case *ast.IndexExpr, *ast.IndexListExpr:
		ix := typeparams.UnpackIndexExpr(e)
		if check.indexExpr(x, ix) {
			if !enableReverseTypeInference {
				T = nil
			}
			check.funcInst(T, e.Pos(), x, ix, true)
		}
		if x.mode == invalid {
			goto Error