// number of index expressions.
//
// For nodes that don't represent index expressions, the first return value of
// UnpackIndexExpr will be nil.
func UnpackIndexExpr(n ast.Node) (x ast.Expr, lbrack token.Pos, indices []ast.Expr, rbrack token.Pos) {
	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
	}

	Lbrack  token.Pos  // position of "["
	Indices []ast.Expr // index expressions
	Rbrack  token.Pos  // position of "]"
}

func (x *IndexExpr) Pos() token.Pos {
	return x.Orig.Pos()
}

func UnpackIndexExpr(n ast.Node) *IndexExpr {
	switch e := n.(type) {
	case *ast.IndexExpr:
		return &IndexExpr{
			Orig:    e,
			X:       e.X,
			Lbrack:  e.Lbrack,
			Indices: []ast.Expr{e.Index},
			Rbrack:  e.Rbrack,
		}

			obj = pass.TypesInfo.Uses[v]
		case *ast.SelectorExpr:
			obj = pass.TypesInfo.Uses[v.Sel]
		case *ast.IndexExpr, *ast.IndexListExpr:
			// Check generic functions such as "f[T1,T2]".
			x, _, _, _ := typeparams.UnpackIndexExpr(v)
			if id, ok := x.(*ast.Ident); ok {
				obj = pass.TypesInfo.Uses[id]
			}
		default:
			return
		}

		// Only want functions.
		if _, ok := obj.(*types.Func); !ok {
			return
		}

		// it is a *types.Func and not a *types.Var.
		// Example: Don't match a slice m within the expression `m[0]()`.
		isInstance = true
		fun, _, _, _ = typeparams.UnpackIndexExpr(fun)
	}

	var obj types.Object
	switch fun := fun.(type) {
	case *ast.Ident:
		obj = info.Uses[fun] // type, var, builtin, or declared func
	case *ast.SelectorExpr:
		if sel, ok := info.Selections[fun]; ok {

			return true
		}
	}
	return false
}

func funcIdent(fun ast.Expr) *ast.Ident {
	switch fun := astutil.Unparen(fun).(type) {
	case *ast.IndexExpr, *ast.IndexListExpr:
		x, _, _, _ := typeparams.UnpackIndexExpr(fun) // necessary?
		id, _ := x.(*ast.Ident)
		return id
	case *ast.Ident:
		return fun
	default:
		return nil
	}
}

// funcLitInScope returns a FuncLit that id is at least initially assigned to.

		WriteExpr(buf, x.X)
		buf.WriteByte(')')

	case *ast.SelectorExpr:
		WriteExpr(buf, x.X)
		buf.WriteByte('.')
		buf.WriteString(x.Sel.Name)

	case *ast.IndexExpr, *ast.IndexListExpr:
		ix := typeparams.UnpackIndexExpr(x)
		WriteExpr(buf, ix.X)
		buf.WriteByte('[')
		writeExprList(buf, ix.Indices)
		buf.WriteByte(']')

	case *ast.SliceExpr:
		WriteExpr(buf, x.X)
		buf.WriteByte('[')
		if x.Low != nil {

		}
	}).describef(atPos(pos), "verify instantiation")

	return inst
}

func (check *Checker) callExpr(x *operand, call *ast.CallExpr) exprKind {
	ix := typeparams.UnpackIndexExpr(call.Fun)
	if ix != nil {
		if check.indexExpr(x, ix) {
			// Delay function instantiation to argument checking,
			// where we combine type and value arguments for type
			// inference.

		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:

			ptr = true
			rtyp = t.X
		default:
			break L
		}
	}

	// unpack type parameters, if any
	switch rtyp.(type) {
	case *ast.IndexExpr, *ast.IndexListExpr:
		ix := typeparams.UnpackIndexExpr(rtyp)
		rtyp = ix.X
		if unpackParams {
			for _, arg := range ix.Indices {
				var par *ast.Ident
				switch arg := arg.(type) {
				case *ast.Ident:
					par = arg
				case *ast.BadExpr:

		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
		}