Function types with and without receiver are interchangeability


In it says

Non-literal values of function types with and without receiver are interchangeable

I want to discuss similar interchangeability further:

class X {
public fun toInt() = 0;

typealias SX1 = (X)->Int;
typealias SX2 = X.()->Int;

var to1:SX1 = X::toInt
var to2:SX2 = X::toInt

to2(with receiver signature) can be invoked using both ways:

val i1=to2(x1)
val i2=x1.to2();

but to1 (with first parameter signature) can be used only:

val i3=to1(x1)

But these doesn’t work:

val i4=x1.to1();

Can one commnet why this limitation, what it breaks if it is allowed ?

(English Isn’t my language, sorry)