[BUG] friend and this for commutative operators

[JohelEGP]

Title: friend and this for commutative operators.

Description:

This is a first write of an operator* in Cpp2:

quantity: @struct <Unit: type> type = {
  number: i32;
  operator*: (this, r: i32) -> quantity = (number * r);
}
operator*: <Unit> (l: i32, r: quantity<Unit>) -> quantity<Unit> = r * l;

The signatures are unfortunately too dissimilar and far apart.
We could remediate:

quantity: @struct <Unit: type> type = {
  number: i32;
  operator/: /* ... */;
  operator%: /* ... */;
}
operator*: <Unit> (l: quantity<Unit>, r: i32) -> quantity<Unit> = (number * r);
operator*: <Unit> (l: i32, r: quantity<Unit>) -> quantity<Unit> = r * l;

Not only is this more verbose,
we've moved them away from similar operators,
operators which still share the dissimilarity.
I think this is the ideal:

quantity: @struct <Unit: type> type = {
  number: i32;
  friend operator*: (this, r: i32) -> quantity = (number * r);
  friend operator*: (l: i32, this) -> quantity = r * l;
}

[JohelEGP]

  friend operator*: (l: i32, this) -> quantity = r * l;

It seems like this implied a hidden friend,
as I can't make make it an out-of-line definition:
https://compiler-explorer.com/z/saf9Gcdob.

[JohelEGP]

Looks like you have to make it a template that doesn't use
the injected-class-name or template parameters of the type:
https://compiler-explorer.com/z/EjaKGx9Mn.

[JohelEGP]

I think this is the ideal:

quantity: @struct <Unit: type> type = {
  number: i32;
  friend operator*: (this, r: i32) -> quantity = (number * r);
  friend operator*: (l: i32, this) -> quantity = r * l;
}

If a non-first parameter this implies a Cpp1 friend or equivalent out-of-line declaration,
there's no need for the friend keyword in Cpp2:

quantity: @struct <Unit: type> type = {
  number: i32;
  operator*: (this, r: i32) -> quantity = (number * r);
  operator*: (l: i32, this) -> quantity = r * l;
}

[JohelEGP]

operator<< was special-cased to emit as a friend: b589f5d#diff-e683e1259bbc0d6de7f1fbf797fb772f31a0a1ef2254ec317d15c298102fa3f3R5514.

[JohelEGP]

Now it's generalized to streaming and arithmetic operators: ca42e2c#diff-e683e1259bbc0d6de7f1fbf797fb772f31a0a1ef2254ec317d15c298102fa3f3R5515.