Allow compound assignment with all binary operators

[DeidreHouse]

I noticed that compound assignment won't work with certain operators, like |> and ∘.

For example, the following code gives the error "ERROR: syntax: unexpected "="" on line 2:

a = 2
a |>= exp 

But if I say, redefine the * operator as |>, it works perfectly:

import Base.*
* = |>
a = 2
a *= exp

outputs 7.389, as it should. The same thing happens with the function composition operator. So clearly the language is able to do this, the parser just doesn't handle it properly with all operators. I tried this in both 0.6.4 and 0.7 and got the same results. Is there some deeper reason why this doesn't currently work?

[stevengj]

It's not really a "deep" reason, but right now the parser code uses an explicit list of assignment operators and a corresponding explicit list of lowering rules. It's impractical to extend this approach to all possible binary operators, so some surgery on the parsing/lowering code would be required.

[JeffBezanson]

duplicate of #15964