From: Peter Hickman <peterhickman386@...>
Date: 2011-01-17T20:45:22+09:00
Subject: Re: Obscure syntax error

Well, and I have to admit I am guessing here, the source is parsed in
the context of the code that precedes it. So 'x :y' is parsed when
nothing (no method or variable) called x exists so the parser is
thinking that x should be a method, because it has not been told
otherwise and the :y makes sense as a parameter to a method.

In the case of 'x = nil; x :y' the 'x = nil' means that we now have
something defined for x (a variable) which the parser can then take
into account when it gets to 'x :y'. Whereas before with 'x :y' it
assumes that x is a method because x has not been defined. But with 'x
= nil; x :y' we know that x is a variable and the parser no longer
assumes that x is a method and tries to make sense of 'a variable'
followed by 'a symbol'. Which make no sense, thus the error.

I don't know if this is the case as I do not routinely browse the Ruby
parser code but from what I know this makes sense - even if I may not
have explained it clearly.