3 @ - <space> 5 is what I object to (and Opal allows)
RBParser allows this one even if old Compiler dissallows it (i.e. in Pharo 2).
I haven't tracked if Opal follows the RBParser on that or if this is the reverse (Opal pushed changes on RBParser). I suspect all things RB in Pharo (and Squeak?) are a port from the Dolphin version of RB, which means this is allowed in quite a few other smalltalks (Dolphin?, VW?).
I haven't checked if the SmaCC Smalltalk parser accept that.
Note: 4 - 5 in RBParser does what you would expect. 4 - - 5 as well.
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3 @-5 and/or 3 @- 5 is (rightly) disallowed by both.
That one is easier and expected.
My position would be twofold: - RBParser is, IMHO, a good parser and I would follow its interpretation. That Opal reuses it is a good point for me.
- As a programming language designer (and parser implementor), accepting - 5 is user friendly, but a bit too contextual in the lexer to be nice to implement.