Interesting user mistake

[Observer via Digitalmars-d]

On Thursday, 3 September 2015 at 17:15:25 UTC, Enamex wrote:
> On Thursday, 3 September 2015 at 16:46:30 UTC, Andrei 
> Alexandrescu wrote:
>> we should disallow during tokenization the sequence "=", "+", 
>> whitespace. Surely it's not a formatting anyone would aim for, 
>> but instead a misspelling of +=.
>
> Wasn't the original design for these operators in whichever 
> language started them actually the swapped form? In any case, 
> I'd personally choose to warn against any operator sequence 
> without separating whitespaces if the sequence isn't in fact 
> one operator (like "=!" (non-warn: "= !") vs "!=", and "=&", 
> and a lot others, given operator overloading).

The += operator originally appeared in C, as the =+ operator 
(K&R, 1/e, p212, "Anachronisms").  Not long afterward, the 
ambiguity of a=+b became apparent, along with the obvious need to 
change the language to resolve the problem.  The issue was dealt 
with over several successive releases of the compiler:

(1) =+ operator is available (original compiler)
(2) =+ and += are both available (I'm not sure whether this step 
existed)
(3) =+ and += are both available; =+ produces a deprecation 
warning
(4) += is available; =+ now produces either a warning/error or 
just changes meaning, not sure which

I don't recall now where I read about that sequence of steps, and 
a quick, incomplete scan of my library doesn't yield it up so I 
could be more precise.  Nonetheless, that's how the transition 
happened.  The Rationale for the original ANSI C (X3.159-1989) 
mentions that =op forms have been dropped, and that in a Quiet 
Change, "expressions of the form x=-3 change meaning with the 
loss of the old-style assignment operators".  Which I suppose 
implies that the Standard itself doesn't require a warning 
message, though presumably high-quality compilers would be free 
to implement one.

K&R C did not contain a unary + operator (K&R, 1/e, p. 37, 
Section 2.5).  It was added by the first ANSI C, "for symmetry 
with unary -" (K&R, 2/e, p204, Section A7.4.4).  "An integral 
operand undergoes integral promotion."

In terms of compiler quality, we have a long history of compilers 
generating warning messages for legal but questionable 
constructions.  The first one that comes quickly to mind is GCC 
complaining about "if(a=b)":

     warning: suggest parentheses around assignment used as truth 
value [-Wparentheses]

The notion here is that a common mistake is handled by:

(a) being warned about, when warnings are enabled (at least, by 
-Wall in GCC)
(b) having an alternate construction suggested (e.g., "if((a=b))")
(c) having a specific compiler flag to control generation of each 
such warning