[Issue 7177] $ should forward to length by default

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http://d.puremagic.com/issues/show_bug.cgi?id=7177



--- Comment #44 from Andrei Alexandrescu <andrei at erdani.com> 2013-03-24 08:24:46 PDT ---
I gave this long thread a read and reached a decision. Sorry for the
dictatorial approach but this seems to be a matter in which reasonable people
may disagree so at some point we need to just choose a way and stick with it.
Among my priorities in choosing a behavior is that of simplicity - we can't
require people to mind a trait such as supportsOpDollar, or require them to
define an alias for opDollar in casual ranges.

So, there are a few cases to look at. Given a type T:

1. hasLength!T || isNarrowString!T, but T does not define a member opDollar.

Then ALL uses of expr.$ in indexing expressions involving values of type T will
automatically alias themselves to expr.length. This includes UFCS, i.e. if user
code defines a module-level property length(T), then expr.$ lowers into
expr.length which in turn may lower to length(expr).

(Note that hasLength!T currently does work with UFCS.)

2. T defines opDollar.

Great - nothing to do, it's all explicit.

3. expr.length has no meaning and T does not define opDollar.

In this case there is a compile-time error.

===========

Regarding having opDollar and/or other operators work as UCSF, this would be
desirable from a completeness/expectability/consistency standpoint. I don't
quite buy the argument that that opens a can of worms. However, seeing as
there's already a compiler change for that, I suggest we go this
least-committal route for now and defer that decision for later.

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