[Issue 7301] RegexMatch opCast!bool not working

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=7301



--- Comment #8 from timon.gehr at gmx.ch 2012-01-17 13:13:19 PST ---
(In reply to comment #7)
> (In reply to comment #6)
> > (In reply to comment #5)
> > > (In reply to comment #4)
> > > > Neither int (outside the range [0,1]) nor RegexMatch implicitly convert to
> > > > bool. Negating counts as an explicit cast to bool.
> > > 
> > opCast means explicit cast, as in cast(bool)regexMatch or !regexMatch or
> > regexMatch && 2.
> 
> I think this is the crux of what we're arguing about.  To me, explicit casting
> happens when you write cast(T) in the code.  When the compiler generates a cast
> for you, that's implicit casting.
> 
> Is this not correct?

In D, cast(T) is an explicit cast as is using an object in a boolean context
such as if(...) !... ...||... ...&&... etc. Those are presumably the cases
referred to on the Operator Overloading page.

> 
> It seems to me that opCast is pretty useless unless the compiler can decide to
> use it to convert an object for you.
> 

It cannot, except in the cases where it inserts cast(bool).

> > > http://www.d-programming-language.org/operatoroverloading.html#Cast claims that
> > > both ! and bare references will get rewritten to opCast!bool:
> > > 
> > >   if (e)   =>  if (e.opCast!(bool))
> > >   if (!e)  =>  if (!e.opCast!(bool))
> > > 
> > > That should take care of the return, no?
> > 
> > That only works for conditions and arguments to boolean operators. If you think
> > it should also work for return values where the enclosing function is declared
> > to return bool, then that is an enhancement request. It might be a reasonable
> > one.
> 
> Why should it only work in those cases? 

It is about disabling some narrowing implicit conversions to bool while still
allowing them were it really makes sense.

bool b = 2; // error, int does not implicitly convert to bool
if(2){...} // fine, because explicit cast(bool)2 works.

> I can't find anything in the language docs that says this. 
> The docs on functions say that when a type is passed in,
> implicit conversion is used as one of the match possibilities.  That's opCast,
> no? 

No. opCast cannot be used for implicit conversions. At some point there was
even the idea of introducing opImplicitCast.

> So if parameter passing is using opCast for implicit conversion, the
> return statement should be too.  Hence my labeling this as a bug.
> 

Parameter passing does _not_ use opCast for implicit conversions.

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