Forbid dynamic arrays in boolean evaluation contexts

[bearophile]

Vladimir Panteleev:

> That would silently break my code.

Right, that's why I have asked to just forbid something. The 
proposal is a little breaking change either way, but turning 
something into an error allows you to find it quickly in most 
cases. (But D also has the compiles __trait, that does not give 
an error, it just silently returns false instead of true).

On the other hand the behaviour change is not meant to happen all 
at once. It has to be a warning, then deprecation and then a 
change, along months and three or more DMD versions. So there is 
time to fix D code. And even if the old D code doesn't get 
changed, I think it's uncommon for code to rely on dynamic array 
to be actually (null,null) instead of being of zero length. So 
the total amount of breakage even for a semantic change is 
probably small. Considering the presence of the compiles __trait, 
the idea of the small behaviour change is not so bad.

So far this proposal was well received (almost everyone in the 
thread seems to agree to change the current behaviour. But some 
persons prefer to change it into an error, while others prefer a 
more handy semantics change. In both cases the change path will 
start at the same way: warning, deprecation, and then an error or 
a behavour change). This proposal is one of the about fifteen 
tiny D breaking changes that I am suggesting since years.

If Andrei Alexandrescu or Walter are reading this thread I'd like 
a comment on this very small proposal mostly meant to avoid 
certain bugs.

Bye,
bearophile