More templated type argument inference

[Timon Gehr]

On 12/07/2012 11:12 PM, bearophile wrote:
> Timon Gehr:
>
>> I consider it necessary.
>
> To consider opening an enhancement request, are you able to explain us
> why?

It is just embarrassing if it does not work. Similar things work even in 
C#. Of course, it is harder in D because the type system is Turing 
complete and stuff.

I think we want to support things like:

[1,2,3].map(a=>2*a);

> And is it possible to implement it? :-)
>

Yes; in fact it almost works satisfactorily in my home-grown front end 
already.
As most of D compilation, it is undecidable, but the following heuristic 
should be fine:

1. Resolve and fix explicit template arguments.
2. Infer types from arguments. Unify. Do not consider known lambda
    return types at this point.
3. Analyze lambdas whose parameter types are known. Fix those types.
4. Resolve lambda return values where known. Unify.
5. Repeat 2-4 until fixed point is reached.
6. Use default template arguments.
7. Repeat 5-6 until fixed point is reached.
8. Check if everything became known and if the argument types are
    actually compatible with the inferred signature.

(Where 'fix' means, if the type should later be unified with something
that is not the same type, fail instantiation.)

Furthermore, probably it should treat curried lambdas a little more
cleverly than the above. It may be argued that this is too complex. :o)