Is this a bug?

[John Colvin via Digitalmars-d]

On Thursday, 26 June 2014 at 12:38:40 UTC, John Colvin wrote:
> On Thursday, 26 June 2014 at 10:09:53 UTC, Rene Zwanenburg 
> wrote:
>> I /think/ this is a bug, but I'm not 100% sure. The following 
>> compiles without any problems, as it should:
>>
>> import std.typecons;
>>
>> alias Handle(T) = RefCounted!(T, RefCountedAutoInitialize.no);
>>
>> auto initialized(T)() if(is(T == RefCounted!S, S...))
>> {
>> 	T refCounted;
>> 	refCounted.refCountedStore.ensureInitialized();
>> 	return refCounted;
>> }
>>
>> alias S = Handle!S_Impl;
>> struct S_Impl
>> {
>> }
>>
>> void main()
>> {
>> 	auto s = initialized!S;
>> }
>>
>>
>> Change 'initialized' to:
>> auto initialized(T)() if(is(T == Handle!S, S))
>> and the compiler will complain:
>>
>> Error: template instance test.initialized!(RefCounted!(S_Impl, 
>> cast(RefCountedAutoInitialize)0)) does not match template 
>> declaration initialized(T)() if (is(T == Handle!S, S))
>
> I don't think that's a bug, but I think the template constraint 
> itself shouldn't be legal to write. It's not possible to 
> support the general case of a template A in is(T == A!X, X) as 
> it would require following all possible instantiation paths of 
> A.
>
> The compiler could keep track of how a type was created on a 
> per-identifier basis and then use that to check, but that would 
> mean rejecting types that had a different instantiation path 
> (e.g. is(RefCounted!(int, RefCountedAutoInitialize.no) == 
> Handle!S, S) == false).
>
> What might be feasible would be to extend is(T == A!X, X) to 
> work with simple tmeplates and paramterised aliases like 
> Handle, while statically disallowing more complicated / 
> undecidable ones..

Ah, it appears that this has already been done. Good.

If I've misunderstood the situation, somebody please correct me.