Re: isBidirectionalRange fails for unknown reasons

[anonymous via Digitalmars-d-learn]

On 16.12.2015 21:43, Jack Stouffer wrote:
I'm trying to add a ReferenceBidirectionalRange range type to
std.internal.test.dummyrange so I can test some range code I'm writing,
but I've hit a wall and I'm not sure why. For some reason, the
isBidirectionalRange check fails even though back and popBack are
present. Any help here would be appreciated.
[...]

class ReferenceForwardRange(T) : ReferenceInputRange!T
{
     this(Range)(Range r) if (isInputRange!Range) { super(r); }
     final @property ReferenceForwardRange save()
     {return new ReferenceForwardRange!T( _payload); }
}

class ReferenceBidirectionalRange(T) : ReferenceForwardRange!T
{
     this(Range)(Range r) if (isInputRange!Range) { super(r); }
     final @property ref T back(){ return _payload.back; }
     final void popBack(){ _payload.popBack(); }
}

The `.save` primitive of forward ranges must return the very same type 
that the range has. But your ReferenceBidirectionalRange!T.save returns 
a ReferenceForwardRange!T, because it's inherited. That makes 
isForwardRange!(ReferenceBidirectionalRange!T) fail, and everything that 
depends on it.

You can override `save` in ReferenceBidirectionalRange or try something 
clever like using a template this parameter:

@property auto save(this This)() {return new This( _payload);}