UDAs - Restrict to User Defined Types?

[Jonathan M Davis]

On Thursday, November 08, 2012 21:10:55 Walter Bright wrote:
> Many algorithms (at least the ones in Phobos do) already do a check to
> ensure the inputs are the correct kind of range. I don't think you'll get
> very far trying to use a range that isn't a range.
> 
> Of course, you can always still have bugs in your range implementation.

Given that a range requires a very specific set of functions, I find it highly 
unlikely that anything which isn't a range will qualify as one. It's far more 
likely that you screw up and a range isn't the right kind of range because one 
of the functions wasn't quite right.

There is some danger in a type being incorrectly used with a function when 
that function requires and tests for only one function, or maybe when it 
requires two functions. But I would expect that as more is required by a 
template constraint, it very quickly becomes the case that there's no way that 
any type would ever pass it with similarly named functions that didn't do the 
same thing as what they were expected to do. It's just too unlikely that the 
exact same set of function names would be used for different things, especially 
as that list grows. And given that ranges are a core part of D's standard 
library, I don't think that there's much excuse for having a type that has the 
range functions but isn't supposed to be a range. So, I really don't see this 
as a problem.

- Jonathan M Davis