On 5/20/2016 6:47 AM, H. S. Teoh via Digitalmars-d-announce wrote:
Not to mention inconsistency in what exactly
is being tested for: if you want to check if something is an input
range, do you use is(typeof(R.empty)), etc., or should you use
__traits(compiles, R.init.empty), or is it is(typeof((R r){r.empty;})),
or any of the 15 or so slightly different ways of testing for the
existence and type of some aggregate member, all subtly different from
each other? Subtly different as in, for instance, testing for
is(typeof((){R r; bool x = r.empty;})) is different from is(typeof(R
r){bool x = r.empty;}), because the former doesn't work with R that has
parameters closing over a local scope, whereas the latter does.

That is not a problem with constraints, it's a result of a dozen people adding constraints in an uncoordinated manner. I.e. it's a library problem.

Whereas if D had concepts, it would have been a simple
matter of defining the prototypical range with struct-like syntax and
calling it a day.

That really isn't good enough. Constraints can address behavior and relationships, concepts do not.

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