On Wednesday, 27 December 2017 at 16:36:59 UTC, H. S. Teoh wrote:
> On Tue, Dec 26, 2017 at 11:28:56AM +0000, Mark via Digitalmars-d wrote:
>> On Monday, 25 December 2017 at 22:48:39 UTC, H. S. Teoh wrote:
>> > [...]
>> 
>> Maybe we can document the interface of the return type using signature constraints? For instance, for the function:
>> 
>> auto map(Range)(Range r) if (isInputRange!(Unqual!Range));
>> 
>> we'd add the following to its constraints:
>> 
>> isInputRange!(ReturnType!(map!R))
>> 
>> However, this does not compile at the moment. I'm not sure why.
>
> I'm not sure why either, but signature constraints are intended to be used for constraining the range of incoming template arguments that the template will accept. It's not intended to establish a contract on what the template is going to produce when instantiated with those arguments, though this could well be an enhancement request. Perhaps this should be filed in bugzilla so that it won't get forgotten.
> 
> T

I guess we could introduce "static in" and "static out" contracts analogous to the existing "in" and "out" contracts, except these would be run at compile-time. But this doesn't seem strictly necessary - the compiler can figure out on its own which constraints depend only on the function's (compile-time) parameters and which constraints also depend on its return type.