Please do not use 'auto' return types without thoroughly describing the interface

[Mark]

On Monday, 25 December 2017 at 22:48:39 UTC, H. S. Teoh wrote:
> While I agree that all template parameters ought to be 
> documented and all auto return types thoroughly described, I 
> disagree with explicit naming of auto return types. The whole 
> point of auto return types is to return an *opaque* type that 
> user code should not depend on, apart from what the 
> documentation says you can do with the type.  It's a matter of 
> encapsulation, i.e., "you can do X, Y, Z with the return value 
> of this function, everything else is none of your business". 
> Or, in other words, if your code can't possibly work without 
> knowing the explicit type of the return value, then you're 
> doing something wrong (using the function wrongly).
>
> T

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.