Bug 4464 – std.range.take does not always return Take!R

When used with a range that supports slicing, take(r, n) returns r[0..n], which unfortunately makes it impossible to declare a variable of type Take!R and have it work in all situations. I do consider this a bug and not an intended part of the design, please correct me if this is not the case. eg. int[] a; Take!R r = take(a, 3); // type mismatch error In most situations this is not a problem as type inference can be used, but when the type need to be known (as in a struct declaration) it won't work. Because take is an auto function, typeof(take(r, n)) does not work either. To fix this without disabling the slicing optimisation: 1. Change the template constraint on 'Take' - struct Take(R) if (isInputRange!(R)) + struct Take(R) if (isInputRange!(R) && !hasSlicing!R) 2. Add a template to handle the other case + template Take(R) if (isInputRange!R && hasSlicing!R) + { + alias typeof(R.init[0..R.init.length]) Take; + }; 3. Change the return type of take from 'auto' to 'Take!R' - auto take(R)(R input, size_t n) if (isInputRange!R && hasSlicing!R) + Take!R take(R)(R input, size_t n) if (isInputRange!R && hasSlicing!R) With these changes take and Take work properly with everything. I _will_ learn how to make real diffs one day.

Fixed SVN.

Unfortunately this has been broken again by: http://www.dsource.org/projects/phobos/changeset/2102 (http://d.puremagic.com/issues/show_bug.cgi?id=5052) which added a new overload for 'take' but does not add a corresponding 'Take' template. Patch: Change the template constraint on the main Take range to match the main take function, and add a 'Take' template for the Take!Take!R case. range.d:2074 - if(isInputRange!(Unqual!Range) && - (!hasSlicing!(Unqual!Range) || isNarrowString!(Unqual!Range))) + if((isInputRange!(Unqual!Range) && (!hasSlicing!(Unqual!Range) || isNarrowString!(Unqual!Range))) + && !is (Range T == Take!T)) + // For the simplified Take!Take!R case + + template Take(R) + if((isInputRange!(Unqual!R) && (!hasSlicing!(Unqual!R) || isNarrowString!(Unqual!R))) + && is (R T == Take!T)) + { + alias R Take; + }

Since I broke it, I'll take care of fixing it as well. (And I'll make sure to add a unittest so it doesn't break again.) But it'll probably have to wait until 2.051 is released.