Bug 12134 – std.string.split(string) assignment to immutable

I have tagged this as a D enhancement. I'd like this code to be accepted: void main() { import std.string: split; immutable r = "AB CD".split; } DMD 2.065beta2 gives: test.d(3): Error: cannot implicitly convert expression (split("AB CD")) of type string[] to immutable(char[][]) split(string) should return a string[] that is implicitly castable to immutable(string)[] and immutable(string[]). split comes from std.array, and the problem boils down to: S[] split(S)(S s) pure { S[] result; return result; } void main() { char[][] r1 = "AB CD".dup.split; const string[] r2 = "AB CD".split; } This is a first try at finding a solution: import std.traits: isMutable, Select, ForeachType; import std.traits: isSomeString; import std.uni: isWhite; Select!(isMutable!(ForeachType!S), S, immutable S)[] split(S)(S s) @safe pure if (isSomeString!S) { size_t istart; bool inword = false; typeof(return) result; foreach (i, dchar c; s) { if (isWhite(c)) { if (inword) { result ~= s[istart .. i]; inword = false; } } else { if (!inword) { istart = i; inword = true; } } } if (inword) result ~= s[istart .. $]; return result; } void main() { string s = "AB CD"; char[] m = s.dup; char[][] r1 = m.split; immutable r2 = s.split; auto r3 = s.split; pragma(msg, typeof(r3)); // Output: immutable(char[])[] const(string)[] r4 = s.split; // OK string[] r5 = s.split; // Error } The code works correctly with r1, you can assign it to a mutable array because the input is mutable. But the problem is shown with r5, what if you instead want a mutable array of regular strings? So, can the D type system be modified for such situations? And is it a good idea to modify it like this?

fixed since 2.066 : https://run.dlang.io/is/kjMKqT