cannot implicitly convert expression (array(map(split("a")))) of type string[][] to immutable(string[][])

[Fangrui Song via Digitalmars-d]

What cannot `string[][]` be implicitly converted to 
`immutable(string[][])` as in `not_compilable`?


import std.stdio, std.range, std.algorithm, std.array;

void main() {
  immutable not_compilable = "a".split.map!(x => x.split).array;
  static immutable compilable0 = "a".split.map!(x => 
x.split).array;
  immutable compilable1 = "a".split.map!"a.split".array;
}



% dmd --version
DMD64 D Compiler v2.073.0
Copyright (c) 1999-2016 by Digital Mars written by Walter Bright

% dmd a.d
a.d(3): Error: cannot implicitly convert expression 
(array(map(split("a")))) of type string[][] to 
immutable(string[][])