[Issue 23319] std.range.Generator does not work with non-mutable elements

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23319

--- Comment #2 from Ali Cehreli <acehreli at yahoo.com> ---
Thank you for an alternative but it has the same limitation, right?

  gen!(() => const(int)(42));

deneme.d(4431): Error: cannot modify `const` expression `this.func`
deneme.d(4448): Error: template instance `deneme.gen!(function () pure nothrow
@nogc @safe => 42)` error instantiating

Line 4431 is this one:

    void popFront() { func = fn(); }

Please note that this bug report is about std.range.Generator. No hand-written
code can magically fix it.

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