mutable keyword

[ag0aep6g via Digitalmars-d-learn]

On 05/21/2016 07:15 PM, Jack Applegame wrote:
> Really? Mutating immutable is UB too, but look at std.typecons.Rebindable.

Rebindable uses a union of a mutable and an immutable variant of the 
type. No access to the mutable union member is provided, and it's never 
dereferenced by Rebindable.

Assignment is done through the mutable member. So the immutable member 
is never overwritten explicitly, but its value does change, of course.

It's supposedly not a problem that the immutable member changes, as long 
as nobody looks. That is, as long as nobody obtains a reference to it. 
Copies of the immutable member are truly immutable.

For this to work, the compiler must recognize that an immutable union 
member isn't actually immutable when the union has mutable members as 
well. I don't know if this is specified anywhere, or if it's perfectly 
sound then.

And Rebindable does give out a reference, actually. That's unacceptable, 
as far as I can see. Filed an issue:
https://issues.dlang.org/show_bug.cgi?id=16054