[Issue 12885] const union wrongly converts implicitly to mutable

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=12885

FeepingCreature <default_357-line at yahoo.de> changed:

           What    |Removed                     |Added
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                 CC|                            |default_357-line at yahoo.de
           Severity|enhancement                 |normal

--- Comment #2 from FeepingCreature <default_357-line at yahoo.de> ---
I just ran into this. This bug breaks std.json quite badly: see
https://issues.dlang.org/show_bug.cgi?id=19256 , in which const(JSONValue)
implicitly converts to JSONValue, allowing us to mutate JSON objects via a
const parameter.

This is definitely unacceptable.

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