[Issue 23523] Allow User-Defined Implicit Conversions for const/immutable containers

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23523

--- Comment #1 from John Hall <john.michael.hall at gmail.com> ---
In the past the argument from Walter against something like opImplicitCast has
always been that users would get confused about random things happening if they
had freedom to do any implicit cast they could think of.

The solution to this would be only to allow the implicit conversions discussed
above. So instead of some general opImplicitCast it might be better described
as opImplicitConstCast.

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