Is there a standard way to define to for user-defined types?

[Jonathan M Davis]

For instance, if I want to make it legal to pass a core.time.TickDuration to 
to!(core.time.Duration) instead of casting it (which is actually why I've been 
think of this issue), what is the standard way to do that? Or isn't there one? 
I'm not aware of one. And if there isn't one, how should we do it?

I can think of 3 possible ways:

1. Overload to in the module with the type being converted from. So, for 
instance, core.time would have an overload for to which takes a TickDuration 
and returns a Duration (either that or std.datetime if it didn't work to have 
that in druntime for some reason). I'm not sure if that'll cause problems with 
overload sets or not though.

2. Make it so that std.conv.to can do its thing based on opCast. If a type 
overloads opCast, then std.conv.to can use that opCast to do the conversion 
(but only if opCast is defined, not for just any cast which may or may not be 
valid).

3. Make it so that user-defined types have a semi-standard member function 
(e.g. to) which std.conv.to looks for and uses for conversions if it's there.


Which of those would you consider to be the best? Or can you think of another, 
better way? It seems to me that we need an essentially standard way of 
defining conversions which use to. Otherwise, the only option is to use 
opCast, and while there's nothing wrong with overloading opCast, it would 
definitely be preferable to use to for safe conversions.

Thoughts?

- Jonathan M Davis