Limiting implicit conversions to right-hand-side (definition arguments) for Dotty

[Blaisorblade]

I’ve had a hard time figuring out what problem this fixes, and we’ve discussed this at https://gitter.im/lampepfl/dotty?at=5939666a142826e972968625.

I suspect the scenario where this is most problematic is the one in this thread:

Canonical "Non-Deterministic" Implicit Conversion Based on Example in Scala Documentation

Non-deterministic has to be understood as a metaphor only in this context. Technically Scala implicit conversions are deterministic. They don’t change program behaviour on each run or re-compile. Non-deterministic is sometime used in arguments against implicit conversions to describe non-obvious behaviour. It is based directly on an example taken right from Scala Documentation Where does Scala look for implicits? in FAQ where it explains Implicit scope of an argument’s type. class A(val n: Int…

In general, given def ===[A](v1 : A, v2: A): Boolean = ..., inverse conversions from A to B and from B to A, and variables a : A and b : B, what does a === b mean?

Overall, I suspect an actual linter/compiler warning/error for such inverse conversions might be a better goal. But I wouldn’t know right away how to specify when to give this warning, if the conversions are more generic, so it’s not obvious how to achieve this goal.