Consider stdlib additions for dotty cross-compilation

[adriaanm]

• ImplicitConverter: Restrict implicit conversions to implicit method or ImplicitConverter instance #447, would already simplify cross-compiling with dotty.

[szeiger]

type AnyConstr[_] = Any would be useful

[smarter]

See https://github.com/lampepfl/dotty/tree/master/library/src for everything that could be added. For stuff that does not require compiler support, we could also consider cross-compiling a modified version of dotty-library to scalac.

[smarter]

A very useful addition for cross-compiling would be https://github.com/lampepfl/dotty/blob/master/library/src/scala/implicits/Not.scala for implicit negation, see the documentation inside to understand the rationale.

[SethTisue]

time is running short for PRs in this area

[smarter]

But additions could go in post-2.13.0, no?

[adriaanm]

No, we're impose forwards and backwards bin compat for the std lib.

[smarter]

What's the purpose of forward bin compat?

[lrytz]

Libraries compiled with 2.N.(x+1) can safely be used on 2.N.x.

[smarter]

Isn't sbt going to select the biggest version anyway?

[adriaanm]

not everyone uses sbt -- it's good to re-evaluate this before we release 3.0, but the 2.x cycle probably can't change its bin compat policy