"Private" impl functions can leak out of a module when defined on a public type

[deadalusai]

"Private" impl functions can leak out of a module when defined on a public type. This can allow private types to appear as part of a modules' public interface.

Example on playground: http://is.gd/aGx2MV

In this example, the type Private leaks out of the_module via the impl Public<Private> impl block. It looks like all functions in that block should be inferred to be private?

Reddit discussion

[deadalusai]

This issue seems similar.

[petrochenkov]

Yep, incarnation of #30476 (although the example is sufficiently different from those given in #30476), one more example of type inference being too smart.

Private-in-public checker assumes that the type Public<Private> is private and therefore its impls are private, because you can't name it or construct its values outside of its module without using Private.
However, type inference is able to infer private type parameters in some cases and it's not something generally fixable with private-in-public checker alone.