False positives with -Wself-implicit

[DavidGregory084]

Enabling the -Wself-implicit flag causes spurious warnings when compiling code in the console4cats project.

Affected code: https://github.com/profunktor/console4cats/blob/master/core/src/main/scala/cats/effect/ConsoleError.scala#L33

[error] /home/circleci/repo/core/src/main/scala/cats/effect/ConsoleError.scala:33:56: Implicit resolves to enclosing value catsStdShowForString
[error]   def putError(str: String): F[Unit] = putError[String](str)
[error]                                                        ^
[error] /home/circleci/repo/core/src/main/scala/cats/effect/ConsoleOut.scala:33:56: Implicit resolves to enclosing value catsStdShowForString
[error]   def putStrLn(str: String): F[Unit] = putStrLn[String](str)
[error]                                                        ^
[error] /home/circleci/repo/core/src/main/scala/cats/effect/ConsoleOut.scala:43:52: Implicit resolves to enclosing value catsStdShowForString
[error]   def putStr(str: String): F[Unit] = putStr[String](str)
[error]                                                    ^
[error] three errors found
[error] (console4cats / Compile / compileIncremental) Compilation failed
[error] Total time: 10 s, completed Nov 27, 2019 7:03:39 AM

See profunktor/console4cats#65

[som-snytt]

Thanks, I'll take a look. In the meantime, maybe change the title of the linked issue to, "Consider not adding -Wself-implicit".

[DavidGregory084]

Thanks @som-snytt! There are some more examples in these PRs:

http4s/http4s#2997
http4s/blaze#360

[som-snytt]

This comment in TreeCheckers is relevant:

/* XXX: lots of syms show up here with accessed == NoSymbol. */

There is more explanatory commentary at def accessed on the perils involved.

Also, hasTransOwner is trivially true for NoSymbol, which is the root of all evil, I mean the root of root.

So avoiding the false positive is easy; not sure about possible false negatives.