2.12.4 regression (?): Spurious unused warnings

[lloydmeta]

Apologies in advance if this has already been filed.

In 2.12.3, private/local values that are used in macro expansions were considered to be used, but in 2.12.4, they trigger unused value warnings.

This is evident when using something like MacWire to wire up dependencies at compile time.

Repro: https://github.com/lloydmeta/scala-2.12.4-unused-test

Main.scala L6-L12, where wire[T] is used triggers the following compile warning/failures:

[error] ~/scala-2.12.4-unused-test/src/main/scala/com/beachape/Main.scala:6:20: private val config in object Main is never used
[error]   private lazy val config = {
[error]                    ^
[error] ~/scala-2.12.4-unused-test/src/main/scala/com/beachape/Main.scala:7:9: local val host in lazy value config is never used
[error]     val host = "https://my-api.com"
[error]         ^
[error] ~/scala-2.12.4-unused-test/src/main/scala/com/beachape/Main.scala:8:9: local val port in lazy value config is never used
[error]     val port = 9000

Workarounds:

1. Go back to 2.12.3 in build.sbt
2. Manually wire up dependencies (e.g. Config(host, port) and new Client(config))
3. Make all members publicly accessible

[SethTisue]

looks related to #6096. paging @som-snytt?

(Lloyd has clarified that the remaining "2.11"s above should be "2.12"s)

[som-snytt]

That is tweaked behavior due to scala/scala#5876

Linting can be done on the unexpanded and/or expanded tree (before/after). It defaults to before (as that represents what was coded directly, as it were).

  "-Xlint",
  "-Ywarn-macros:after",
  "-Ywarn-unused:params,patvars",

[lloydmeta]

It defaults to before (as that represents what was coded directly, as it were)

Sorry to nitpick, but shouldn't it default to after, given that was the prior behaviour? As it stands, 2.11.4 breaks builds that compiled under 2.11.3.

[SethTisue]

it's just a warning; we're free to change that stuff around in minor releases, and we routinely do. if you think the new default is poorly chosen, you need a better argument than “it was something else before”

[lloydmeta]

Nope, was just asking given there wasn't much of an argument for the current behaviour presented. Breaking builds when going up a patch version is generally surprising, but given it's partly caused by turning on fatal warnings, I suppose it's understandable :)

[SethTisue]

(I guess #10296 was considered self-explanatory, but I haven’t thought about it too deeply)

[som-snytt]

It's true that someone has to suffer in the name of progress. Compare the linked ticket. The motivation for not linting the expanded tree is that a macro can emit arbitrary cruft, but linting is to help people avoid mistakes. Some kinds of magic aren't lintable at all, for example generating different code depending on whether an implicit value is in scope (like language features). It's been suggested to let a macro tell the compiler, "I used this symbol."

An improvement might be to look only for usages in the expanded tree, ignoring newly introduced cruft. That would be a different mode from "it passes lint before and after."