Make -Ywarn-unused:params sensitive to usages in extending classes

[som-snytt]

Tweaks to linting params as noted by @lrytz on community build.

$ skala -Xlint -Ywarn-unused
Welcome to Scala 2.12.2-20170406-193519-6614931 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_111).
Type in expressions for evaluation. Or try :help.

scala> def f(x: Int)(y: Int = 1) = x + y
<console>:11: warning: parameter value x in method f$default$2 is never used
       def f(x: Int)(y: Int = 1) = x + y
             ^
f: (x: Int)(y: Int)Int

scala> :pa
// Entering paste mode (ctrl-D to finish)

class C { def f(x: Int, y: Int) = y }
class D extends C { override def f(x: Int, y: Int) = x + y }

// Exiting paste mode, now interpreting.

<pastie>:11: warning: parameter value x in method f is never used
       class C { def f(x: Int, y: Int) = y }
                       ^
defined class C
defined class D

[som-snytt]

scala/scala#5402 for default getters

[SethTisue]

@som-snytt should this stay open? I transferred it from scala-dev

[som-snytt]

The second case still warns. The improvement is to (optionally) warn only for sealed or final classes, if param is used in no override. Don't warn (optionally) for abstract classes.

[som-snytt]

Relatedly, unused ctor of "effectively sealed" (sealed/private) class does not warn. Under the rubric of things to notice about such classes.

[SethTisue]

The second case still warns

I would not consider that a bug. Or rather, I would require convincing?

The parameter is in fact unused. The fact that D does something different seems irrelevant to me.

Scala 3 also warns in this case.

The improvement is to (optionally) warn only for sealed or final classes, if param is used in no override

I doubt this level of ambition is desirable.

There aren't other places in unused analysis where you do this kind of inter-class analysis.... are there?

[som-snytt]

The sentiment at one point (maybe Jason said something) was that warning for unused params would be too noisy to be useful.

I would like all the unused warnings to be lints, just because remembering which are enabled by -Xlint is an encumbrance. That is possible if they work well with some heuristics (which can be turned off).

[SethTisue]

Hmm. Maybe in Scala 3 I'll conclude that -Wunused:all casts too wide a net and I should enable the categories individually.

[som-snytt]

Currently, an override is absolved (for unused param) because you can't change the signature you inherit.

Closing because not inclined to convince Seth (by plying him with beers). The nowarn and WConf facilities obviate too much cleverness.

Probably the other heuristics to mitigate warnings should be removed, except for simple "RHS is ???".