Eta-expand 0-arity method if expected type is Function0

[adriaanm]

Reverse course from the deprecation introduced in 2.12.

(4.3) condition for eta-expansion by -Xsource level:

• until 2.13:
  • for arity > 0: function or sam type is expected
  • for arity == 0: Function0 is expected -- SAM types do not eta-expand
    because it could be an accidental SAM Sammy gets confused with Java classes bug#9489
• 2.14:
  • for arity > 0: unconditional
  • for arity == 0: a function-ish type of arity 0 is expected (including SAM)

warnings:

• 2.12: eta-expansion of zero-arg methods was deprecated (eta expansion should not precede empty application  bug#7187)
• 2.13: deprecation dropped in favor of setting the scene for uniform eta-expansion in 3.0
  warning still available under -Xlint:eta-zero
• 2.14: expected type is a SAM that is not annotated with @FunctionalInterface
  if it's a Java-defined interface (under -Xlint:eta-sam)

(4.2) condition for auto-application by -Xsource level:

• until 2.14: none (assuming condition for (4.3) was not met)

• in 3.0: meth.isJavaDefined
  TODO decide -- currently the condition is more involved to give slack to
  Scala methods overriding Java-defined ones; I think we should resolve that
  by introducing slack in overriding e.g. a Java-defined def toString()
  by a Scala-defined def toString. This also works better for dealing
  with accessors overriding Java-defined methods.

  The current strategy in methodSig is problematic:

  // Add a () parameter section if this overrides some method with () parameters
  val vparamSymssOrEmptyParamsFromOverride =

  This means an accessor that overrides a Java-defined method gets a
  MethodType instead of a NullaryMethodType, which breaks lots of
  assumptions about accessors

[dwijnand]

Do you need to redo #7614 too?

[adriaanm]

Yep, thanks!

[som-snytt]

The PR to undo is the one where -Ymoors undoes the other PRs and forks the language.

[adriaanm]

PRs to implement -Yunmoorsed welcome :-p

[adriaanm]

Interesting issues in future-spec -- the other two test fails are due to a bug this introduces under -Xsource:3.0

[adriaanm]

I think I addressed all review comments.

[SethTisue]

@adriaanm we should un-deprecate in 2.12.9, then?

[adriaanm]

Good point: #7740

[som-snytt]

It warns on usual usages of varianceInType. I'm not sure if the code comment is about whether to require the annotation, or whether it's too annoying to warn.

Implicits.scala:701:78: Eta-expansion performed to meet expected type scala.tools.nsc.Variance.Extractor[Implicits.this.global.Symbol], which is SAM-equivalent to Implicits.this.global.Symbol => scala.reflect.internal.Variance,
[warn] even though trait Extractor is not annotated with `@FunctionalInterface`;
[warn] to suppress warning, add the annotation or write out the equivalent function literal.
[warn]                 val targs = solvedTypes(tvars, allUndetparams, varianceInType(wildPt), upper = false, lubDepth(tpInstantiated :: wildPt :: Nil))