Un-deprecate default floating point Orderings; issue migration warning instead under -Xmigration

[eed3si9n]

Fixes scala/bug#11844
Ref scala/bug#10511
Ref #6410
Ref #76

This changes the deprecation of DeprecatedDoubleOrdering to a migration warning to avoid List(1.0, -1.0).sorted giving a deprecation warning.

This also provides some documentation on the ordering instances in Scaladoc:

The behavior of the comparison operations provided by the default (implicit) ordering on Double changed in 2.10.0 and 2.13.0. Prior to Scala 2.10.0, the Ordering instance used semantics consistent with java.lang.Double.compare.

Scala 2.10.0 changed the implementation of lt, equiv, min, etc., to be IEEE 754 compliant, while keeping the compare method NOT compliant, creating an internally inconsistent instance. IEEE 754 specifies that 0.0 == -0.0. In addition, it requires all comparisons with Double.NaN return false thus 0.0 < Double.NaN, 0.0 > Double.NaN, and Double.NaN == Double.NaN all yield false, analogous None in flatMap.

Recognizing the limitation of the IEEE 754 semantics in terms of ordering, Scala 2.13.0 created two instances: Ordering.Double.IeeeOrdering, which retains the IEEE 754 semantics from Scala 2.12.x, and Ordering.Double.TotalOrdering, which brings back the java.lang.Double.compare semantics for all operations. The default extends TotalOrdering.

List(0.0, 1.0, 0.0 / 0.0, -1.0 / 0.0).sorted      // List(-Infinity, 0.0, 1.0, NaN)
List(0.0, 1.0, 0.0 / 0.0, -1.0 / 0.0).min         // -Infinity
implicitly[Ordering[Double]].lt(0.0, 0.0 / 0.0)   // true
{
  import Ordering.Double.IeeeOrdering
  List(0.0, 1.0, 0.0 / 0.0, -1.0 / 0.0).sorted    // List(-Infinity, 0.0, 1.0, NaN)
  List(0.0, 1.0, 0.0 / 0.0, -1.0 / 0.0).min       // NaN
  implicitly[Ordering[Double]].lt(0.0, 0.0 / 0.0) // false
}

[NthPortal]

I like it! 👍

[dwijnand]

Suggested some tweaks: eed3si9n#1

[SethTisue]

squash before merge, otherwise LGTM, thanks Eugene, and thanks @NthPortal @Ichoran @odersky and everyone else who helped this along

[dwijnand]

Suggested some tweaks eed3si9n#1

[eed3si9n]

I just pulled @dwijnand's fix ups in. I'll squash them all now.