Add Eq instances of standard types to Predef

To make tests pass, this required a looser specification of
`assumedCanEquals`, so that an abstract type T can be compared to
arbitrary values, as long as its upper bound can be compared. E.g.

    T == null
    T == "abc"

2 files changed, 38 insertions, 2 deletions

diff --git a/src/dotty/DottyPredef.scala b/src/dotty/DottyPredef.scala
index 9170da476..9e1f2102c 100644
--- a/src/dotty/DottyPredef.scala
+++ b/src/dotty/DottyPredef.scala
@@ -3,6 +3,7 @@ package dotty
 import scala.reflect.runtime.universe.TypeTag
 import scala.reflect.ClassTag
 import scala.Predef.???
+import scala.collection.Seq
 
 /** unimplemented implicit for TypeTag */
 object DottyPredef {
@@ -10,4 +11,27 @@ object DottyPredef {
 
   implicit def arrayTag[T](implicit ctag: ClassTag[T]): ClassTag[Array[T]] =
     ctag.wrap
+
+  implicit def eqNumber   : Eq[Number, Number] = Eq
+  implicit def eqString   : Eq[String, String] = Eq
+
+  // true asymmetry, modeling the (somewhat problematic) nature of equals on Proxies
+  implicit def eqProxy    : Eq[Proxy, Any]     = Eq
+
+  implicit def eqSeq[T, U](implicit eq: Eq[T, U]): Eq[Seq[T], Seq[U]] = Eq
+
+  implicit def eqByteNum  : Eq[Byte, Number]   = Eq
+  implicit def eqNumByte  : Eq[Number, Byte]   = Eq
+  implicit def eqCharNum  : Eq[Char, Number]   = Eq
+  implicit def eqNumChar  : Eq[Number, Char]   = Eq
+  implicit def eqShortNum : Eq[Short, Number]  = Eq
+  implicit def eqNumShort : Eq[Number, Short]  = Eq
+  implicit def eqIntNum   : Eq[Int, Number]    = Eq
+  implicit def eqNumInt   : Eq[Number, Int]    = Eq
+  implicit def eqLongNum  : Eq[Long, Number]   = Eq
+  implicit def eqNumLong  : Eq[Number, Long]   = Eq
+  implicit def eqFloatNum : Eq[Float, Number]  = Eq
+  implicit def eqNumFloat : Eq[Number, Float]  = Eq
+  implicit def eqDoubleNum: Eq[Double, Number] = Eq
+  implicit def eqNumDouble: Eq[Number, Double] = Eq
 }
diff --git a/src/dotty/tools/dotc/typer/Implicits.scala b/src/dotty/tools/dotc/typer/Implicits.scala
index bbad799e0..a5d415d97 100644
--- a/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/src/dotty/tools/dotc/typer/Implicits.scala
@@ -472,8 +472,20 @@ trait Implicits { self: Typer =>
     EmptyTree
   }
 
-  private def assumedCanEqual(ltp: Type, rtp: Type)(implicit ctx: Context) =
-    ltp.isError || rtp.isError || ltp <:< rtp || rtp <:< ltp
+  private def assumedCanEqual(ltp: Type, rtp: Type)(implicit ctx: Context) = {
+    val lift = new TypeMap {
+      def apply(t: Type) = t match {
+        case t: TypeRef =>
+          t.info match {
+            case TypeBounds(lo, hi) if lo ne hi => hi
+            case _ => t
+          }
+        case _ =>
+          if (variance > 0) mapOver(t) else t
+      }
+    }
+    ltp.isError || rtp.isError || ltp <:< lift(rtp) || rtp <:< lift(ltp)
+  }
 
   /** Check that equality tests between types `ltp` and `rtp` make sense */
   def checkCanEqual(ltp: Type, rtp: Type, pos: Position)(implicit ctx: Context): Unit =