Fix comparisons for primitive types

Need reflexive instances so that eqAny does not kick in.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -656,7 +656,7 @@ trait Implicits { self: Typer =>
     if (!ctx.isAfterTyper && !assumedCanEqual(ltp, rtp)) {
       val res = inferImplicitArg(
         defn.EqType.appliedTo(ltp, rtp), msgFun => ctx.error(msgFun(""), pos), pos)
-      implicits.println(i"Eq witness found: $res: ${res.tpe}")
+      implicits.println(i"Eq witness found for $ltp / $rtp: $res: ${res.tpe}")
     }
 
   /** Find an implicit parameter or conversion.

library/src/dotty/DottyPredef.scala

@@ -16,6 +16,15 @@ object DottyPredef {
 
   implicit def eqNumber   : Eq[Number, Number] = Eq
   implicit def eqString   : Eq[String, String] = Eq
+  implicit def eqBoolean  : Eq[Boolean, Boolean] = Eq
+  implicit def eqByte     : Eq[Byte, Byte] = Eq
+  implicit def eqShort    : Eq[Short, Short] = Eq
+  implicit def eqChar     : Eq[Char, Char] = Eq
+  implicit def eqInt      : Eq[Int, Int] = Eq
+  implicit def eqLong     : Eq[Long, Long] = Eq
+  implicit def eqFloat    : Eq[Float, Float] = Eq
+  implicit def eqDouble   : Eq[Double, Double] = Eq
+  implicit def eqUnit     : Eq[Unit, Unit] = Eq
 
   // true asymmetry, modeling the (somewhat problematic) nature of equals on Proxies
   implicit def eqProxy    : Eq[Proxy, Any]     = Eq

tests/neg/equality2.scala

@@ -0,0 +1,12 @@
+object equality2 {
+  class A
+
+  val x: Int = 3
+  x == new A // error: cannot compare
+  new A == x // error: cannot compare
+  1 == new A // error: cannot compare
+  new A == 1 // error: cannot compare
+
+  true == new A // error: cannot compare
+  new A == true // error: cannot compare
+}