Merge pull request #1833 from dotty-staging/fix-#1793

Fix #1793: allow multiversal comparisons between Null and X

Author: odersky

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

@@ -542,6 +542,15 @@ trait Implicits { self: Typer =>
   }
 
   private def assumedCanEqual(ltp: Type, rtp: Type)(implicit ctx: Context) = {
+    def eqNullable: Boolean = {
+      val other =
+        if (ltp.isRef(defn.NullClass)) rtp
+        else if (rtp.isRef(defn.NullClass)) ltp
+        else NoType
+
+      (other ne NoType) && !other.derivesFrom(defn.AnyValClass)
+    }
+
     val lift = new TypeMap {
       def apply(t: Type) = t match {
         case t: TypeRef =>
@@ -553,7 +562,7 @@ trait Implicits { self: Typer =>
           if (variance > 0) mapOver(t) else t
       }
     }
-    ltp.isError || rtp.isError || ltp <:< lift(rtp) || rtp <:< lift(ltp)
+    ltp.isError || rtp.isError || ltp <:< lift(rtp) || rtp <:< lift(ltp) || eqNullable
   }
 
   /** Check that equality tests between types `ltp` and `rtp` make sense */
@@ -669,6 +678,7 @@ trait Implicits { self: Typer =>
             case result: AmbiguousImplicits => true
             case _ => false
           }
+
         def validEqAnyArgs(tp1: Type, tp2: Type) = {
           List(tp1, tp2).foreach(fullyDefinedType(_, "eqAny argument", pos))
           assumedCanEqual(tp1, tp2) || !hasEq(tp1) && !hasEq(tp2) ||

compiler/test/dotc/scala-collections.blacklist

@@ -164,11 +164,6 @@
 #     |                ^^^
 #     |                not found: msg
 
-../scala-scala/src/library/scala/ref/WeakReference.scala
-# 33 |    if (x != null) Some(x) else None
-#    |       ^^^^^^^^^^^
-#    |       Values of types wr.underlying.java$lang$ref$WeakReference$$T and Null cannot be compared with == or !=
-
 ../scala-scala/src/library/scala/reflect/ClassManifestDeprecatedApis.scala
 # 51 |      import Manifest._
 #    |             ^^^^^^^^

compiler/test/dotc/scala-collections.whitelist

@@ -423,6 +423,7 @@
 ../scala-scala/src/library/scala/ref/ReferenceQueue.scala
 ../scala-scala/src/library/scala/ref/ReferenceWrapper.scala
 ../scala-scala/src/library/scala/ref/SoftReference.scala
+../scala-scala/src/library/scala/ref/WeakReference.scala
 
 ../scala-scala/src/library/scala/reflect/macros/internal/macroImpl.scala
 ../scala-scala/src/library/scala/reflect/NoManifest.scala

tests/neg/i1793.scala

@@ -0,0 +1,7 @@
+object Test {
+  import scala.ref.WeakReference
+  def unapply[T <: AnyVal](wr: WeakReference[T]): Option[T] = {
+    val x = wr.underlying.get
+    if (x != null) Some(x) else None // error
+  }
+}

tests/pos/i1793.scala

@@ -0,0 +1,7 @@
+object Test {
+  import scala.ref.WeakReference
+  def unapply[T <: AnyRef](wr: WeakReference[T]): Option[T] = {
+    val x = wr.underlying.get
+    if (x != null) Some(x) else None
+  }
+}