SI-6899, prohibit dangerous, useless implicit conversions.

src/compiler/scala/tools/nsc/typechecker/Implicits.scala

@@ -144,6 +144,15 @@ trait Implicits {
   private val infoMapCache = new LinkedHashMap[Symbol, InfoMap]
   private val improvesCache = perRunCaches.newMap[(ImplicitInfo, ImplicitInfo), Boolean]()
 
+  private def isInvalidConversionTarget(tpe: Type): Boolean = tpe match {
+    case Function1(_, out) => AnyRefClass.tpe <:< out
+    case _                 => false
+  }
+  private def isInvalidConversionSource(tpe: Type): Boolean = tpe match {
+    case Function1(in, _) => in <:< NullClass.tpe
+    case _                => false
+  }
+
   def resetImplicits() {
     implicitsCache.clear()
     infoMapCache.clear()
@@ -1357,10 +1366,10 @@ trait Implicits {
 
         val wasAmbigious = result.isAmbiguousFailure // SI-6667, never search companions after an ambiguous error in in-scope implicits
         result = materializeImplicit(pt)
-
         // `materializeImplicit` does some preprocessing for `pt`
         // is it only meant for manifests/tags or we need to do the same for `implicitsOfExpectedType`?
-        if (result.isFailure && !wasAmbigious) result = searchImplicit(implicitsOfExpectedType, isLocal = false)
+        if (result.isFailure && !wasAmbigious)
+          result = searchImplicit(implicitsOfExpectedType, isLocal = false)
 
         if (result.isFailure) {
           context.updateBuffer(previousErrs)
@@ -1370,9 +1379,18 @@ trait Implicits {
           if (Statistics.canEnable) Statistics.incCounter(oftypeImplicitHits)
         }
       }
-
-      if (result.isFailure && settings.debug)
-        log("no implicits found for "+pt+" "+pt.typeSymbol.info.baseClasses+" "+implicitsOfExpectedType)
+      if (result.isSuccess && isView) {
+        if (isInvalidConversionTarget(pt)) {
+          context.issueAmbiguousError(AmbiguousImplicitTypeError(tree, "the result type of an implicit conversion must be more specific than AnyRef"))
+          result = SearchFailure
+        }
+        else if (isInvalidConversionSource(pt)) {
+          context.issueAmbiguousError(AmbiguousImplicitTypeError(tree, "an expression of type Null is ineligible for implicit conversion"))
+          result = SearchFailure
+        }
+      }
+      if (result.isFailure)
+        debuglog("no implicits found for "+pt+" "+pt.typeSymbol.info.baseClasses+" "+implicitsOfExpectedType)
 
       result
     }

src/library/scala/Option.scala

@@ -128,7 +128,7 @@ sealed abstract class Option[+A] extends Product with Serializable {
    * val textField = new JComponent(initalText.orNull,20)
    * }}}
    */
-  @inline final def orNull[A1 >: A](implicit ev: Null <:< A1): A1 = this getOrElse null
+  @inline final def orNull[A1 >: A](implicit ev: Null <:< A1): A1 = this getOrElse ev(null)
 
   /** Returns a $some containing the result of applying $f to this $option's
    * value if this $option is nonempty.
@@ -210,7 +210,7 @@ sealed abstract class Option[+A] extends Product with Serializable {
   }
 
   /** Tests whether the option contains a given value as an element.
-   * 
+   *
    *  @param elem the element to test.
    *  @return `true` if the option has an element that is equal (as
    *  determined by `==`) to `elem`, `false` otherwise.

src/library/scala/Predef.scala

@@ -346,19 +346,6 @@ object Predef extends LowPriorityImplicits with DeprecatedPredef {
   implicit def double2Double(x: Double)     = java.lang.Double.valueOf(x)
   implicit def boolean2Boolean(x: Boolean)  = java.lang.Boolean.valueOf(x)
 
-  // These next eight implicits exist solely to exclude AnyRef methods from the
-  // eight implicits above so that primitives are not coerced to AnyRefs.  They
-  // only create such conflict for AnyRef methods, so the methods on the java.lang
-  // boxed types are unambiguously reachable.
-  implicit def byte2ByteConflict(x: Byte)           = new AnyRef
-  implicit def short2ShortConflict(x: Short)        = new AnyRef
-  implicit def char2CharacterConflict(x: Char)      = new AnyRef
-  implicit def int2IntegerConflict(x: Int)          = new AnyRef
-  implicit def long2LongConflict(x: Long)           = new AnyRef
-  implicit def float2FloatConflict(x: Float)        = new AnyRef
-  implicit def double2DoubleConflict(x: Double)     = new AnyRef
-  implicit def boolean2BooleanConflict(x: Boolean)  = new AnyRef
-
   implicit def Byte2byte(x: java.lang.Byte): Byte             = x.byteValue
   implicit def Short2short(x: java.lang.Short): Short         = x.shortValue
   implicit def Character2char(x: java.lang.Character): Char   = x.charValue
@@ -481,24 +468,6 @@ private[scala] abstract class LowPriorityImplicits {
   @inline implicit def doubleWrapper(x: Double)   = new runtime.RichDouble(x)
   @inline implicit def booleanWrapper(x: Boolean) = new runtime.RichBoolean(x)
 
-  // These eight implicits exist solely to exclude Null from the domain of
-  // the boxed types, so that e.g. "var x: Int = null" is a compile time
-  // error rather than a delayed null pointer exception by way of the
-  // conversion from java.lang.Integer.  If defined in the same template as
-  // Integer2int, they would have higher priority because Null is a subtype
-  // of Integer.  We balance that out and create conflict by moving the
-  // definition into the superclass.
-  //
-  // Caution: do not adjust tightrope tension without safety goggles in place.
-  implicit def Byte2byteNullConflict(x: Null): Byte          = sys.error("value error")
-  implicit def Short2shortNullConflict(x: Null): Short       = sys.error("value error")
-  implicit def Character2charNullConflict(x: Null): Char     = sys.error("value error")
-  implicit def Integer2intNullConflict(x: Null): Int         = sys.error("value error")
-  implicit def Long2longNullConflict(x: Null): Long          = sys.error("value error")
-  implicit def Float2floatNullConflict(x: Null): Float       = sys.error("value error")
-  implicit def Double2doubleNullConflict(x: Null): Double    = sys.error("value error")
-  implicit def Boolean2booleanNullConflict(x: Null): Boolean = sys.error("value error")
-
   implicit def genericWrapArray[T](xs: Array[T]): WrappedArray[T] =
     if (xs eq null) null
     else WrappedArray.make(xs)

test/files/neg/no-implicit-to-anyref.check

@@ -1,10 +1,4 @@
-no-implicit-to-anyref.scala:11: error: type mismatch;
- found   : Int(1)
- required: AnyRef
-Note: an implicit exists from scala.Int => java.lang.Integer, but
-methods inherited from Object are rendered ambiguous.  This is to avoid
-a blanket implicit which would convert any scala.Int to any AnyRef.
-You may wish to use a type ascription: `x: java.lang.Integer`.
+no-implicit-to-anyref.scala:11: error: the result type of an implicit conversion must be more specific than AnyRef
     1: AnyRef
     ^
 no-implicit-to-anyref.scala:17: error: type mismatch;

test/files/neg/t4158.check

@@ -1,19 +1,7 @@
-t4158.scala:3: error: type mismatch;
- found   : Null(null)
- required: Int
-Note that implicit conversions are not applicable because they are ambiguous:
- both method Integer2intNullConflict in class LowPriorityImplicits of type (x: Null)Int
- and method Integer2int in object Predef of type (x: Integer)Int
- are possible conversion functions from Null(null) to Int
+t4158.scala:3: error: an expression of type Null is ineligible for implicit conversion
   var y = null: Int
           ^
-t4158.scala:2: error: type mismatch;
- found   : Null(null)
- required: Int
-Note that implicit conversions are not applicable because they are ambiguous:
- both method Integer2intNullConflict in class LowPriorityImplicits of type (x: Null)Int
- and method Integer2int in object Predef of type (x: Integer)Int
- are possible conversion functions from Null(null) to Int
+t4158.scala:2: error: an expression of type Null is ineligible for implicit conversion
   var x: Int = null
                ^
 two errors found

test/files/neg/t4727.check

@@ -1,10 +1,4 @@
-t4727.scala:5: error: type mismatch;
- found   : Null
- required: Int
-Note that implicit conversions are not applicable because they are ambiguous:
- both method Integer2intNullConflict in class LowPriorityImplicits of type (x: Null)Int
- and method Integer2int in object Predef of type (x: Integer)Int
- are possible conversion functions from Null to Int
+t4727.scala:5: error: an expression of type Null is ineligible for implicit conversion
 Error occurred in an application involving default arguments.
     new C[Int]
     ^

test/files/neg/t6889.check

@@ -0,0 +1,7 @@
+t6889.scala:16: error: the result type of an implicit conversion must be more specific than AnyRef
+  def f(x: Dingo): AnyRef = x   // fail - no conversion to AnyRef
+                            ^
+t6889.scala:17: error: an expression of type Null is ineligible for implicit conversion
+  var x: Int = null             // fail - no conversion from Null
+               ^
+two errors found

test/files/neg/t6889.scala

@@ -0,0 +1,18 @@
+package bippy {
+  trait Bippy[A] extends Any
+}
+package foo {
+  package object unrelated {
+    implicit def bippyDingo[A](x: bippy.Bippy[A]): AnyRef = Nil
+  }
+  package unrelated {
+    trait Unrelated
+  }
+}
+
+object Test {
+  trait Dingo extends Any with bippy.Bippy[foo.unrelated.Unrelated]
+
+  def f(x: Dingo): AnyRef = x   // fail - no conversion to AnyRef
+  var x: Int = null             // fail - no conversion from Null
+}