Fix #876: Allow implicit conversions from singleton types

compiler/src/dotty/tools/dotc/core/Mode.scala

@@ -37,6 +37,7 @@ object Mode {
    *  that TypeParamRefs can be sub- and supertypes of anything. See TypeComparer.
    */
   val TypevarsMissContext = newMode(4, "TypevarsMissContext")
+
   val CheckCyclic = newMode(5, "CheckCyclic")
 
   /** We are looking at the arguments of a supercall */

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

@@ -598,15 +598,6 @@ trait Checking {
         defn.ObjectType
     }
 
-  /** Check that a non-implicit parameter making up the first parameter section of an
-   *  implicit conversion is not a singleton type.
-   */
-  def checkImplicitParamsNotSingletons(vparamss: List[List[ValDef]])(implicit ctx: Context): Unit = vparamss match {
-    case (vparam :: Nil) :: _ if !(vparam.symbol is Implicit) =>
-      checkNotSingleton(vparam.tpt, " to be parameter type of an implicit conversion")
-    case _ =>
-  }
-
   /** If `sym` is an implicit conversion, check that implicit conversions are enabled.
    *  @pre  sym.is(Implicit)
    */
@@ -948,7 +939,6 @@ trait NoChecking extends ReChecking {
   override def checkValue(tree: Tree, proto: Type)(implicit ctx: Context): tree.type = tree
   override def checkStable(tp: Type, pos: Position)(implicit ctx: Context): Unit = ()
   override def checkClassType(tp: Type, pos: Position, traitReq: Boolean, stablePrefixReq: Boolean)(implicit ctx: Context): Type = tp
-  override def checkImplicitParamsNotSingletons(vparamss: List[List[ValDef]])(implicit ctx: Context): Unit = ()
   override def checkImplicitConversionDefOK(sym: Symbol)(implicit ctx: Context): Unit = ()
   override def checkImplicitConversionUseOK(sym: Symbol, pos: Position)(implicit ctx: Context): Unit = ()
   override def checkFeasibleParent(tp: Type, pos: Position, where: => String = "")(implicit ctx: Context): Type = tp

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

@@ -71,6 +71,15 @@ object Implicits {
     /** The implicit references */
     def refs: List[ImplicitRef]
 
+    private var SingletonClass: ClassSymbol = null
+
+    /** Widen type so that it is neither a singleton type nor it inherits from scala.Singleton. */
+    private def widenSingleton(tp: Type)(implicit ctx: Context): Type = {
+      if (SingletonClass == null) SingletonClass = defn.SingletonClass
+      val wtp = tp.widenSingleton
+      if (wtp.derivesFrom(SingletonClass)) defn.AnyType else wtp
+    }
+
     /** Return those references in `refs` that are compatible with type `pt`. */
     protected def filterMatching(pt: Type)(implicit ctx: Context): List[Candidate] = track("filterMatching") {
 
@@ -80,15 +89,17 @@ object Implicits {
           case mt: MethodType =>
             mt.isImplicitMethod ||
             mt.paramInfos.lengthCompare(1) != 0 ||
-            !ctx.test(implicit ctx => argType relaxed_<:< mt.paramInfos.head)
+            !ctx.test(implicit ctx =>
+              argType relaxed_<:< widenSingleton(mt.paramInfos.head))
           case poly: PolyType =>
             // We do not need to call ProtoTypes#constrained on `poly` because
             // `refMatches` is always called with mode TypevarsMissContext enabled.
             poly.resultType match {
               case mt: MethodType =>
                 mt.isImplicitMethod ||
                 mt.paramInfos.length != 1 ||
-                !ctx.test(implicit ctx => argType relaxed_<:< wildApprox(mt.paramInfos.head, null, Set.empty))
+                !ctx.test(implicit ctx =>
+                  argType relaxed_<:< wildApprox(widenSingleton(mt.paramInfos.head), null, Set.empty))
               case rtp =>
                 discardForView(wildApprox(rtp, null, Set.empty), argType)
             }
@@ -132,14 +143,32 @@ object Implicits {
           case _ => false
         }
 
+        /** Widen singleton arguments of implicit conversions to their underlying type.
+         *  This is necessary so that they can be found eligible for the argument type.
+         *  Note that we always take the underlying type of a singleton type as the argument
+         *  type, so that we get a reasonable implicit cache hit ratio.
+         */
+        def adjustSingletonArg(tp: Type): Type = tp match {
+          case tp: PolyType =>
+            val res = adjustSingletonArg(tp.resType)
+            if (res `eq` tp.resType) tp else tp.derivedLambdaType(resType = res)
+          case tp: MethodType =>
+            tp.derivedLambdaType(paramInfos = tp.paramInfos.mapConserve(widenSingleton))
+          case _ => tp
+        }
+
         (ref.symbol isAccessibleFrom ref.prefix) && {
           if (discard) {
             record("discarded eligible")
             false
           }
           else {
             val ptNorm = normalize(pt, pt) // `pt` could be implicit function types, check i2749
-            NoViewsAllowed.isCompatible(normalize(ref, pt), ptNorm)
+            val refAdjusted =
+              if (pt.isInstanceOf[ViewProto]) adjustSingletonArg(ref.widenSingleton)
+              else ref
+            val refNorm = normalize(refAdjusted, pt)
+            NoViewsAllowed.isCompatible(refNorm, ptNorm)
           }
         }
       }

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

@@ -1413,10 +1413,7 @@ class Typer extends Namer
     val tparams1 = tparams mapconserve (typed(_).asInstanceOf[TypeDef])
     val vparamss1 = vparamss nestedMapconserve (typed(_).asInstanceOf[ValDef])
     vparamss1.foreach(checkNoForwardDependencies)
-    if (sym is Implicit) {
-      checkImplicitParamsNotSingletons(vparamss1)
-      checkImplicitConversionDefOK(sym)
-    }
+    if (sym is Implicit) checkImplicitConversionDefOK(sym)
     val tpt1 = checkSimpleKinded(typedType(tpt))
 
     var rhsCtx = ctx

tests/neg/implicitDefs.scala

@@ -7,7 +7,7 @@ object implicitDefs {
 
   implicit val x = 2 // error: type of implicit definition needs to be given explicitly
   implicit def y(x: Int) = 3 // error: result type of implicit definition needs to be given explicitly
-  implicit def z(a: x.type): String = "" // error: implicit conversion may not have a parameter of singleton type
+  implicit def z(a: x.type): String = "" // ok, used to be: implicit conversion may not have a parameter of singleton type
 
   def foo(implicit x: String) = 1
 

tests/run/i876.scala

@@ -0,0 +1,22 @@
+object Test extends App {
+  object O
+  implicit def foo(x: O.type): String = "hello"
+  val s: String = O
+  implicit def bar(x: s.type): Int = s.length
+  //implicit def bar2(x: String): Int = s.length
+  val l: Int = s
+  assert(s == "hello")
+  assert(l == 5)
+}
+
+object Test3781 {
+  class Foo[T](val value : T)
+  object Foo {
+    implicit def fromXInt[T <: Int with Singleton](i : T): Foo[T] = new Foo[T](i)
+  }
+  class FooUser[T] {
+    def op[T2](that : Foo[T2]) : FooUser[T2] = new FooUser[T2]
+  }
+  val f = new FooUser[1]
+  val f2 = f op 2
+}