Add error for partial functions with refined terms

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/ExpandSAMs.scala

@@ -35,6 +35,7 @@ class ExpandSAMs extends MiniPhaseTransform { thisTransformer =>
       tpt.tpe match {
         case NoType => tree // it's a plain function
         case tpe @ SAMType(_) if tpe.isRef(defn.PartialFunctionClass) =>
+          checkRefinements(tpe, fn.pos)
           toPartialFunction(tree)
         case tpe @ SAMType(_) if isPlatformSam(tpe.classSymbol.asClass) =>
           checkRefinements(tpe, fn.pos)
@@ -90,7 +91,7 @@ class ExpandSAMs extends MiniPhaseTransform { thisTransformer =>
   private def checkRefinements(tpe: Type, pos: Position)(implicit ctx: Context): Unit = tpe match {
     case RefinedType(parent, name, _) =>
       if (name.isTermName && tpe.member(name).symbol.ownersIterator.isEmpty) // if member defined in the refinement
-        ctx.error(s"Cannot refine $name on a lambda", pos)
+        ctx.error("Lambda does not define " + name, pos)
       checkRefinements(parent, pos)
     case _ =>
   }

tests/neg/i2539.scala

@@ -12,4 +12,6 @@ object Foo {
   val fun0a: Fun0[Int] = () => 42
   val fun0b: Fun0[Int] { def foo(): Int } = () => 42 // error
 
+  val pf: PartialFunction[Int, Int]{ def foo(): Int } = { case x1 => 42 } // error
+
 }