Fix #4339: Dealias when checking refinements

Author: allanrenucci

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

@@ -88,7 +88,7 @@ class ExpandSAMs extends MiniPhase {
     cpy.Block(tree)(List(applyDef, isDefinedAtDef), anonCls)
   }
 
-  private def checkRefinements(tpe: Type, pos: Position)(implicit ctx: Context): Type = tpe match {
+  private def checkRefinements(tpe: Type, pos: Position)(implicit ctx: Context): Type = tpe.dealias match {
     case RefinedType(parent, name, _) =>
       if (name.isTermName && tpe.member(name).symbol.ownersIterator.isEmpty) // if member defined in the refinement
         ctx.error("Lambda does not define " + name, pos)

tests/neg/i4339.scala

@@ -0,0 +1,19 @@
+class Test {
+  def test: Unit = {
+    val a: PartialFunction[Int, Int]{ def foo: Int } = { case x => x } // error
+
+    type PF = PartialFunction[Int, Int] { def foo: Int }
+    val b: PF = { case x => x } // error
+
+    type PF1 = PartialFunction[Int, Int] {def foo: Int }
+    val c: PF1 {} = { case x => x } // error
+
+    type PF2 = PartialFunction[Int, Int]
+    val d: PF2 {} = { case x => x }
+
+    type PF3 = PartialFunction[Int, Int] {}
+    val e: PF3 = { case x => x }
+
+    val f: PartialFunction[Int, Int] {} { def foo: Int } = { case x => x } // error
+  }
+}