Loosen skolem comparisons when rechecking

Treat a SolemType(T) as a supertype (as well as a subtype) of T.

Author: odersky

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

@@ -295,6 +295,9 @@ object Phases {
     /** If set, implicit search is enabled */
     def allowsImplicitSearch: Boolean = false
 
+    /** If set equate Skolem types with underlying types */
+    def widenSkolems: Boolean = false
+
     /** List of names of phases that should precede this phase */
     def runsAfter: Set[String] = Set.empty
 

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

@@ -745,6 +745,8 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             false
         }
         compareClassInfo
+      case tp2: SkolemType =>
+        ctx.phase.widenSkolems && recur(tp1, tp2.info) || fourthTry
       case _ =>
         fourthTry
     }

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

@@ -33,6 +33,8 @@ abstract class Recheck extends Phase, IdentityDenotTransformer:
     // TODO: investigate what goes wrong we Ycheck directly after rechecking.
     // One failing test is pos/i583a.scala
 
+  override def widenSkolems = true
+
   def run(using Context): Unit =
     newRechecker().checkUnit(ctx.compilationUnit)
 
@@ -315,6 +317,7 @@ abstract class Recheck extends Phase, IdentityDenotTransformer:
           || expected.isRepeatedParam
              && actual <:< expected.translateFromRepeated(toArray = tree.tpe.isRef(defn.ArrayClass))
         if !isCompatible then
+          println(i"err at ${ctx.phase}")
           err.typeMismatch(tree.withType(tpe), pt)
 
     def checkUnit(unit: CompilationUnit)(using Context): Unit =

compiler/test/dotc/pos-test-recheck.excludes

@@ -1,8 +1,3 @@
 # Cannot compensate dealiasing due to false result dependency
 i6635a.scala
 i6682a.scala
-
-# Cannot handle closures with skolem types
-i6199b.scala
-i6199c.scala
-i11247.scala

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -229,9 +229,11 @@ class CompilationTests {
     ).checkCompile()
 
 
-  /** The purpose of this test is three-fold, being able to compile dotty
-   *  bootstrapped, and making sure that TASTY can link against a compiled
-   *  version of Dotty, and compiling the compiler using the SemanticDB generation
+  /** This test serves several purposes:
+   *   - being able to compile dotty bootstrapped,
+   *   - making sure that TASTY can link against a compiled version of Dotty,
+   *   - compiling the compiler using the SemanticDB generation
+   *   - compiling the compiler under -Yrecheck mode.
    */
   @Test def tastyBootstrap: Unit = {
     implicit val testGroup: TestGroup = TestGroup("tastyBootstrap/tests")