SI-9542 Unify different reprs. of module type refs

The new unit test shows failures in transitivity of subtyping
and type equivalence, which boil down the the inconsistent
handling of the semantically equivalent:

   ThisType(pre, ModuleClass)
   ModuleTypeRef(pre, ModuleClass)
   SingleType(pre, Module)

This commit:

   - adds a case to `normalizePlus` to unwrap a `ThisType` to
     a `ModuleTypeRef`
   - Use `normalizePlus` more widely during subtype comparison
   - refactor `fourthTry` (part of `isSubType`) to remove code
     that becomes obviated by the use of `normalizePlus`.

This fixes the regression in the extension methods phase which
was triggered by https://github.com/scala/scala/pull/4749.
We can also fix that regression by tweaking the extension methods
phase itself to emit the `ThisType` representation of the owner
of the value class, as before.

I plan to demonstrate the two approaches to fixing the regression
on separate branches, and the propose that the merged result of these
two is useds.

2 files changed, 35 insertions, 1 deletions

diff --git a/test/files/pos/t9542.scala b/test/files/pos/t9542.scala
new file mode 100644
index 0000000000..d65f7ac4c6
--- /dev/null
+++ b/test/files/pos/t9542.scala
@@ -0,0 +1,8 @@
+object O {
+  trait T
+
+  class VC(val self: Any) extends AnyVal {
+    def extMethod(f: F1[T, Any]) = ()
+  }
+}
+trait F1[A, B]
diff --git a/test/junit/scala/reflect/internal/TypesTest.scala b/test/junit/scala/reflect/internal/TypesTest.scala
index 95194ef0a4..05a77cfb47 100644
--- a/test/junit/scala/reflect/internal/TypesTest.scala
+++ b/test/junit/scala/reflect/internal/TypesTest.scala
@@ -1,9 +1,10 @@
 package scala.reflect.internal
 
 import org.junit.Assert._
-import org.junit.Test
+import org.junit.{Assert, Test}
 import org.junit.runner.RunWith
 import org.junit.runners.JUnit4
+import scala.collection.mutable
 import scala.tools.nsc.symtab.SymbolTableForUnitTesting
 
 @RunWith(classOf[JUnit4])
@@ -32,4 +33,29 @@ class TypesTest {
     val uniquelyNarrowed2 = refinedType(boolWithString1narrow2 :: Nil, NoSymbol)
     assert(uniquelyNarrowed1 =:= uniquelyNarrowed2)
   }
+
+  @Test
+  def testTransitivityWithModuleTypeRef(): Unit = {
+    import rootMirror.EmptyPackageClass
+    val (module, moduleClass) = EmptyPackageClass.newModuleAndClassSymbol(TermName("O"), NoPosition, 0L)
+    val minfo = ClassInfoType(List(ObjectTpe), newScope, moduleClass)
+    module.moduleClass setInfo minfo
+    module setInfo module.moduleClass.tpe
+    val tp1 = TypeRef(ThisType(EmptyPackageClass), moduleClass, Nil)
+    val tp2 = SingleType(ThisType(EmptyPackageClass), module)
+    val tp3 = ThisType(moduleClass)
+    val tps = List(tp1, tp2, tp3)
+    val results = mutable.Buffer[String]()
+    tps.permutations.foreach {
+      case ts @ List(a, b, c) =>
+        def tsShownRaw = ts.map(t => showRaw(t)).mkString(", ")
+        if (a <:< b && b <:< c && !(a <:< c)) results += s"<:< intransitive: $tsShownRaw"
+        if (a =:= b && b =:= c && !(a =:= c)) results += s"=:= intransitive: $tsShownRaw"
+    }
+    results.toList match {
+      case Nil => // okay
+      case xs =>
+        Assert.fail(xs.mkString("\n"))
+    }
+  }
 }