Add missing case in TypeComparer

TypeArgRefs with equal prefix, classes and indices should compare as equal.
Previously this probably was not detected because equal prefixes happened to
be identical and then the `eq` case in isSubType would kick in. The problematic
case happened when comparing `ParMapLike[<lots of arguments>]#<typearg Sequential>` with
itself except that one side had a Lazy(CC) where the other had a CC. It was in compileStdLib,
in case you were wondering.

Author: odersky

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

@@ -528,7 +528,13 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       }
       compareExpr
     case tp2: TypeArgRef =>
-      isSubType(tp1, tp2.underlying.loBound) || fourthTry(tp1, tp2)
+      def sameTypeArgRef = tp1 match {
+        case tp1: TypeArgRef =>
+          tp1.clsRef == tp2.clsRef && tp1.idx == tp2.idx && tp1.prefix =:= tp2.prefix
+        case _ =>
+          false
+      }
+      sameTypeArgRef || isSubType(tp1, tp2.underlying.loBound) || fourthTry(tp1, tp2)
     case tp2 @ TypeBounds(lo2, hi2) =>
       def compareTypeBounds = tp1 match {
         case tp1 @ TypeBounds(lo1, hi1) =>

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

@@ -32,8 +32,8 @@ class CompilationTests extends ParallelTesting {
   // Positive tests ------------------------------------------------------------
 
   // @Test  // enable to test compileStdLib separately with detailed stats
-  def compileStdLib: Unit = {
-    implicit val testGroup: TestGroup = TestGroup("compileStdLib")
+  def compileStdLibOnly: Unit = {
+    implicit val testGroup: TestGroup = TestGroup("compileStdLibOnly")
     compileList("compileStdLib", StdLibSources.whitelisted, scala2Mode.and("-migration", "-Yno-inline", "-Ydetailed-stats"))
   }.checkCompile()