Make forwardParamBindings a Config setting

It was a command line setting before. But since we do not
need to selective suppress param forwarding for making
things compile anymore, it's better not to expose it as such.

Author: odersky

compiler/src/dotty/tools/dotc/config/Config.scala

@@ -75,6 +75,13 @@ object Config {
   /** If this flag is set, take the fast path when comparing same-named type-aliases and types */
   final val fastPathForRefinedSubtype = true
 
+  /** If this flag is set, `TypeOps.normalizeToClassRefs` will insert forwarders
+   *  for type parameters of base classes. This is an optimization, which avoids
+   *  long alias chains. We should  not rely on the optimization, though. So changing
+   *  the flag to false can be used for checking that everything works OK without it.
+   */
+  final val forwardTypeParams = true
+
   /** If this flag is set, and we compute `T1 { X = S1 }` & `T2 { X = S2 }` as a new
    *  upper bound of a constrained parameter, try to align the refinements by computing
    *  `S1 =:= S2` (which might instantiate type parameters).

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -96,7 +96,6 @@ class ScalaSettings extends Settings.SettingGroup {
   val YforceSbtPhases = BooleanSetting("-Yforce-sbt-phases", "Run the phases used by sbt for incremental compilation (ExtractDependencies and ExtractAPI) even if the compiler is ran outside of sbt, for debugging.")
   val YdumpSbtInc = BooleanSetting("-Ydump-sbt-inc", "For every compiled foo.scala, output the API representation and dependencies used for sbt incremental compilation in foo.inc, implies -Yforce-sbt-phases.")
   val YcheckAllPatmat = BooleanSetting("-Ycheck-all-patmat", "Check exhaustivity and redundancy of all pattern matching (used for testing the algorithm)")
-  val YsuppressParamForwarding = BooleanSetting("-Ysuppress-param-forwarding", "Don't install aliases for base type parameters.")
 
   /** Area-specific debug output */
   val Yexplainlowlevel = BooleanSetting("-Yexplain-lowlevel", "When explaining type errors, show types at a lower level.")

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

@@ -400,9 +400,9 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
       enterArgBinding(formals(name), refinedInfo, cls, decls)
     }
 
-    if (!ctx.settings.YsuppressParamForwarding.value)
+    if (Config.forwardTypeParams)
       forwardParamBindings(parentRefs, refinements, cls, decls)
- 
+
     parentRefs
   }