Revert "Even hackier fix"

This reverts commit 1b2a5aa. I'll add next a
test that breaks it.

Author: Blaisorblade

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

@@ -198,40 +198,6 @@ class TypeApplications(val self: Type) extends AnyVal {
     }
   }
 
-  // Argh, we'll need in fact multiple substitutions.
-  final def typeParamsAndPrefix(maybePrefix: Type)(implicit ctx: Context): (List[TypeParamInfo], Type) = /*>|>*/ track("typeParams") /*<|<*/ {
-    try self match {
-      case self: TypeRef =>
-        val tsym = self.symbol
-        if (tsym.isClass) (tsym.typeParams, self.prefix)
-        else if (!tsym.exists) self.info.typeParamsAndPrefix(self.prefix)
-        else tsym.infoOrCompleter match {
-          case info: LazyType =>
-            (info.completerTypeParams(tsym), self.prefix) //XXX
-          case info => info.typeParamsAndPrefix(self.prefix)
-        }
-      case self: AppliedType =>
-        if (self.tycon.typeSymbol.isClass)
-          (Nil, maybePrefix)
-        else self.superType.typeParamsAndPrefix(maybePrefix)
-      case self: ClassInfo =>
-        (self.cls.typeParams, self)
-      case self: HKTypeLambda =>
-        (self.typeParams, maybePrefix)
-      case _: SingletonType | _: RefinedType | _: RecType =>
-        (Nil, maybePrefix)
-      case self: WildcardType =>
-        self.optBounds.typeParamsAndPrefix(maybePrefix)
-      case self: TypeProxy =>
-        self.superType.typeParamsAndPrefix(maybePrefix)
-      case _ =>
-        (Nil, maybePrefix)
-    }
-    catch {
-      case ex: Throwable => handleRecursive("type parameters and prefix of", self.show, ex)
-    }
-  }
-
   /** If `self` is a higher-kinded type, its type parameters, otherwise Nil */
   final def hkTypeParams(implicit ctx: Context): List[TypeParamInfo] =
     if (isLambdaSub) typeParams else Nil

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -74,13 +74,13 @@ object Checking {
     val AppliedTypeTree(tycon, args) = tree
     // If `args` is a list of named arguments, return corresponding type parameters,
     // otherwise return type parameters unchanged
-    val (tparams, prefix) = tycon.tpe.typeParamsAndPrefix(tree.tpe)
+    val tparams = kind(tycon.tpe).typeParams
     def argNamed(tparam: ParamInfo) = args.find {
       case NamedArg(name, _) => name == tparam.paramName
       case _ => false
     }.getOrElse(TypeTree(tparam.paramRef))
     val orderedArgs = if (hasNamedArg(args)) tparams.map(argNamed) else args
-    val bounds = tparams.map(_.paramInfoAsSeenFrom(prefix.widen).bounds)
+    val bounds = tparams.map(_.paramInfoAsSeenFrom(tree.tpe).bounds)
     def instantiate(bound: Type, args: List[Type]) =
       HKTypeLambda.fromParams(tparams, bound).appliedTo(args)
     if (boundsCheck) checkBounds(orderedArgs, bounds, instantiate)