New method substApprox on types

Author: odersky

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

@@ -1790,7 +1790,7 @@ object SymDenotations {
                   case LambdaParam(_, _) :: _ =>
                     recur(tp.superType)
                   case tparams: List[Symbol @unchecked] =>
-                    new ctx.SubstApproxMap(tparams, args).apply(recur(tycon))
+                    recur(tycon).substApprox(tparams, args)
                 }
               record(tp, baseTp)
               baseTp

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

@@ -1443,6 +1443,13 @@ object Types {
     final def substSym(from: List[Symbol], to: List[Symbol])(implicit ctx: Context): Type =
       ctx.substSym(this, from, to, null)
 
+    /** Substitute all occurrences of symbols in `from` by corresponding types in a`to`.
+     *  Unlike `subst`, the `to` types here can be type bounds. A TypeBounds target
+     *  will be replaced by range that gets absorbed in an approximating type map.
+     */
+    final def substApprox(from: List[Symbol], to: List[Type])(implicit ctx: Context): Type =
+      new ctx.SubstApproxMap(from, to).apply(this)
+
 // ----- misc -----------------------------------------------------------
 
     /** Turn type into a function type.