closes #1569, #3731: refactored dependent metho...

closes #1569, #3731: refactored dependent method types to get rid of
debruijn indices and use singleton types instead.

this is the core of the dependent types refactoring, no implicit or
inference changes

(one baffling discovery: resultType should drop annotations that don't subclass TypeConstraint, even in the trivial case... wow -- thanks to Tiark for helping me figure it out on a terrace in Barcelona
TODO: probably need a more principled approach to the propagation of plugin type-annotations)

review by odersky

1 files changed, 5 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 7ef494ea73..e71ad5475c 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -1751,7 +1751,7 @@ trait Typers { self: Analyzer =>
           error(vparam1.pos, "*-parameter must come last")
 
       var tpt1 = checkNoEscaping.privates(meth, typedType(ddef.tpt))
-      if (!settings.Xexperimental.value) {
+      if (!settings.YdepMethTpes.value) {
         for (vparams <- vparamss1; vparam <- vparams) {
           checkNoEscaping.locals(context.scope, WildcardType, vparam.tpt); ()
         }
@@ -2420,7 +2420,10 @@ trait Typers { self: Analyzer =>
             val tparams = context.extractUndetparams()
             if (tparams.isEmpty) { // all type params are defined
               val args1 = typedArgs(args, argMode(fun, mode), paramTypes, formals)
-              val restpe = mt.resultType(args1 map (_.tpe)) // instantiate dependent method types
+              // instantiate dependent method types, must preserve singleton types where possible (stableTypeFor) -- example use case:
+              // val foo = "foo"; def precise(x: String)(y: x.type): x.type = {...}; val bar : foo.type = precise(foo)(foo)
+              // precise(foo) : foo.type => foo.type
+              val restpe = mt.resultType(args1 map (arg => gen.stableTypeFor(arg) getOrElse arg.tpe))
               def ifPatternSkipFormals(tp: Type) = tp match {
                 case MethodType(_, rtp) if ((mode & PATTERNmode) != 0) => rtp
                 case _ => tp