introduce NullaryMethodType to disambiguate Pol...

introduce NullaryMethodType to disambiguate PolyType

motivation:
given `def foo[T]: (T, T)` and `type Foo[T] = (T, T)`,

`foo.info` and `TypeRef(_, Foo, Nil).normalize` are both
`PolyType(List(T), Pair[T, T])`

uncurry has been relying on an ugly hack to distinguish these
cases based on ad-hoc kind inference without this distinction,
the type alias's info (a type function) would be transformed to
`PolyType(List(T), MethodType(Nil, Pair[T, T]))`

anonymous type functions are being used more often (see #2741, #4017,
#4079, #3443, #3106), which makes a proper treatment of PolyTypes more
pressing

change to type representation:
PolyType(Nil, tp) -> NullaryMethodType(tp)
PolyType(tps, tp) -> PolyType(tps, NullaryMethodType(tp)) (if the polytype denoted a polymorphic nullary method)

PolyType(Nil, tp) is now invalid

the kind of a PolyType is * iff its resulttype is a NullaryMethodType or a MethodType (i.e., it's a polymorphic value)
in all other cases a PolyType now denotes a type constructor

NullaryMethodType is eliminated during uncurry

pickling:
for backwards compatibility, a NullaryMethodType(tp) is still pickled as a PolyType(Nil, tp),
unpickling rewrites pre-2.9-pickled PolyTypes according to the expected kind of the unpickled type (similar to what we used to do in uncurry)
a pickled PolyType(Nil, restpe) is unpickled to NullaryMethodType(restpe)
a pickled PolyType(tps, restpe) is unpickled to PolyType(tps, NullaryMethodType(restpe)) when the type is expected to have kind *

the rewrite probably isn't complete, but was validated by compiling
against the old scalacheck jar (which has plenty of polymorphic nullary
methods) nevertheless, this commit includes a new scalacheck jar

summary of the refactoring:
* PolyType(List(), tp) or PolyType(Nil, tp) or PolyType(parms, tp) if params.isEmpty ==> NullaryMethodType(tp)
* whenever there was a case PolyType(tps, tp) (irrespective of tps isEmpty), now need to consider the
  case PolyType(tps, NullaryMethodType(tp)); just add a case NullaryMethodType(tp), since usually:
    - there already is a PolyType case that recurses on the result type,
    - the polytype case applied to empty and non-empty type parameter lists alike
* tp.resultType, where tp was assumed to be a PolyType that represents a polymorphic nullary method type
  before, tp == PolyType(tps, res), now tp == PolyType(tps, NullaryMethodType(res))

* got bitten again (last time was dependent-method types refactoring)
by a TypeMap not being the identity when dropNonConstraintAnnotations
is true (despite having an identity apply method). Since asSeenFrom
is skipped when isTrivial, the annotations aren't dropped. The
cps plugin relies on asSeenFrom dropping these annotations for
trivial types though. Therefore, NullaryMethodType pretends to
never be trivial. Better fix(?) in AsSeenFromMap: `if(tp.isTrivial)
dropNonContraintAnnotations(tp) else ...`

TODO: scalap and eclipse

review by odersky, rytz

1 files changed, 2 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
index 6a708a873c..8ad6830e4c 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
@@ -114,6 +114,8 @@ trait Implicits {
     private def containsError(tp: Type): Boolean = tp match {
       case PolyType(tparams, restpe) =>
         containsError(restpe)
+      case NullaryMethodType(restpe) =>
+        containsError(restpe)
       case MethodType(params, restpe) =>
         params.exists(_.tpe.isError) || containsError(restpe)
       case _ =>