Details
-
Type:
Bug
-
Status: Open
-
Priority:
Major
-
Resolution: Unresolved
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Affects Version/s: Scala 2.9.1, Scala 2.10.0
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Fix Version/s: None
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Component/s: Type Inference
Description
class Result[+A]
case class Success[A](x: A) extends Result[A]
class Apply[A]
object Apply {
def apply[A](f: Int => Result[A]): Apply[A] = new Apply[A]
}
object TestUnit {
def goo : Apply[Option[Int]] = Apply { i =>
/* val p = */i match {
case 1 => Success(Some(1))
case _ => Success(None)
}
//p
}
}
This code compiles wуll. However with uncommented it fails. I checked all generics under the clojure are inferred in the same way. So I can't explain this different behavior.
Issue Links
Activity
With the intermediate assignment, it seems to infer "Success[_ >: Some[Int] with None.type <: Option[Int]]" as the result of the match. Then it decides this is incompatible with the typevar. Without the intermediate assignment, it is able to connect the dots between ?A and the match.
I guess my question is, why doesn't it simplify Success[_ >: Some[Int] with None.type <: Option[Int]] to Success[Option[Int]]. Of course if you do that by hand with type annotations, everything works.
In my experience using wildcards a lot more than appropriate, the error message seems normal. It just says that you cannot pass a BoundedWildcardType as a type parameter, as it often happens (in my experience).
Reading the source, this does not make sense since recursion should end here:
https://github.com/scala/scala/blob/d8ba5d091e5641553b438ef9930a6023a2709dcd/src/compiler/scala/reflect/internal/Types.scala#L5347
However, it needs to pass a couple of layers before. For the error to happen, in fact, it is required a double mismatch: Success vs Result and type variable vs bounded wildcard. Either making Success covariant or exchanging Success with Result either at the Apply.apply signature or at its caller hides the bug.
<console>:13: error: no type parameters for method apply: (f: Int => Result[A])Apply[A] in object Apply exist so that it can be applied to arguments (Int => Success[_ >: Some[Int] with None.type <: Option[Int]])
--- because ---
argument expression's type is not compatible with formal parameter type;
found : Int => Success[_ >: Some[Int] with None.type <: Option[Int]]
required: Int => Result[?A]
def goo : Apply[Option[Int]] = Apply { i =>
Here's another look at this:
% ~/code/scala2 cat sandbox/t5585.scala
class Result[+A]
case class Success[A](x: A) extends Result[A]
class Apply[A]
object Apply {
def apply[A](f: Int => Result[A]): Apply[A] = new Apply[A]
}
object TestUnit {
def goo : Apply[Option[Int]] = Apply.apply { i =>
val p = (0: Int) match {
case 1 => Success(Some(1))
case _ => Success(None)
}
p
}
//
// OK
//
def f[A](r: Result[A]): Apply[A] = ???
f {
(0: Int) match {
case 1 => Success(Some(1))
case _ => Success(None)
}
}: Apply[Option[Int]]
f {
val p = (0: Int) match {
case 1 => Success(Some(1))
case _ => Success(None)
}
p
}: Apply[Option[Int]]
case class H[-A, +B](b: B)
def h[A](r: H[Int, Result[A]]): Apply[A] = ???
h {
val p = (0: Int) match {
case 1 => Success(Some(1))
case _ => Success(None)
}
H(p)
}: Apply[Option[Int]]
//
// KO
//
def g[A](r: Function1[Int, Result[A]]): Apply[A] = ???
g {
val p = (0: Int) match {
case 1 => Success(Some(1))
case _ => Success(None)
}
(i: Int) => p
}: Apply[Option[Int]]
}
% qbin/scalac sandbox/t5585.scala
% git diff
diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index dc5491a..00af937 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -2837,7 +2837,7 @@ trait Typers extends Modes with Adaptations with Tags {
// }
val formals = vparamSyms map (_.tpe)
val body1 = typed(fun.body, respt)
- val restpe = packedType(body1, fun.symbol).deconst.resultType
+ val restpe = body1.tpe.deconst
val funtpe = typeRef(clazz.tpe.prefix, clazz, formals :+ restpe)
// body = checkNoEscaping.locals(context.scope, restpe, body)
treeCopy.Function(fun, vparams, body1).setType(funtpe)
An understanding of `packedType` seems important here.
> In my experience using wildcards a lot more than appropriate, the error message seems normal. It just says that you cannot pass a BoundedWildcardType as a type parameter, as it often happens (in my experience).
I should clarify: with "normal" I meant "a bug I see all the time", not "the right thing to do". At least, that's what I should have meant.
It's very similar to https://issues.scala-lang.org/browse/SI-5579