Imported From: https://issues.scala-lang.org/browse/SI-9970?orig=1
Reporter: Diogo Sousa (orium)
Affected Versions: 2.11.8
See #8884

@som-snytt said:
If it implied that, then we could leave off the param type ascription:

scala> class A[T] { def f: T => Map[Int, Int] = _ => Map.empty } // print

class A[T] {^J  def f: _root_.scala.Function1[T, scala.Predef.Map[scala.Int, scala.Int]] = ((x$1: T) => scala.Predef.Map.empty[Int, Nothing])^J} // : <notype>

Unfortunately, // print completion doesn't work for the example.

@SethTisue said:
[~orium] what passage in the Scala Language Specification justifies considering this a bug?

Diogo Sousa (orium) said:
I'm didn't study the Scala language spec (but I would say that if it agrees with this behavior then the bug still exists but it is in the spec).

In fact, as @apm said, it didn't infer Map.empty to be of type Map[Int, Int] as I said, but it must, at least, infer Map.empty to be of type Map[Int, _], since Map is invariant in the key. However, the error on instatiation says that it got a Map[Nothing, Nothing]. How could it even typecheck at the type of the class definition?

To be more general: I don't think it is acceptable to get a type error when a default parameter is used. It was typechecked before, it must fail at definition of the class/method if the types don't checkout

(Another way this is weird is that class A(foo: String => Map[Int, Int] = (_: String) => Map.empty) ; new A works. Having the type parameterized somehow makes a difference here. This is like breaking "referential transparency" for types.)

@som-snytt said:

        class C[A] extends scala.AnyRef {
          <paramaccessor> private[this] val f: A => Map[Int,Int] = _;
          def <init>(f: A => Map[Int,Int] = ((x$1: A) => scala.Predef.Map.empty[Int, Nothing])): C[A] = {
            C.super.<init>();
            ()
          }
        };
        <synthetic> object C extends AnyRef {
          def <init>(): C.type = {
            C.super.<init>();
            ()
          };
          <synthetic> def <init>$default$1[A]: A => scala.collection.immutable.Map[Nothing,Nothing] = ((x$1: A) => scala.Predef.Map.empty[Nothing, Nothing])
        }
      }

I think there's an open ticket for default args, of course I can't find it. Maybe similar to #8884.

Spec behavior.

@lrytz said:
{quote}I don't think it is acceptable to get a type error when a default parameter is used. It was typechecked before, it must fail at definition of the class/method if the types don't checkout{quote}

There's a special case with default arguments for parametric types, to support the following:

scala> def f[T](x: T = 1) = x
f: [T](x: T)T

scala> f()
res0: Int = 1

scala> f[String]()
<console>:13: error: type mismatch;
 found   : Int
 required: String
Error occurred in an application involving default arguments.
       f[String]()
        ^

See http://www.scala-lang.org/files/archive/spec/2.12/04-basic-declarations-and-definitions.html#default-arguments

For the original example in your report, I think it's a valid bug.

could you pls confirm that the following behavior is related to this issue

  def x(i: Int, m: Map[String, Boolean]) = ???
  def x(i: String, m: Map[String, Boolean]) = ???
  
  x(1, Map.empty[String, Boolean]) // compiles well
  x(1, Map.empty) // caused error
[error] /Users/ievgen/scalaprojects/pantheon/app/controllers/X.scala:41:3: overloaded method value x with alternatives:
[error]   (i: String,m: Map[String,Boolean])Nothing <and>
[error]   (i: Int,m: Map[String,Boolean])Nothing
[error]  cannot be applied to (Int, scala.collection.immutable.Map[Nothing,Nothing])
[error]   x(1, Map.empty) // caused error