Compiler does not compile a function with a default argument initialized using an expression with the same name field

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The following trivial example produces a compiler error:

class A {
  val b = ""
  def f(b: Option[String] = Some(b))
}

The error is

Error:(3, 34) type mismatch;
 found   : Option[String]
 required: String
  def f(b: Option[String] = Some(b))
                                 ^

The error seems to suggest the compiler is trying to initialize b argument with itself, not with the b field, when the b field should be the only possible valid interpretation in this context, as b argument is not visible in the parameter list itself.

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Imported From: https://issues.scala-lang.org/browse/SI-9413?orig=1
Reporter: Ondřej Španěl (OSpanel)
Affected Versions: 2.11.7

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@lrytz said (edited on Jul 23, 2015 7:19:46 PM UTC):
Hah. In fact, default argument expressions are always type-checked in a context where the parameter is in scope - this is a bug.
The reason: normal value definitions are allowed to be recursive, so we need a special case for parameters.

Now, unfortunately we cannot just use the outer context, because parameters of a previous parameter list are visible: def f(x: Int)(y: Int = x) is valid.
The type checker doesn't have a scope per parameter list, only one per method, so there's no easy fix.

Here's another bug that has the same underlying reason:

class A {
  type T
}

class C {
  val a: A
  def f(a: String, b: a.T) = 0
}

By the spec (http://www.scala-lang.org/files/archive/spec/2.11/04-basic-declarations-and-definitions.html#default-arguments):
{quote}The scope of a formal value parameter name x comprises all subsequent parameter clauses, as well as the method return type and the function body.
{quote}

So the parameter a should not be in scope in the type definition of b. However, in reality, the type checker will resolve a in a.T to the parameter instead of the field and yield an error:

error: type T is not a member of String
         def f(a: String, b: a.T) = 0
                               ^