Better Implicit Search Errors: problematic cases wanted!

Compiler
13

Here you are:

import org.scalatest.FunSuite
import play.api.libs.json.{Format, Json}

class ImplicitOrderingTest extends FunSuite {

  implicit val BarFormat: Format[Bar] = Json.format
  implicit val FooFormat: Format[Foo] = Json.format
  test("Writing Bar - right order: compile and crash at runtime") {

    assert(BarFormat.writes(Bar(Foo(1))).toString() === """{"f":{"i":1}}""")  //throws!!
  }

}

case class Foo(i: Int)

case class Bar(f: Foo)

This compiles but throws NPE at runtime. Ideally it shouldn’t compile, just like this doesn’t compile :

import org.scalatest.FunSuite
import play.api.libs.json.{Format, Json}

class ImplicitOrderingTest extends FunSuite {

  test("Writing Bar - right order: compile and crash at runtime") {
    implicit val BarFormat: Format[Bar] = Json.format
    implicit val FooFormat: Format[Foo] = Json.format

    assert(BarFormat.writes(Bar(Foo(1))).toString() === """{"f":{"i":1}}""")  
  }

}

case class Foo(i: Int)

case class Bar(f: Foo)

The compiler error being “forward reference extends over definition of value BarFormat
[error] implicit val BarFormat: Format[Bar] = Json.format”