`iterableWrites2` causes Map.map(...) to be written as a seq of tuples

[vhminh]

Play JSON Version (2.5.x / etc)

3.0.4

API (Scala / Java / Neither / Both)

Scala

Operating System (Ubuntu 15.10 / MacOS 10.10 / Windows 10)

MacOS

JDK (Oracle 1.8.0_72, OpenJDK 1.8.x, Azul Zing)

openjdk version "17.0.11" 2024-04-16 LTS
OpenJDK Runtime Environment Corretto-17.0.11.9.1 (build 17.0.11+9-LTS)
OpenJDK 64-Bit Server VM Corretto-17.0.11.9.1 (build 17.0.11+9-LTS, mixed mode, sharing)

Library Dependencies

None

Expected Behavior

Map.map(...) should be written as a json object
For example, this test should pass

"JsValueWrapper conversion" should {
  "convert Map.map(...) to json object, not json array" in {
    import Writes._
    Json.obj("data" -> Map("key" -> "value").map(identity))
      .mustEqual(Json.obj("data" -> Json.obj("key" -> "value")))
  }
}

Actual Behavior

The above test case fails with output:

[info] - should convert Map.map(...) to json object, not json array *** FAILED *** (2 milliseconds)
[info]   {"data":[["key","value"]]} did not equal {"data":{"key":"value"}} (JsonSharedSpec.scala:464)
[info]   Analysis:
[info]   JsObject(underlying: ImmutableLinkedHashMap("data": [["key","value"]] -> {"key":"value"}))

• This is likely a Scala 2 compiler bug. I have opened a ticket Type inference failure while picking among conversions scala/bug#13102
• Through trial and error, I found that the below signature may work. We can try it if the bug can't be fixed in the compiler

implicit def iterableWrites3[A, I[_]](implicit ev: I[A] <:< Iterable[A], w: Writes[A]): Writes[I[A]]