LowPriorityKeyReads.readableKeyReads should not exist

[arturaz]

Play JSON Version (2.5.x / etc)

3.0.1

API (Scala / Java / Neither / Both)

Scala

Problem

Map serialization/deserialization is broken if the key doesn't have a KeyReads instance.

The problem stems from the LowPriorityKeyReads.readableKeyReads which implies that if you have a Reads[A] then you have KeyReads[A], which does not seem like a logical conclusion. After all, the Reads can expect JsArray, JsObject or any other type, none of which are JsString.

import play.api.libs.json.*

val map = Map((1, 2) -> "foo")

val asJson = Json.prettyPrint(Json.toJson(map)) 
println(asJson)
// Gets serialized to: [ [ [ 1, 2 ], "foo" ] ]

val fromJson = Json.parse(asJson).validate[Map[(Int, Int), String]]
println(fromJson)
// Fails to deserialize, as a failing `KeyReads[(Int, Int)]` will be created from `LowPriorityKeyReads.readableKeyReads`

Suggested solution

Remove LowPriorityKeyReads.readableKeyReads. This breaks backwards compatibility, but I argue that this function is useless (as in it almost never produces a useful Reads instance anyway).

Reproducible Test Case

https://scastie.scala-lang.org/8ePgBH6lQ5i4GmCOPksvyw