Automatic OpenAPI type renaming can break discriminators

Tapir version: 0.17.0-M11

Scala version: 2.13.4

Describe the bug

Type name collisions in OpenAPI are handled by renaming types. Unfortunately, the default discriminator behavior is to use type names in the schema as values. This means that the actual values of the discriminator are difficult to predict, and may differ from what Circe uses

How to reproduce?

See the full Scastie here.

Let's say we have a setup like this:

implicit val tapirConfig: Configuration = Configuration.default.withDiscriminator("__tag")
implicit val circeConfig: CirceConfiguration = CirceConfiguration.default.withDiscriminator("__tag")

sealed trait Foo

object Foo {
  final case class Bar(x: Int) extends Foo
}

final case class Bar()

val e = endpoint.get.in("foo").in(jsonBody[Bar]).out(jsonBody[Foo])

An example of Foo JSON generated by Circe:

{
  "x" : 123,
  "__tag" : "Bar"
}

The corresponding OpenAPI type:

Foo:
  oneOf:
  - $ref: '#/components/schemas/Bar1'
  discriminator:
    propertyName: __tag

Note that the implicit discriminator value (Bar1) does not match the actual JSON (Bar).

Additional information

This can be worked around using discriminatorMappingOverride in SchemaType, but it is painful to say the least.

Start with the linked Scastie reproduction and inspect the discriminator configuration and SchemaType.discriminatorMappingOverride behavior. Compare the generated OpenAPI schema with the Circe JSON discriminator value; done means automatic type renaming no longer produces a mismatched discriminator value in this case.