Newly added TransactionOperations.execute(Runnable) breaks existing Kotlin code

[dittos]

Affects: 5.2.0.RC1+

---

2c33c11 / #23250 added the new TransactionOperations.execute(Runnable) method for Java lambda usage without return value.

However it breaks existing Kotlin code using TransactionTemplate with return values:

val result = transactionTemplate.execute {
  ...
  1
}
// result becomes Unit type, which used to be Int? type

This happens because Kotlin allows omitting single parameter (the implicit it parameter). Kotlin compiler cannot distinguish no-arg Runnable with unary TransactionCallback<T>.

In order to disambiguate, a parameter or the callback interface should be explicitly stated:

val result = transactionTemplate.execute { _ ->
  ...
  1
}
// or
val result = transactionTemplate.execute(TransactionCallback {
  ...
  1
})

I'm not sure how to fix it. Maybe let it break for bigger convenience in Java?