Unable to compile an extension function with reified type parameter in Kotlin

Question

I have a kotlin library project and use Spek for testing. In my tests I try to use the following function:

inline fun JsonElement?.cast<reified T>(gson: Gson): T {
  return if (this != null)
    gson.fromJson<T>(this, javaClass<T>())
  else null
}

When I try to compile the above code, I get a compilation error:

Error: Gradle: java.lang.IllegalStateException: Error type encountered:
org.jetbrains.kotlin.types.ErrorUtils$UninferredParameterTypeConstructor@144bac7f (ErrorTypeImpl).
One of the possible reasons may be that this type is not directly accessible from this module.
To workaround this error, try adding an explicit dependency on the module
or library which contains this type to the classpath

However this code compiles correctly:

inline fun JsonElement?.cast<reified T>(gson: Gson): T? {
  return if (this != null)
    gson.fromJson<T>(this, javaClass<T>())
  else null
}

You notice, the return type was changed from T to T?. Even though it compiles, I get a hint message:

'T' has nullable upper bound. This means that a value of type may be null.
Using 'T?' is likely to mislead the reader.

Is it a correct behavior?

I don't want to mislead the reader, so what should I write instead? — ntoskrnl, Apr 21 '15 at 07:17
To make this more correct and include the generic type information, you need to change to use a Gson TypeToken as well. Please see http://stackoverflow.com/a/33420043/3679676 for a answer that does this. — Jayson Minard, Jan 04 '16 at 21:47
The `gradle` tag should be removed, this is unrelated to Gradle and would have shown up in any use of the Kotlin compiler at that time. — Jayson Minard, Jan 04 '16 at 21:49

score 2 · Accepted Answer · answered Apr 21 '15 at 11:15

2

The exception you are getting is a bug in the compiler.

The warning you are getting is correct, and to eliminate it, write <reified T: Any>, this way a plain T is not nullable any more.

answered Apr 21 '15 at 11:15

Andrey Breslav

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To make this more correct and include the generic type information, you need to change to use a Gson TypeToken as well. Please see http://stackoverflow.com/a/33420043/3679676 for a answer that does this. – Jayson Minard Jan 04 '16 at 21:47

Unable to compile an extension function with reified type parameter in Kotlin

1 Answers1