Mocking unsigned Kotlin types ends in `java.lang.ClassCastException`

I’m mocking simple function returning UInt but after that method is executed in the test it crashes with java.lang.ClassCastException.

every { calculator.compute(any()) } returns 23859762u

Expected Behavior

No crash, mocked function returns unsigned number result.

Current Behavior

Exception java.lang.ClassCastException: kotlin.UInt cannot be cast to java.lang.Integer is thrown in case of UInt return type.

Steps to Reproduce

1. Write mockk every function for a method returning unsigned number like UInt
2. Call this function in the test
3. Exception is thrown

Context

• MockK version: 1.10.0
• OS: macOS Catalina
• Kotlin version: 1.3.72
• JUnit version: 4.12
• Type of test: unit test

Minimal reproducible code (the gist of this issue)

    every { calculator.compute(any()) } returns 23859762u
    assertEquals(23859762u, calculator.compute(42))

[Raibaz]

Unsigned integer support in Kotlin is still in beta, so I guess @d35h is right.

[migueltorcha]

Same issue here trying to mock a UByte:

interface MyInterface {
  suspend fun myMethod(): UByte?
}

....

coEvery { myInterface.myMethod() } returns Random.nextInt().toUByte()

As result, I have the next: class java.lang.Byte cannot be cast to class kotlin.UByte (java.lang.Byte is in module java.base of loader 'bootstrap'; kotlin.UByte is in unnamed module of loader 'app') java.lang.ClassCastException: class java.lang.Byte cannot be cast to class kotlin.UByte (java.lang.Byte is in module java.base of loader 'bootstrap'; kotlin.UByte is in unnamed module of loader 'app')