Is it expected for `shouldBeInstanceOf` not to work with generics? This test passes when I don’t thi...

# kotest

[Kirill Zhukov]

Is it expected for shouldBeInstanceOf not to work with generics? This test passes when I don’t think it should?

test("test") {
  listOf(1).shouldBeInstanceOf<List<String>>()
}

[Kirill Zhukov]

filed ticket https://github.com/kotest/kotest/issues/3524

[Ivan Pavlov]

It's not possible due to type erasure. Generic type isn't stored in runtime, so List<Int> is the same type as List<String> in such case
https://kotlinlang.org/docs/generics.html#type-erasure

[Kirill Zhukov]

Docs make it sound like it should work 🙃
https://kotest.io/docs/blog/release_4.2/#generic-contracts-in-matchers

[Kirill Zhukov]

When using shouldBeInstanceOf<T> or shouldBeTypeOf<T>, the assertions can now use generic contracts to smart case down to generic instances.

For example, consider the following example where we are given an Any. After invoking shouldBeTypeOf with a generic type, the type is smart casted if the assertion passes.

```val list: Any = arrayListOf(1, 2, 3)

list.shouldBeTypeOf<ArrayList<Int>>()

list[0] shouldBe 1 // can only work with a smart case```

[Adam S]

type erasure would mean it’s not possible to assume that it’s a List<Int>, but it should be possible to verify all the elements are Strings

[Adam S]

as an alternative, you could try this @Kirill Zhukov

listOf(1, 2, 3).shouldForAll { it.shouldBeInstanceOf<String>() }

[Adam S]

in that blog post it uses shouldBeTypeOf<ArrayList<Int>>(), not shouldBeInstanceOf<ArrayList<Int>>() - could you try both?

[Ivan Pavlov]

@Kirill Zhukov docs may be confusing in such case if you're not aware of type erasure 🙂 It actually can't check generic type, but can smart cast argument of this check

[Kirill Zhukov]

shouldBeTypeOf doesn’t work either

[Kirill Zhukov]

I see, makes sense

[Kirill Zhukov]

In my use case I actually have a Err<T>: Result generic data class that I want to assert: someResult.shouldBeOfType<Err<SomeError>>()

[Kirill Zhukov]

Since that doesn’t work, think will just create a custom extension: someResult.shouldBeErrOfType<SomeError>()

[Javier]

this shouldn't compile directly, I think there is an issue about adding an annotation to the kotest matcher to force this check

[Javier]

don't remember the annotation name tho 😕