Is it expected for `shouldBeInstanceOf` not to work with gen

Question

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 lt List lt String gt gt ```

Kirill Zhukov · Answer

filed ticket <https github com kotest kotest issues 3524>

Ivan Pavlov · Answer

It s not possible due to type erasure Generic type isn t stored in runtime so List lt Int gt is the same type as List lt String gt in such case <https kotlinlang org docs generics html type erasure|https kotlinlang org docs generics html type erasure>

Kirill Zhukov · Answer

Docs make it sound like it should work upside down face <https kotest io docs blog release 4 2 generic contracts in matchers>

Kirill Zhukov · Answer

gt When using shouldBeInstanceOf lt T gt or shouldBeTypeOf lt T gt the assertions can now use generic contracts to smart case down to generic instances gt 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 gt ```val list Any = arrayListOf 1 2 3 gt list shouldBeTypeOf lt ArrayList lt Int gt gt gt list 0 shouldBe 1 can only work with a smart case```

Adam S · Answer

type erasure would mean it s not possible to assume that it s a `List lt Int gt ` but it should be possible to verify all the elements are Strings

Adam S · Answer

as an alternative you could try this < Kirill Zhukov> ```listOf 1 2 3 shouldForAll it shouldBeInstanceOf lt String gt ```

Adam S · Answer

in that blog post it uses `shouldBeTypeOf lt ArrayList lt Int gt gt ` not `shouldBeInstanceOf lt ArrayList lt Int gt gt ` could you try both

Ivan Pavlov · Answer

< Kirill Zhukov> docs may be confusing in such case if you re not aware of type erasure slightly smiling face It actually can t check generic type but can smart cast argument of this check

Kirill Zhukov · Answer

`shouldBeTypeOf` doesn t work either

Kirill Zhukov · Answer

I see makes sense

Kirill Zhukov · Answer

In my use case I actually have a `Err lt T gt Result` generic data class that I want to assert `someResult shouldBeOfType lt Err lt SomeError gt gt `

Kirill Zhukov · Answer

Since that doesn t work think will just create a custom extension `someResult shouldBeErrOfType lt SomeError gt `

Javier · Answer

this shouldn t compile directly I think there is an issue about adding an annotation to the kotest matcher to force this check

Javier · Answer

don t remember the annotation name tho confused