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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;()
}```

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

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>

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

&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```


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

as an alternative, you could try this <@U2Y54SWLS>

```listOf(1, 2, 3).shouldForAll { it.shouldBeInstanceOf&lt;String&gt;() }```

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?

<@U2Y54SWLS> 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

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;()`

Since that doesn’t work, think will just create a custom extension: `someResult.shouldBeErrOfType&lt;SomeError&gt;()`

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

don't remember the annotation name tho :confused: