```if (e is Error && e !is AssertionError && e !is NotImplementedError // wh...

# compiler

[PHondogo]

if (e is Error
    && e !is AssertionError
    && e !is NotImplementedError // why compiler complains about "Check for instance is always 'true'." ?
) {
    throw e
}

[dmitriy.novozhilov]

It's an IDE inspection, not a compiler warning

[PHondogo]

Hello, Dmitriy! It is also compiler warning.

[dmitriy.novozhilov]

To distinguish them you can do the following: enable every FIR checker that we have (and output kotlin.git-searchable internal IDs of reported diagnostics) by chanting the -Wextra -Xuse-fir-experimental-checkers -Xrender-internal-diagnostic-names incantation in compiler arguments.
If a warning does not appear in compilation logs after that — it's an IDE inspection.

You can also enable IJ's internal mode; then the aforementioned IDs should be displayed right in IDE (pic related).
If a warning does not have an ID after that — it's an IDE inspection.

[dmitriy.novozhilov]

What is the full code then?
I failed to reproduce it for the compiler

[PHondogo]

fun ensureNonCritical(e: Exception) {
    if (e is Error
        && e !is NotImplementedError
    ) {
        throw e
    }
}

executing using Gradle cmd

[PHondogo]

kotlin=2.1.20

[dmitriy.novozhilov]

Ah, that's simple.
Error is not a subtype of Exception.
They both inherit from Throwable, but there is no subtyping relation between them

[dmitriy.novozhilov]

I got a little confused, since you mentioned true in the original message, not false

[PHondogo]

😂 Sorry! My bad!

[PHondogo]

But it is strange that 'e is Error' is not marked as 'Always false'

[PHondogo]

fun ensureNonCritical(e: Exception) {
    if (e is Error) {
        throw e
    }
}

[dmitriy.novozhilov]

Indeed, looks like a little bug. Could you please report it to YouTrack?
Also cc @Nick Lunyak

[PHondogo]

https://youtrack.jetbrains.com/issue/KT-76766/Warning-is-missing-for-wrong-subclass-checking