Kotlin: `tailrec` on Instance method vs Extension function

Question

In the below code, I see a warning no tail calls found, but the same function, when written as extension function, doesn't have that warning. Now I am confused whether my IDE is wrong, or my Extension method is not actually tail-recursive or is there a difference between how compiler deals with Instance methods vs Extension functions.

Answer 1

As the second warning

Recursive call is not a tail call​

indicates, this call is recursive, but is not a tail call.

This is because the function is being invoked with a different instance as its target (next and not this), so tailrec doesn't really have an effect in this case. If you replace the target with this, the compiler stops issuing any warnings.

I assume the extension function compiles without any warnings because they are compiled as static (i.e. non-instance) functions, where the target is simply another parameter, meaning that the transformed code looks (roughly) like

@Nullable
public static final SLLNode2 getNodeForValue2(@NotNull SLLNode2 $this$getNodeForValue2, int valToFind) {
    Intrinsics.checkParameterIsNotNull($this$getNodeForValue2, "$this$getNodeForValue2");
    if ($this$getNodeForValue2.getValue() == valToFind) {
        return $this$getNodeForValue2;
    } else {
        SLLNode2 var10000 = $this$getNodeForValue2.getNext();
        return var10000 != null ? getNodeForValue2(var10000, valToFind) : null;
    }
}

which quite clearly contains a recursive tail call.

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However, I'm not sure this distinction is intentional, since the way extension functions are compiled should be an implementation detail, and the instance function example should(?) be equivalently optimizable.

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Edit: it seems this is in fact a bug.