Can't use self of a readonly or isolated object as a captured variable in an isolated function

[hasithaa]

Description

The following program gives a compile time error.

import ballerina/io;

readonly class Test {
    
    string[] words;
    int length;

    isolated function init(string[] & readonly words, int length) {
        self.words = words;
        self.length = length;
    }

    isolated function getCount() returns int {
       // An Error at self.length.
        return self.words.filter(  isolated function (string word) returns boolean  =>  word.length() == self.length ).length();
    }
}

public function main() {
    string[] words = ["hello", "world", "this", "is", "a", "test"];
    Test test = new Test(words.cloneReadOnly(), 4);
    int count = test.getCount();
    io:println(count);
}

But it should work.

Workaround: Assign the affected field locally and pass that to the closure.

Steps to Reproduce

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Affected Version(s)

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OS, DB, other environment details and versions

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Related area

-> Compilation

Related issue(s) (optional)

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Suggested label(s) (optional)

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Suggested assignee(s) (optional)

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[MaryamZi]

I believe this is because self isn't considered final. The following seems to be allowed atm too, so the error in the isolated function is correct atm (because it's not safe).

readonly class Class {
    int i;

    function init(int i) {
        self.i = i;
    }

    function updateSelf() {
        self = new (2); // assigns to self
    }
}

The spec doesn't seem to mention this, will create an issue.

[MaryamZi]

Spec issue - ballerina-platform/ballerina-spec#1235

[MaryamZi]

This should get fixed once self is marked as implicitly final as finalized in the spec. Changing the type since this isn't quite a bug given self isn't final atm.

Issue to make self implicitly final - #40308