[Bug]: No error when assinging a mutable value to a union where the relevent member type is immutable

[rdulmina]

Description

Consider the below sample

type Foo record {
    int i;
};

type Bar record {
    int i;
    int j;
};


public function main() {
    Bar b = {i: 1, j: 2};
    Bar & readonly|Foo a = b; // no error
    Bar & readonly|string a = b; // error as expected
    Bar & readonly a = b; // error as expected
}

Steps to Reproduce

No response

Affected Version(s)

No response

OS, DB, other environment details and versions

No response

Related area

-> Compilation

Related issue(s) (optional)

No response

Suggested label(s) (optional)

No response

Suggested assignee(s) (optional)

No response

[LakshanWeerasinghe]

The Foo type is assignable to the Bar type, making the Bar & readonly|Foo union type also assignable to the Bar type.
This is due to the fact that Foo is an open record, with its fields constituting a subset of the fields present in the Bar type.

Bar b = {i: 1, j: 2};
Foo foo = b;
Bar & readonly|Foo a = b;

If we add a new field to the Foo type that is not present in the Bar type, it will result in an error.

type Foo record {
    int i;
    int z;
};

type Bar record {
    int i;
    int j;
};

public function main() {
    Bar b = {i: 1, j: 2};
    Bar & readonly|Foo a = b; // error as expected
}

[github-actions]

This issue is NOT closed with a proper Reason/ label. Make sure to add proper reason label before closing. Please add or leave a comment with the proper reason label now.

      - Reason/EngineeringMistake - The issue occurred due to a mistake made in the past.
      - Reason/Regression - The issue has introduced a regression.
      - Reason/MultipleComponentInteraction - Issue occured due to interactions in multiple components.
      - Reason/Complex - Issue occurred due to complex scenario.
      - Reason/Invalid - Issue is invalid.
      - Reason/Other - None of the above cases.