Even after ensuring type, the compiler does not pick up the variable

[sacheeramesh]

Description:
A record type has a nullable attribute, It can either be a string or a null. In the code, I checked the attribute is a string, but within the if condition, I cannot treat the variable as a string. the compiler still picks up the variable as a string or null.

Steps to reproduce:

Record type :

# Employee record.
type Employee record {
    # First name of the employee
    string firstName;
    # Compensation details or null
    string? compensation;
};

# Compensation record.
public type Compensation record {|
    # Compensation type
    string key;
    # Compensation value
    string value;
    # Type of the value
    CompensationType 'type;
|};

Code:

Employee|error result = databaseClient->queryRow(query);
 types:Compensation[]|error compensations = [];
    if result.compensation is string {
        compensations = result.compensation.fromJsonStringWithType();
    }

Error:
incompatible types: expected 'string', found 'string?'

Affected Versions:
Ballerina 2201.8.4
OS, DB, other environment details and versions:

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[MaryamZi]

Type narrowing happens only for parameters and local variables. Since the record value is mutable, we can't guarantee that the field will continue to be a string even if the is check evaluates to true when that check is done.

You can assign result.compensation to a variable and use that instead, narrowing will then work as you expect here.

string? compensation = result.compensation;
if compensation is string {
    compensations = compensation.fromJsonStringWithType();
}

[sacheeramesh]

HI @MaryamZi Understood and thank you for the update