Add a Conditional Access Operator (Null Conditional Operator)

[kindlich]

In order to make working with optional types easier without losing the information that we are using optionals, we want to introduce a Conditional Access Operator.

Null conditional operator: (x?.field or x?.method(...))

• Used to call a method or load a field on an optional value
• Can be used only on optional values (T?), usage on other types is an error
• Always returns an optional - null if the value if null, the result of the method call / field otherwise
• If the resulting value is optional, it is flattened - there are no doubly optionals
• Not supported for operators, including [ ] and ( )
• No short-circuit, if the operand before the ?. is null, any chained calls are stil evaluated.
  • a?.getB().c would error unless B? had a member c
• Uses the members from the base-type (T if invoked on T?)

Acceptance Criteria:

• Decide: Should this be usable on member-get and member-set operator => Requires: Member getter / setter in zc #144 ; If that issue has been completed by then, implement if we decide we want this, otherwise create new issue to track that implementation.
• Test Cases:

  • Simple test case with null- and non-null-value

  • ?. flattens optional return type

  • "Collision:" Member exists on both Nullable-type and on base-type, which one does the null-safe operator use?

    public expand string? { public const get internalized as string => this == null ? "NULL" : this.toUpperCase(); }
    public expand string  { public const get internalized as string => this; }
    
    var x as string? = null;
    println(x?.internalized); // null
    println(x.internalized); // "NULL"
    
    var y as string? = "Hello";
    println(y?.internalized); // "Hello"
    println(y.internalized); // "HELLO"
    

  • Properly converts back and forth between null and -1 for null as usize

  • If decision on member-get and member-set operator: Tests for that

  • Fails for non-optional type

  • Operator cannot be overridden in a custom type