Support alternate receive action

[jclark]

When send actions can be conditional #1273, it becomes useful to have an alternate-receive-action, analogous to the alternate-wait-action that already exists.

[lochana-chathura]

For case 1, case 2 and case 3, do we need to omit the error:NoMessageError? case 4 is correct.

function case1() {
    boolean foo = true;

    worker w1 {
        if foo {
            10 -> w2;
        } else {
            20 -> w2;
        }
    }

    worker w2 {
        int|error:NoMessageError a = <- w1 | w1; // is enforcing NoMessageError here correct? 
    }

    _ = wait {a: w1, b: w2};
}

function case2() {
    boolean foo = true;

    worker w1 {
        if foo {
            10 -> w2;
        }
        20 -> w2;
    }

    worker w2 {
        int|error:NoMessageError a = <- w1 | w1; // is enforcing NoMessageError here correct?
    }

    _ = wait {a: w1, b: w2};
}

function case3() {
    boolean foo = true;
    boolean bar = true;

    worker w1 {
        if foo {
            if bar {
                10 -> w2;
            } else {
                20 -> w2;
            }
            
        } else {
            if bar {
                20 -> w2;
            } else {
                30 -> w2;
            }
        } 
        
    }

    worker w2 {
        int|error:NoMessageError a = <- w1 | w1 | w1 | w1; // is enforcing NoMessageError here correct? 
    }

    _ = wait {a: w1, b: w2};
}

function case4() {
    boolean foo = true;
    boolean bar = true;

    worker w1 {
        if foo {
            10 -> w2;
        }
        if bar {
            20 -> w2;
        }
    }

    worker w2 {
        int|error:NoMessageError a = <- w1 | w1; // OK here
    }

    _ = wait {a: w1, b: w2};
}

[jclark]

1. foo is not const, so compile-time know that it must be one of the two, but not which; we want to avoid error:NoMessage here, but we haven't defined how
2. second alternate is compile-time guaranteed to produce a value, so no need for error:NoMessage
3. same as 1

[lochana-chathura]

I am just updating the issue with the current jBallerina behavior.

1. foo is not const, so compile-time know that it must be one of the two, but not which; we want to avoid error:NoMessage here, but we haven't defined how

We enforce error:NoMessage for both 1 and 3 cases as we haven't figured out how to avoid it.

2. second alternate is compile-time guaranteed to produce a value, so no need for error:NoMessage

We fixed this. error:NoMessage is added iff all receives in the alternate receive have the error:NoMessage.

[jclark]

The static type of alternative-receive-action isn't right in the spec at the moment. error:NoMessage is included if any of the alternate receives can fail, rather than if all of them can fail. Need to change the definition of failure type to handle this.

Now fixed by 45f08c3