support flow-typing improvement

[gavinking]

The backend needs to support the improvement in ceylon/ceylon-spec/issues/891.

An example is:

Integer fun(Integer? int) {
    if (!exists int) {
        return 0;
    }
    return int+1;
}

Which currently results in this backend error:

Bad operand types for binary operator +
  first type:  ceylon.language.Integer
  second type: long

[quintesse]

Could you give a small explanation of how you implemented it? Because I intuitively understood the SyntheticAssert but you removed and improved it and now it's not as obvious to me how to implement it on the backend.

[gavinking]

I used the same strategy I use for narrowing in else clauses. I add a Variable. In this case the Variable gets slotted into block just after the if statement, and it gets a special new scope.

[FroMage]

OK I'll bite.

[FroMage]

Done!

[gavinking]

Excellent, thanks :)