Allow to assign null AST childs after non-null

[leventov]

See #249

[GerardPaligot]

unused.

[GerardPaligot]

This feature seems necessary. We shouldn't have a NPE when we set a target with null. But my first reaction was that it is awful to check everywhere if the parameter is null. After reflection, if we want to keep the setters of parents and/or target on the AST, we don't have any choices. So, I'm ok with this PR.

[leventov]

@GerardPaligot fixed

[GerardPaligot]

An expression of an assertion can be null? For me, an assertion must have an expression so I propose to change for something like:

public void setExpression(CtExpression<T> value) {
    if (value == null) {
        throw new SpoonException("An assertion must have an expression.");
    }
    value.setParent(this);
    this.value = value;
}

What do you think?

[leventov]

There is an "assertionExpression" and just "expression". Null expression corresponds to the statement assert assertionExpression;. Or it might be assert assertionExpression : expression;

[GerardPaligot]

For now, you have only one test in your PR to test a target null but there aren't tests for other checks. It could be interesting to add new tests for all others cases. We know now why we add these check but not necessarily in a few years.

So, can you add tests?

[leventov]

Ok

[leventov]

@GerardPaligot please look why CI build is failing. I don't understand. Locally it is ok.