No completions in lambda blocks above return

```
public static void main(String[] args) {
FutureTask task = new FutureTask<>(() -> {
Sys
return null;
});
}
```

No completion behind `Sys`, Even without the return statement completions doesn't work. But if the code is casted

```
public static void main(String[] args) {
FutureTask task = new FutureTask<>((Callable) () -> {
Sys
return null;
});
}
```

Then it works. So may be we should try to deduce the correct functional type of overloaded methods based on the return value of the lambda.

@stephan-herrmann what do you think ? Is above suggested a good idea to tryout ?

Reproduce the two Java snippets and compare completion behind `Sys` with and without the explicit `Callable` cast, including the version without `return null`. Investigate lambda functional-type deduction as it affects completion; done means completion works in the uncast lambda block as well as the cast case.

Written by the indexing model from the issue text.