This fixes a failing qltest and makes the exclusion similar to what's in
`PointerOverflow.ql`. It's possible we should exclude based on both `+`
and `<`, but we can revisit that if false positives show up.
Before this change, evaluating `cpp/constant-comparison` followed by
`cpp/signed-overflow-check` would result in re-evaluation of almost all
the cached stages they share: CFG, basic blocks, SSA, and range
analysis. The same effect could be seen on `cpp/bad-strncpy-size`, which
also uses the GVN library.
We were privately importing `semmle.code.<lang>.ir.internal.Overlap`, but `PrintSSA.qll` was depending on it being public. This is made a little more complicated by the presence of cross-langage pyrameterized modules.
Should fix#1833, #2137, and #2187.
Internally, comprehensions are (at present) elaborated into local functions and
iterators as described in [PEP-289](https://www.python.org/dev/peps/pep-0289/).
That is, something like:
```
g = (x**2 for x in range(10))
```
becomes something akin to
```
def __gen(exp):
for x in exp:
yield x**2
g = __gen(iter(range(10)))
```
In the context of the top-level of a class, this means `__gen` looks as if it is
a method of the class, and in particular `exp` looks like it's the `self`
argument of this method, which leads the points-to analysis to think that `exp`
is an instance of the surrounding class itself.
The fix in this case is pretty simple: we look for occurrences of `exp` (in fact
called `.0` internally -- carefully chosen to _not_ be a valid Python
identifier) and explicitly exclude this parameter from being classified as a
`self` parameter.
The `SignedOverflowCheck.ql` query was very slow on certain snapshots
(jluttine/suitesparse and Chromium) due to bad magic in
`MacroInvocation::getAnAffectedElement_dispred#fb`. This commit doesn't
fix the bad magic but changes the exclusion mechanism to use a predicate
where we can better control the magic and optimization.
The query should also give more good results due to this new exclusion
mechanism, which is the same one used in its sibling,
`PointerOverflow.ql`.
