C++: Add tests for experimental cpp/guarded-free query

[jketema]

I investigated a bit if the query can be promoted (instead of moving it to a separate repo). The tests highlight some issues.

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[paldepind]

Is the comma operator what's causing the issue? Or does this also fail with { inspect(x); free(x); }? If it does then maybe just use semicolons? If not, would it make sense to add a comment to clarify that?

[jketema]

The problem is that inspect may assume that x is non-null, so if the remove the if, which is what the query is about, the thing would start crashing. Note that this derives from code I saw in the wild. Replacing it by { inspect(x); free(x); } would not give an FP.

[paldepind]

Then what about changing the comment to:

// GOOD [FALSE POSITIVE]: x might be accessed in the first operand of the comma operator

Or something else to make it clear that , is part of the problem/what is being tested?

[jketema]

Done.

[paldepind]

Looks good. Just a few comments 😄