JonasToth added inline comments.
================
Comment at: clang-tidy/hicpp/SignedBitwiseCheck.cpp:23
+ const auto SignedIntegerOperand =
+
expr(ignoringImpCasts(hasType(isSignedInteger()))).bind("signed_operand");
+
----------------
JonasToth wrote:
> aaron.ballman wrote:
> > JonasToth wrote:
> > > aaron.ballman wrote:
> > > > JonasToth wrote:
> > > > > aaron.ballman wrote:
> > > > > > Is ignoring implicit casts the correct behavior here as far as the
> > > > > > coding standard is concerned? Consider:
> > > > > > ```
> > > > > > unsigned char c = 128;
> > > > > > f(~c); // c promotes to int before ~ is applied
> > > > > > ```
> > > > > > Looking at the coding standard, I get the impression this is not
> > > > > > allowed, but I'm not *super* familiar with HIC++.
> > > > > I first implemented it without ignoring the implicit integer casts,
> > > > > the result was, that most cases (in test cases) where not found.
> > > > > therefore i implemented it that way. I add an testcase for this and
> > > > > see how i need to adjust the matcher.
> > > > >
> > > > > Could you help me there with the semantic, since i am not so fluent
> > > > > in C/C++ standardese, but i think the findings are reasonable.
> > > > It kind of boils down to the intention from the HIC++. Consider a test
> > > > case like:
> > > > ```
> > > > void f(int i);
> > > >
> > > > void g() {
> > > > unsigned char c = 127;
> > > > f(~c);
> > > > }
> > > >
> > > > ```
> > > > Does `f()` expect to receive `-128` or `128`? I think this code will
> > > > pass your check (ignoring the promotion means the type is `unsigned
> > > > char`), but the actual bitwise operation is on a signed integer value
> > > > because there is an integer promotion. So 127 is promoted to int, then
> > > > ~ is applied, resulting in the value `-128` being passed to the
> > > > function.
> > > Yeah i see, i have such cases added in the tests.
> > > TBH. i don't know if the standard wants this covered, but the
> > > demonstrated case is definitly bad.
> > >
> > > Would it be a good idea, to warn on assigning/initializing `signed`
> > > integers with `unsigned` integers?
> > >
> > > The CppCoreGuidelines have some sections on that as well: [[
> > > https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#arithmetic
> > > | Section ES.100-103 ]]
> > >
> > > Not sure if this check should care. On the other hand, would it be nice
> > > to have a check that covers all "integer problems".
> > > Yeah i see, i have such cases added in the tests.
> > > TBH. i don't know if the standard wants this covered, but the
> > > demonstrated case is definitly bad.
> >
> > I think you should ask the HIC++ people what they think; the rule text does
> > not make it clear what the behavior should be here.
> >
> > > Would it be a good idea, to warn on assigning/initializing signed
> > > integers with unsigned integers?
> >
> > We already have such a warning in clang (-Wsign-conversion).
> >
> > >The CppCoreGuidelines have some sections on that as well: Section
> > >ES.100-103
> > >
> > >Not sure if this check should care. On the other hand, would it be nice to
> > >have a check that covers all "integer problems".
> >
> > Any such check will require so many options that it is likely to be almost
> > unusable. However, I would not be opposed to seeing a clang-tidy module
> > that has a bunch of checks in it that relate to integer problems.
> i think, those could land in `bugprone-`, and be aliases to hicpp and the
> cppcoreguidelines if appropriate.
>
> depending on my time (i should have a lot of free time for 2 months)
> i will try to implement some.
> is there a resource with all common problems found with integers?
The response from Christof:
> Hi Jonas,
>
> using bitwise operators with unsigned operands is compliant with this
> rule, even if the operand is promoted to signed int.
>
> expr.unary.op in the C++ standard says "The operand of ~ shall have
> integral or unscoped enumeration type; [...] Integral promotions are
> performed. The type of the result is the type of the promoted operand."
>
> The HICPP rule refers to the type of the operand (and not the type of
> the promoted operand), it therefore refers to the type before promotion
> (which is "unsigned char" in the example).
>
>
> Regards,
> Christof
Your requested case is therefore conforming and i think the current
implementation handles these things correct.
https://reviews.llvm.org/D36586
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