[cfe-dev] RFC: Nullability qualifiers

[Aaron Ballman]

On Sat, Jun 27, 2015 at 8:06 AM, b17 c0de <b17c0de at gmail.com> wrote:
> I figured out my issue. I was compiling with -std=c++14 and the nullability
> and assume_nonnull features are only enabled for ObjC and GNU mode. Why are
> these only supported in GNU mode? I thought GNU mode was only for features
> that contradict the standard. How does this feature contradict the standard
> given that the names are double and single underscore prefixed? I would
> rather not have to compile my code in GNU mode just to enable nullability. I
> can check with __has_extension() but at least Apple headers seem to only use
> __has_feature so the checks there won't be enabled when not compiling in GNU
> mode. If the consensus is that __has_feature(nullability) should only be
> enabled for GNU mode, would it make sense to have an f-group flag like
> -fnullability to enable it for __has_feature when not compiling in GNU mode?
>
> Also I found a bug in clang. __has_extension(assume_nonnull) doesn't work
> properly. It is missing from the StringCase at the end of the HasExtension()
> function in lib/Lex/PPMacroExpansion.cpp. I think it should be there.

__has_feature(assume_nonnull) is the way to test for that feature
(which is also GNU and Obj-C only).

~Aaron

>
> On Sat, Jun 27, 2015 at 12:44 AM, b17 c0de <b17c0de at gmail.com> wrote:
>>
>> Apple please implement  __has_feature(nullability) in clang for Xcode 7
>> release. :-)
>>
>>
>> On Sat, Jun 27, 2015 at 12:31 AM, Aaron Ballman <aaron at aaronballman.com>
>> wrote:
>>>
>>> On Fri, Jun 26, 2015 at 6:29 PM, b17 c0de <b17c0de at gmail.com> wrote:
>>> >  It also appears that the current versions of Apple clang (even the
>>> > newest
>>> > beta) don't even support __has_feature(nullability). I take it this has
>>> > been
>>> > fixed in trunk?
>>>
>>> Correct, trunk is likely also the only place that has _Nonnull and
>>> friends, too. If you're developing on OS X and don't need cross
>>> compiler support for your code base, I would stick with __nonnull
>>> there and you'll be fine. If you need cross compiler support, you'll
>>> likely have to piece it together with macros.
>>>
>>> ~Aaron
>>>
>>> >
>>> > On Fri, Jun 26, 2015 at 11:53 PM, Aaron Ballman
>>> > <aaron at aaronballman.com>
>>> > wrote:
>>> >>
>>> >> On Fri, Jun 26, 2015 at 5:44 PM, b17 c0de <b17c0de at gmail.com> wrote:
>>> >> > OK. What would be the best way to detect if Apple clang supports
>>> >> > _Nonnull or
>>> >> > only __nonnull though.
>>> >>
>>> >> I cannot speak for how Apple's Clang works in this regard, but perhaps
>>> >> Doug can.
>>> >>
>>> >> ~Aaron
>>> >>
>>> >> >
>>> >> > On Fri, Jun 26, 2015 at 11:40 PM, Aaron Ballman
>>> >> > <aaron at aaronballman.com>
>>> >> > wrote:
>>> >> >>
>>> >> >> On Fri, Jun 26, 2015 at 5:36 PM, b17 c0de <b17c0de at gmail.com>
>>> >> >> wrote:
>>> >> >> > How can one detect if an Apple clang supports the new nullability
>>> >> >> > attributes. I tried something like:
>>> >> >> >
>>> >> >> > #if __has_attribute(_Nonnull)
>>> >> >> > #elif __has_attribute(__nonnull)
>>> >> >> > #define _Nonnull __nonnull
>>> >> >> > #else
>>> >> >> > #define _Nonnull
>>> >> >> > #endif
>>> >> >> >
>>> >> >> > But this didn't work. Why doesn't _Nonnull/__nonnull work with
>>> >> >> > __has_attribute?
>>> >> >>
>>> >> >> __has_attribute is used to test for GNU-style attribute support
>>> >> >> only.
>>> >> >> To test for nullability, you should use: __has_feature(nullability)
>>> >> >>
>>> >> >> ~Aaron
>>> >> >>
>>> >> >> >
>>> >> >> > On Wed, Jun 24, 2015 at 10:39 PM, Douglas Gregor
>>> >> >> > <dgregor at apple.com>
>>> >> >> > wrote:
>>> >> >> >>
>>> >> >> >> Another addendum: due to the conflict with glibc’s __nonnull,
>>> >> >> >> we’ll
>>> >> >> >> be
>>> >> >> >> renaming the __double_underscored keywords to _Big_underscored
>>> >> >> >> keywords,
>>> >> >> >> e.g.,
>>> >> >> >>
>>> >> >> >> __nonnull -> _Nonnull
>>> >> >> >> __nullable -> _Nullable
>>> >> >> >> __null_unspecified -> _Null_unspecified
>>> >> >> >>
>>> >> >> >> On Darwin, we’ll add predefines
>>> >> >> >>
>>> >> >> >> #define __nonnull _Nonnull
>>> >> >> >> #define __nullable _Nullable
>>> >> >> >> #define __null_unspecified _Null_unspecified
>>> >> >> >>
>>> >> >> >> to keep the existing headers working.
>>> >> >> >>
>>> >> >> >> - Doug
>>> >> >> >>
>>> >> >> >> On Mar 2, 2015, at 1:22 PM, Douglas Gregor <dgregor at apple.com>
>>> >> >> >> wrote:
>>> >> >> >>
>>> >> >> >> Hello all,
>>> >> >> >>
>>> >> >> >> Null pointers are a significant source of problems in
>>> >> >> >> applications.
>>> >> >> >> Whether it’s SIGSEGV taking down a process or a foolhardy
>>> >> >> >> attempt to
>>> >> >> >> recover
>>> >> >> >> from NullPointerException breaking invariants everywhere, it’s a
>>> >> >> >> problem
>>> >> >> >> that’s bad enough for Tony Hoare to call the invention of the
>>> >> >> >> null
>>> >> >> >> reference
>>> >> >> >> his billion dollar mistake [1]. It’s not the ability to create a
>>> >> >> >> null
>>> >> >> >> pointer that is a problem—having a common sentinel value meaning
>>> >> >> >> “no
>>> >> >> >> value”
>>> >> >> >> is extremely useful—but that it’s very hard to determine
>>> >> >> >> whether,
>>> >> >> >> for a
>>> >> >> >> particular pointer, one is expected to be able to use null. C
>>> >> >> >> doesn’t
>>> >> >> >> distinguish between “nullable” and “nonnull” pointers, so we
>>> >> >> >> turn to
>>> >> >> >> documentation and experimentation. Consider strchr from the C
>>> >> >> >> standard
>>> >> >> >> library:
>>> >> >> >>
>>> >> >> >> char *strchr(const char *s, int c);
>>> >> >> >>
>>> >> >> >> It is “obvious” to a programmer who knows the semantics of
>>> >> >> >> strchr
>>> >> >> >> that
>>> >> >> >> it’s important to check for a returned null, because null is
>>> >> >> >> used as
>>> >> >> >> the
>>> >> >> >> sentinel for “not found”. Of course, your tools don’t know that,
>>> >> >> >> so
>>> >> >> >> they
>>> >> >> >> cannot help when you completely forget to check for the null
>>> >> >> >> case.
>>> >> >> >> Bugs
>>> >> >> >> ensue.
>>> >> >> >>
>>> >> >> >> Can I pass a null string to strchr? The standard is unclear [2],
>>> >> >> >> and
>>> >> >> >> my
>>> >> >> >> platform’s implementation happily accepts a null parameter and
>>> >> >> >> returns
>>> >> >> >> null,
>>> >> >> >> so obviously I shouldn’t worry about it… until I port my code,
>>> >> >> >> or
>>> >> >> >> the
>>> >> >> >> underlying implementation changes because my expectations and
>>> >> >> >> the
>>> >> >> >> library
>>> >> >> >> implementor’s expectations differ. Given the age of strchr, I
>>> >> >> >> suspect
>>> >> >> >> that
>>> >> >> >> every implementation out there has an explicit, defensive check
>>> >> >> >> for
>>> >> >> >> a
>>> >> >> >> null
>>> >> >> >> string, because it’s easier to add yet more defensive (and
>>> >> >> >> generally
>>> >> >> >> useless) null checks than it is to ask your clients to fix their
>>> >> >> >> code.
>>> >> >> >> Scale
>>> >> >> >> this up, and code bloat ensues, as well as wasted programmer
>>> >> >> >> effort
>>> >> >> >> that
>>> >> >> >> obscures the places where checking for null really does matter.
>>> >> >> >>
>>> >> >> >> In a recent version of Xcode, Apple introduced an extension to
>>> >> >> >> C/C++/Objective-C that expresses the nullability of pointers in
>>> >> >> >> the
>>> >> >> >> type
>>> >> >> >> system via new nullability qualifiers . Nullability qualifiers
>>> >> >> >> express
>>> >> >> >> nullability as part of the declaration of strchr  [2]:
>>> >> >> >>
>>> >> >> >> __nullable char *strchr(__nonnull const char *s, int c);
>>> >> >> >>
>>> >> >> >> With this, programmers and tools alike can better reason about
>>> >> >> >> the
>>> >> >> >> use
>>> >> >> >> of
>>> >> >> >> strchr with null pointers.
>>> >> >> >>
>>> >> >> >> We’d like to contribute the implementation (and there is a patch
>>> >> >> >> attached
>>> >> >> >> at the end [3]), but since this is a nontrivial extension to all
>>> >> >> >> of
>>> >> >> >> the
>>> >> >> >> C
>>> >> >> >> family of languages that Clang supports, we believe that it
>>> >> >> >> needs to
>>> >> >> >> be
>>> >> >> >> discussed here first.
>>> >> >> >>
>>> >> >> >> Goals
>>> >> >> >> We have several specific goals that informed the design of this
>>> >> >> >> feature.
>>> >> >> >>
>>> >> >> >> Allow the intended nullability to be expressed on all pointers:
>>> >> >> >> Pointers
>>> >> >> >> are used throughout library interfaces, and the nullability of
>>> >> >> >> those
>>> >> >> >> pointers is an important part of the API contract with users.
>>> >> >> >> It’s
>>> >> >> >> too
>>> >> >> >> simplistic to only allow function parameters to have
>>> >> >> >> nullability,
>>> >> >> >> for
>>> >> >> >> example, because it’s also important information for data
>>> >> >> >> members,
>>> >> >> >> pointers-to-pointers (e.g., "a nonnull pointer to a nullable
>>> >> >> >> pointer
>>> >> >> >> to
>>> >> >> >> an
>>> >> >> >> integer”), arrays of pointers, etc.
>>> >> >> >> Enable better tools support for detecting nullability problems:
>>> >> >> >> The
>>> >> >> >> nullability annotations should be useful for tools (especially
>>> >> >> >> the
>>> >> >> >> static
>>> >> >> >> analyzer) that can reason about the use of null, to give
>>> >> >> >> warnings
>>> >> >> >> about
>>> >> >> >> both
>>> >> >> >> missed null checks (the result of strchr could be null…) as well
>>> >> >> >> as
>>> >> >> >> for
>>> >> >> >> unnecessarily-defensive code.
>>> >> >> >> Support workflows where all interfaces provide nullability
>>> >> >> >> annotations:
>>> >> >> >> In
>>> >> >> >> moving from a world where there are no nullability annotations
>>> >> >> >> to
>>> >> >> >> one
>>> >> >> >> where
>>> >> >> >> we hope to see many such annotations, we’ve found it helpful to
>>> >> >> >> move
>>> >> >> >> header-by-header, auditing a complete header to give it
>>> >> >> >> nullability
>>> >> >> >> qualifiers. Once one has done that, additions to the header need
>>> >> >> >> to
>>> >> >> >> be
>>> >> >> >> held
>>> >> >> >> to the same standard, so we need a design that allows us to warn
>>> >> >> >> about
>>> >> >> >> pointers that don’t provide nullability annotations for some
>>> >> >> >> declarations in
>>> >> >> >> a header that already has some nullability annotations.
>>> >> >> >>
>>> >> >> >> Zero effect on ABI or code generation: There are a huge number
>>> >> >> >> of
>>> >> >> >> interfaces that could benefit from the use of nullability
>>> >> >> >> qualifiers,
>>> >> >> >> but we
>>> >> >> >> won’t get widespread adoption if introducing the nullability
>>> >> >> >> qualifiers
>>> >> >> >> means breaking existing code, either in the ABI (say, because
>>> >> >> >> nullability
>>> >> >> >> qualifiers are mangled into the type) or at execution time
>>> >> >> >> (e.g.,
>>> >> >> >> because a
>>> >> >> >> non-null pointer ends up being null along some error path and
>>> >> >> >> causes
>>> >> >> >> undefined behavior).
>>> >> >> >>
>>> >> >> >>
>>> >> >> >>
>>> >> >> >>
>>> >> >> >> Why not __attribute__((nonnull))?
>>> >> >> >> Clang already has an attribute to express nullability,
>>> >> >> >> “nonnull”,
>>> >> >> >> which
>>> >> >> >> we
>>> >> >> >> inherited from GCC [4]. The “nonnull” attribute can be placed on
>>> >> >> >> functions
>>> >> >> >> to indicate which parameters cannot be null: one either
>>> >> >> >> specifies
>>> >> >> >> the
>>> >> >> >> indices of the arguments that cannot be null, e.g.,
>>> >> >> >>
>>> >> >> >> extern void *my_memcpy (void *dest, const void *src, size_t len)
>>> >> >> >> __attribute__((nonnull (1, 2)));
>>> >> >> >>
>>> >> >> >> or omits the list of indices to state that all pointer arguments
>>> >> >> >> cannot
>>> >> >> >> be
>>> >> >> >> null, e.g.,
>>> >> >> >>
>>> >> >> >> extern void *my_memcpy (void *dest, const void *src, size_t len)
>>> >> >> >> __attribute__((nonnull));
>>> >> >> >>
>>> >> >> >> More recently, “nonnull”  has grown the ability to be applied to
>>> >> >> >> parameters, and one can use the companion attribute
>>> >> >> >> returns_nonnull
>>> >> >> >> to
>>> >> >> >> state
>>> >> >> >> that a function returns a non-null pointer:
>>> >> >> >>
>>> >> >> >> extern void *my_memcpy (__attribute__((nonnull)) void *dest,
>>> >> >> >> __attribute__((nonnull)) const void *src, size_t len)
>>> >> >> >> __attribute__((returns_nonnull));
>>> >> >> >>
>>> >> >> >> There are a number of problems here. First, there are different
>>> >> >> >> attributes
>>> >> >> >> to express the same idea at different places in the grammar, and
>>> >> >> >> the
>>> >> >> >> use of
>>> >> >> >> the “nonnull” attribute on the function actually has an effect
>>> >> >> >> on
>>> >> >> >> the
>>> >> >> >> function parameters can get very, very confusing. Quick, which
>>> >> >> >> pointers
>>> >> >> >> are
>>> >> >> >> nullable vs. non-null in this example?
>>> >> >> >>
>>> >> >> >> __attribute__((nonnull)) void *my_realloc (void *ptr, size_t
>>> >> >> >> size);
>>> >> >> >>
>>> >> >> >> According to that declaration, ptr is nonnull and the function
>>> >> >> >> returns
>>> >> >> >> a
>>> >> >> >> nullable pointer… but that’s the opposite of how it reads (and
>>> >> >> >> behaves,
>>> >> >> >> if
>>> >> >> >> this is anything like a realloc that cannot fail). Moreover,
>>> >> >> >> because
>>> >> >> >> these
>>> >> >> >> two attributes are declaration attributes, not type attributes,
>>> >> >> >> you
>>> >> >> >> cannot
>>> >> >> >> express that nullability of the inner pointer in a multi-level
>>> >> >> >> pointer
>>> >> >> >> or an
>>> >> >> >> array of pointers, which makes these attributes verbose,
>>> >> >> >> confusing,
>>> >> >> >> and
>>> >> >> >> not
>>> >> >> >> sufficiently generally. These attributes fail the first of our
>>> >> >> >> goals.
>>> >> >> >>
>>> >> >> >> These attributes aren’t as useful as they could be for tools
>>> >> >> >> support
>>> >> >> >> (the
>>> >> >> >> second and third goals), because they only express the nonnull
>>> >> >> >> case,
>>> >> >> >> leaving
>>> >> >> >> no way to distinguish between the unannotated case (nobody has
>>> >> >> >> documented
>>> >> >> >> the nullability of some parameter) and the nullable case (we
>>> >> >> >> know
>>> >> >> >> the
>>> >> >> >> pointer can be null). From a tooling perspective, this is a
>>> >> >> >> killer:
>>> >> >> >> the
>>> >> >> >> static analyzer absolutely cannot warn that one has forgotten to
>>> >> >> >> check
>>> >> >> >> for
>>> >> >> >> null for every unannotated pointer, because the false-positive
>>> >> >> >> rate
>>> >> >> >> would be
>>> >> >> >> astronomical.
>>> >> >> >>
>>> >> >> >> Finally, we’ve recently started considering violations of the
>>> >> >> >> __attribute__((nonnull)) contract to be undefined behavior,
>>> >> >> >> which
>>> >> >> >> fails
>>> >> >> >> the
>>> >> >> >> last of our goals. This is something we could debate further if
>>> >> >> >> it
>>> >> >> >> were
>>> >> >> >> the
>>> >> >> >> only problem, but these declaration attributes fall all of our
>>> >> >> >> criteria, so
>>> >> >> >> it’s not worth discussing.
>>> >> >> >>
>>> >> >> >> Nullability Qualifiers
>>> >> >> >> We propose the addition of a new set of type qualifiers,
>>> >> >> >> spelled
>>> >> >> >> __nullable, __nonnull, and __null_unspecified, to Clang. These
>>> >> >> >> are
>>> >> >> >> collectively known as nullability qualifiers and may be written
>>> >> >> >> anywhere any
>>> >> >> >> other type qualifier may be written (such as const) on any type
>>> >> >> >> subject
>>> >> >> >> to
>>> >> >> >> the following restrictions:
>>> >> >> >>
>>> >> >> >> Two nullability qualifiers shall not appear in the same set of
>>> >> >> >> qualifiers.
>>> >> >> >> A nullability qualifier shall qualify any pointer type,
>>> >> >> >> including
>>> >> >> >> pointers
>>> >> >> >> to objects, pointers to functions, C++ pointers to members,
>>> >> >> >> block
>>> >> >> >> pointers,
>>> >> >> >> and Objective-C object pointers.
>>> >> >> >> A nullability qualifier in the declaration-specifiers applies to
>>> >> >> >> the
>>> >> >> >> innermost pointer type of each declarator (e.g., __nonnull int *
>>> >> >> >> is
>>> >> >> >> equivalent to int * __nonnull).
>>> >> >> >> A nullability qualifier applied to a typedef of a
>>> >> >> >> nullability-qualified
>>> >> >> >> pointer type shall specify the same nullability as the
>>> >> >> >> underlying
>>> >> >> >> type
>>> >> >> >> of
>>> >> >> >> the typedef.
>>> >> >> >>
>>> >> >> >>
>>> >> >> >> The meanings of the three nullability qualifiers are as follows:
>>> >> >> >>
>>> >> >> >> __nullable: the pointer may store a null value at runtime (as
>>> >> >> >> part
>>> >> >> >> of
>>> >> >> >> the
>>> >> >> >> API contract)
>>> >> >> >> __nonnull: the pointer should not store a null value at runtime
>>> >> >> >> (as
>>> >> >> >> part
>>> >> >> >> of the API contract). it is possible that the value can be null,
>>> >> >> >> e.g.,
>>> >> >> >> in
>>> >> >> >> erroneous historic uses of an API, and it is up to the library
>>> >> >> >> implementor
>>> >> >> >> to decide to what degree she will accommodate such clients.
>>> >> >> >> __null_unspecified: it is unclear whether the pointer can be
>>> >> >> >> null or
>>> >> >> >> not.
>>> >> >> >> Use of this type qualifier is extremely rare in practice, but it
>>> >> >> >> fills
>>> >> >> >> a
>>> >> >> >> small but important niche when auditing a particular header to
>>> >> >> >> add
>>> >> >> >> nullability qualifiers: sometimes the nullability contract for a
>>> >> >> >> few
>>> >> >> >> APIs in
>>> >> >> >> the header is unclear even when looking at the implementation
>>> >> >> >> for
>>> >> >> >> historical
>>> >> >> >> reasons, and establishing the contract requires more extensive
>>> >> >> >> study.
>>> >> >> >> In
>>> >> >> >> such cases, it’s often best to mark that pointer as
>>> >> >> >> __null_unspecified
>>> >> >> >> (which will help silence the warning about unannotated pointers
>>> >> >> >> in a
>>> >> >> >> header)
>>> >> >> >> and move on, coming back to __null_unspecified pointers when the
>>> >> >> >> appropriate
>>> >> >> >> graybeard has been summoned out of retirement [5].
>>> >> >> >>
>>> >> >> >> Assumes-nonnull Regions
>>> >> >> >> We’ve found that it's fairly common for the majority of pointers
>>> >> >> >> within
>>> >> >> >> a
>>> >> >> >> particular header to be __nonnull. Therefore, we’ve introduced
>>> >> >> >> assumes-nonnull regions that assume that certain unannotated
>>> >> >> >> pointers
>>> >> >> >> implicitly get the __nonnull nullability qualifiers.
>>> >> >> >> Assumes-nonnull
>>> >> >> >> regions
>>> >> >> >> are marked by pragmas:
>>> >> >> >>
>>> >> >> >> #pragma clang assume_nonnull begin
>>> >> >> >>         __nullable char *strchr(const char *s, int c); // s is
>>> >> >> >> inferred
>>> >> >> >> to
>>> >> >> >> be __nonnull
>>> >> >> >> void *my_realloc (__nullable void *ptr, size_t size); //
>>> >> >> >> my_realloc
>>> >> >> >> is
>>> >> >> >> inferred to return __nonnull
>>> >> >> >> #pragma clang assume_nonnull end
>>> >> >> >>
>>> >> >> >> We infer __nonnull within an assumes_nonnull region when:
>>> >> >> >>
>>> >> >> >> The pointer is a non-typedef declaration, such as a function
>>> >> >> >> parameter,
>>> >> >> >> variable, or data member, or the result type of a function. It’s
>>> >> >> >> very
>>> >> >> >> rare
>>> >> >> >> for one to warn typedefs to specify nullability information;
>>> >> >> >> rather,
>>> >> >> >> it’s
>>> >> >> >> usually the user of the typedef that needs to specify
>>> >> >> >> nullability.
>>> >> >> >> The pointer is a single-level pointer, e.g., int* but not int**,
>>> >> >> >> because
>>> >> >> >> we’ve found that programmers can get confused about the
>>> >> >> >> nullability
>>> >> >> >> of
>>> >> >> >> multi-level pointers (is it a __nullable pointer to __nonnull
>>> >> >> >> pointers,
>>> >> >> >> or
>>> >> >> >> the other way around?) and inferring nullability for any of the
>>> >> >> >> pointers in
>>> >> >> >> a multi-level pointer compounds the situation.
>>> >> >> >>
>>> >> >> >>
>>> >> >> >> Note that no #include may occur within an assumes_nonnull
>>> >> >> >> region,
>>> >> >> >> and
>>> >> >> >> assumes_nonnull regions cannot cross header boundaries.
>>> >> >> >>
>>> >> >> >> Type System Impact
>>> >> >> >> Nullability qualifiers are mapped to type attributes within the
>>> >> >> >> Clang
>>> >> >> >> type
>>> >> >> >> system, but a nullability-qualified pointer type is not
>>> >> >> >> semantically
>>> >> >> >> distinct from its unqualified pointer type. Therefore, one may
>>> >> >> >> freely
>>> >> >> >> convert between nullability-qualified and
>>> >> >> >> non-nullability-qualified
>>> >> >> >> pointers, or between nullability-qualified pointers with
>>> >> >> >> different
>>> >> >> >> nullability qualifiers. One cannot overload on nullability
>>> >> >> >> qualifiers,
>>> >> >> >> write
>>> >> >> >> C++ class template partial specializations that identify
>>> >> >> >> nullability
>>> >> >> >> qualifiers, or inspect nullability via type traits in any way.
>>> >> >> >>
>>> >> >> >> Said more strongly, removing nullability qualifiers from a
>>> >> >> >> well-formed
>>> >> >> >> program will not change its behavior in any way, nor will the
>>> >> >> >> semantics
>>> >> >> >> of a
>>> >> >> >> program change when any set of (well-formed) nullability
>>> >> >> >> qualifiers
>>> >> >> >> are
>>> >> >> >> added to it. Operationally, this means that nullability
>>> >> >> >> qualifiers
>>> >> >> >> are
>>> >> >> >> not
>>> >> >> >> part of the canonical type in Clang’s type system, and that any
>>> >> >> >> warnings we
>>> >> >> >> produce based on nullability information will necessarily be
>>> >> >> >> dependent
>>> >> >> >> on
>>> >> >> >> Clang’s ability to retain type sugar during semantic analysis.
>>> >> >> >>
>>> >> >> >> While it’s somewhat exceptional for us to introduce new type
>>> >> >> >> qualifiers
>>> >> >> >> that don’t produce semantically distinct types, we feel that
>>> >> >> >> this is
>>> >> >> >> the
>>> >> >> >> only plausible design and implementation strategy for this
>>> >> >> >> feature:
>>> >> >> >> pushing
>>> >> >> >> nullability qualifiers into the type system semantically would
>>> >> >> >> cause
>>> >> >> >> significant changes to the language (e.g., overloading, partial
>>> >> >> >> specialization) and break ABI (due to name mangling) that would
>>> >> >> >> drastically
>>> >> >> >> reduce the number of potential users, and we feel that Clang’s
>>> >> >> >> support
>>> >> >> >> for
>>> >> >> >> maintaining type sugar throughout semantic analysis is generally
>>> >> >> >> good
>>> >> >> >> enough
>>> >> >> >> [6] to get the benefits of nullability annotations in our tools.
>>> >> >> >>
>>> >> >> >> Looking forward to our discussion.
>>> >> >> >>
>>> >> >> >> - Doug (with Jordan Rose and Anna Zaks)
>>> >> >> >>
>>> >> >> >> [1]
>>> >> >> >>
>>> >> >> >> http://en.wikipedia.org/wiki/Tony_Hoare#Apologies_and_retractions
>>> >> >> >> [2] The standard description of strchr seems to imply that the
>>> >> >> >> parameter
>>> >> >> >> cannot be null
>>> >> >> >> [3] The patch is complete, but should be reviewed on cfe-commits
>>> >> >> >> rather
>>> >> >> >> than here. There are also several logic parts to this monolithic
>>> >> >> >> patch:
>>> >> >> >> (a) __nonnull/__nullable/__null_unspecified type specifiers
>>> >> >> >> (b) nonnull/nullable/null_unspecified syntactic sugar for
>>> >> >> >> Objective-C
>>> >> >> >> (c) Warning about inconsistent application of nullability
>>> >> >> >> specifiers
>>> >> >> >> within a given header
>>> >> >> >> (d) assume_nonnnull begin/end pragmas
>>> >> >> >> (e) Objective-C null_resettable property attribute
>>> >> >> >> [4] https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html
>>> >> >> >> (search
>>> >> >> >> for “nonnull”)
>>> >> >> >> [5] No graybeards were harmed in the making of this feature.
>>> >> >> >> [6] Template instantiation is the notable exception here,
>>> >> >> >> because it
>>> >> >> >> always canonicalizes types.
>>> >> >> >>
>>> >> >> >> <nullability.patch>
>>> >> >> >> _______________________________________________
>>> >> >> >> cfe-dev mailing list
>>> >> >> >> cfe-dev at cs.uiuc.edu
>>> >> >> >> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>>> >> >> >>
>>> >> >> >>
>>> >> >> >>
>>> >> >> >> _______________________________________________
>>> >> >> >> cfe-dev mailing list
>>> >> >> >> cfe-dev at cs.uiuc.edu
>>> >> >> >> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>>> >> >> >>
>>> >> >> >
>>> >> >> >
>>> >> >> > _______________________________________________
>>> >> >> > cfe-dev mailing list
>>> >> >> > cfe-dev at cs.uiuc.edu
>>> >> >> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>>> >> >> >
>>> >> >
>>> >> >
>>> >
>>> >
>>
>>
>
>
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