Quuxplusone added inline comments.
================
Comment at: docs/LanguageExtensions.rst:1096
+ equivalent to copying the underlying bytes and then dropping the source
object
+ on the floor.
* ``__is_destructible`` (MSVC 2013)
----------------
dblaikie wrote:
> Quuxplusone wrote:
> > rjmccall wrote:
> > > Quuxplusone wrote:
> > > > rjmccall wrote:
> > > > > Quuxplusone wrote:
> > > > > > rjmccall wrote:
> > > > > > > Quuxplusone wrote:
> > > > > > > > rjmccall wrote:
> > > > > > > > > Quuxplusone wrote:
> > > > > > > > > > @rjmccall wrote:
> > > > > > > > > > > trivial_abi permits annotated types to be passed and
> > > > > > > > > > > returned in registers, which is ABI-breaking. Skimming
> > > > > > > > > > > the blog post, it looks like trivially_relocatable does
> > > > > > > > > > > not permit this — it merely signifies that destruction is
> > > > > > > > > > > a no-op after a move construction or assignment.
> > > > > > > > > >
> > > > > > > > > > Not necessarily a "no-op"; my canonical example is a
> > > > > > > > > > CopyOnlyCXX03SharedPtr which increments a refcount on
> > > > > > > > > > construction and decrements on destruction. But
> > > > > > > > > > move-construction plus destruction should "balance out" and
> > > > > > > > > > result in no observable side effects.
> > > > > > > > > >
> > > > > > > > > > > This is usefully different in the design space, since it
> > > > > > > > > > > means you can safely add the attribute retroactively to
> > > > > > > > > > > e.g. std::unique_ptr, and other templates can then detect
> > > > > > > > > > > that std::unique_ptr is trivially-relocatable and
> > > > > > > > > > > optimize themselves to use memcpy or realloc or whatever
> > > > > > > > > > > it is that they want to do. So in that sense trivial_abi
> > > > > > > > > > > is a *stronger* attribute, not a *weaker* one: the
> > > > > > > > > > > property it determines ought to imply
> > > > > > > > > > > trivially_relocatable.
> > > > > > > > > >
> > > > > > > > > > `trivial_abi` is an "orthogonal" attribute: you can have
> > > > > > > > > > `trivial_abi` types with non-trivial constructors and
> > > > > > > > > > destructors, which can have observable side effects. For
> > > > > > > > > > example,
> > > > > > > > > > ```
> > > > > > > > > > struct [[clang::trivial_abi]] DestructionAnnouncer {
> > > > > > > > > > ~DestructionAnnouncer() { puts("hello!"); }
> > > > > > > > > > };
> > > > > > > > > > ```
> > > > > > > > > > is `trivial_abi` (because of the annotation) yet not
> > > > > > > > > > trivially relocatable, because its "move plus destroy"
> > > > > > > > > > operation has observable side effects.
> > > > > > > > > >
> > > > > > > > > > > The only interesting question in the language design that
> > > > > > > > > > > I know of is what happens if you put the attribute on a
> > > > > > > > > > > template that's instantiated to contain a sub-object that
> > > > > > > > > > > is definitely not trivially relocatable / trivial-ABI.
> > > > > > > > > > > For trivial_abi, we decided that the attribute is simply
> > > > > > > > > > > ignored — it implicitly only applies to specializations
> > > > > > > > > > > where the attribute would be legal. I haven't dug into
> > > > > > > > > > > the design enough to know what trivially_relocatable
> > > > > > > > > > > decides in this situation, but the three basic options
> > > > > > > > > > > are:
> > > > > > > > > > >
> > > > > > > > > > > - the attribute always has effect and allows trivial
> > > > > > > > > > > relocation regardless of the subobject types; this is
> > > > > > > > > > > obviously unsafe, so it limits the safe applicability of
> > > > > > > > > > > the attribute to templates
> > > > > > > > > > > - the attribute is ignored, like trivial_abi is
> > > > > > > > > > > - the attribute is ill-formed, and you'll need to add a
> > > > > > > > > > > [[trivially_relocatable(bool)]] version to support
> > > > > > > > > > > templates
> > > > > > > > > >
> > > > > > > > > > What happens is basically the first thing you said, except
> > > > > > > > > > that I disagree that it's "obviously unsafe." Right now,
> > > > > > > > > > conditionally trivial relocation is possible via template
> > > > > > > > > > metaprogramming; see the libcxx patch at e.g.
> > > > > > > > > > https://github.com/Quuxplusone/libcxx/commit/6524822c009e#diff-38adc80cec663f2f29c22e9ffc0de912
> > > > > > > > > > Since the attribute is an opt-in mechanism, it makes
> > > > > > > > > > perfect sense to me that if you put it on a class (or class
> > > > > > > > > > template), then it applies to the class, without any
> > > > > > > > > > further sanity-checking by the compiler. The compiler has
> > > > > > > > > > no reason to second-guess the programmer here.
> > > > > > > > > >
> > > > > > > > > > However, there's one more interesting case. Suppose the
> > > > > > > > > > programmer puts the attribute on a class that isn't
> > > > > > > > > > relocatable at all! (For example, the union case
> > > > > > > > > > @erichkeane mentioned, or a class type with a deleted
> > > > > > > > > > destructor.) In that case, this patch *does* give an
> > > > > > > > > > error... *unless* the class was produced by instantiating a
> > > > > > > > > > template, in which case we *don't* give an error, because
> > > > > > > > > > it's not the template-writer's fault.
> > > > > > > > > > https://p1144.godbolt.org/z/wSZPba
> > > > > > > > > > trivial_abi is an "orthogonal" attribute: you can have
> > > > > > > > > > trivial_abi types with non-trivial constructors and
> > > > > > > > > > destructors, which can have observable side effects.
> > > > > > > > >
> > > > > > > > > Let me cut this conversation short. `trivial_abi` is not
> > > > > > > > > such an old and widely-established attribute that we are
> > > > > > > > > unable to revise its definition. I am comfortable making the
> > > > > > > > > same semantic guarantees for `trivial_abi` that you're making
> > > > > > > > > for `trivially_relocatable`, because I think it is in the
> > > > > > > > > language's interest for `trivial_abi` to be strictly stronger
> > > > > > > > > than `trivially_relocatable`.
> > > > > > > > >
> > > > > > > > > > What happens is basically the first thing you said, except
> > > > > > > > > > that I disagree that it's "obviously unsafe."
> > > > > > > > >
> > > > > > > > > Under your semantics, the attribute is an unchecked assertion
> > > > > > > > > about all of a class's subobjects. A class template which
> > > > > > > > > fails to correctly apply the template metaprogramming trick
> > > > > > > > > to all of its dependently-typed subobjects — which can be
> > > > > > > > > quite awkward because it creates an extra dimension of
> > > > > > > > > partial specialization, and which breaks ABI by adding extra
> > > > > > > > > template parameters — will be silently miscompiled to allow
> > > > > > > > > objects to be memcpy'ed when they're potentially not legal to
> > > > > > > > > memcpy. That is a footgun, and it is indeed "obviously
> > > > > > > > > unsafe".
> > > > > > > > >
> > > > > > > > > Now, it's fair to say that it's unsafe in a useful way:
> > > > > > > > > because the attribute isn't checked, you can wrap a type you
> > > > > > > > > don't control in a `trivially_relocatable` struct and thereby
> > > > > > > > > get the advantages of triviality on the wrapper. The model
> > > > > > > > > used by `trivial_abi` doesn't allow that. But I feel pretty
> > > > > > > > > strongly that that is not the right default behavior for the
> > > > > > > > > language.
> > > > > > > > > Under your semantics, the attribute is an unchecked assertion
> > > > > > > > > about all of a class's subobjects.
> > > > > > > >
> > > > > > > > The attribute is an unchecked assertion about the class's
> > > > > > > > //special member functions//. The attribute doesn't have
> > > > > > > > anything to do with subobjects, period.
> > > > > > > > Vice versa, the property currently expressed by
> > > > > > > > "IsNaturallyTriviallyRelocatable" is deduced from all of the
> > > > > > > > class's subobjects. The programmer can overrule the "natural"
> > > > > > > > property in an "unnatural" way by annotating their class with
> > > > > > > > the attribute.
> > > > > > > >
> > > > > > > > And we know this is true because it is possible to make a
> > > > > > > > trivially-relocatable class type containing
> > > > > > > > non-trivially-relocatable members (e.g. a class having a member
> > > > > > > > of type boost::interprocess::offset_ptr), and vice versa it is
> > > > > > > > possible to make a non-trivially-relocatable class containing
> > > > > > > > trivially-relocatable members (e.g.
> > > > > > > > boost::interprocess::offset_ptr itself, which has only one
> > > > > > > > member, of integral type).
> > > > > > > >
> > > > > > > > > A class template which fails to correctly apply the template
> > > > > > > > > metaprogramming trick to all of its dependently-typed
> > > > > > > > > subobjects — which can be quite awkward because it creates an
> > > > > > > > > extra dimension of partial specialization
> > > > > > > >
> > > > > > > > Agreed that it's awkward. The libc++ implementation was
> > > > > > > > awkward, but definitely not challenging. The only thing that
> > > > > > > > makes it at all tricky in the STL is that the STL allocator
> > > > > > > > model permits fancy "pointer" types that can make e.g.
> > > > > > > > std::vector non-trivially relocatable. If it weren't for fancy
> > > > > > > > pointers, you wouldn't need the extra dimension.
> > > > > > > >
> > > > > > > > > and which breaks ABI by adding extra template parameters
> > > > > > > >
> > > > > > > > The libc++ implementation does not break ABI. The extra
> > > > > > > > template parameter is concealed in a private base class.
> > > > > > > > https://github.com/Quuxplusone/libcxx/commit/6524822c009e#diff-38adc80cec663f2f29c22e9ffc0de912
> > > > > > > >
> > > > > > > > > I feel pretty strongly that that is not the right default
> > > > > > > > > behavior for the language.
> > > > > > > >
> > > > > > > > Can you elaborate on that feeling (maybe in private email)? My
> > > > > > > > intent with P1144 is that no industry programmer should ever
> > > > > > > > see this attribute; the right default for industry programmers
> > > > > > > > is to use the Rule of Zero. The reason we need the attribute is
> > > > > > > > as an opt-in mechanism for the implementor of `unique_ptr`,
> > > > > > > > `shared_ptr`, `vector`, and so on, //so that// the end-user can
> > > > > > > > just use the Rule of Zero and everything will work fine.
> > > > > > > > End-users shouldn't be messing with attributes.
> > > > > > > > And we know this is true because it is possible to make a
> > > > > > > > trivially-relocatable class type containing
> > > > > > > > non-trivially-relocatable members (e.g. a class having a member
> > > > > > > > of type boost::interprocess::offset_ptr), and vice versa it is
> > > > > > > > possible to make a non-trivially-relocatable class containing
> > > > > > > > trivially-relocatable members (e.g.
> > > > > > > > boost::interprocess::offset_ptr itself, which has only one
> > > > > > > > member, of integral type).
> > > > > > >
> > > > > > > Why would a class containing a member of type
> > > > > > > `boost::interprocess::offset_ptr` be trivially-relocatable? If
> > > > > > > you actually trivially relocate an object of the class, the
> > > > > > > pointer will not be rebased and so will be invalidated. It would
> > > > > > > have to be an `offset_ptr` where you happen to know that the
> > > > > > > referent will always be copied simultaneously, e.g. because it's
> > > > > > > a member of the object itself. Of course that's possible, but
> > > > > > > it's also such a corner case that we shouldn't balk at saying
> > > > > > > that the programmer ought to be more explicit about recognizing
> > > > > > > it.
> > > > > > >
> > > > > > > > Agreed that it's awkward. The libc++ implementation was
> > > > > > > > awkward, but definitely not challenging. The only thing that
> > > > > > > > makes it at all tricky in the STL is that the STL allocator
> > > > > > > > model permits fancy "pointer" types that can make e.g.
> > > > > > > > std::vector non-trivially relocatable. If it weren't for fancy
> > > > > > > > pointers, you wouldn't need the extra dimension.
> > > > > > >
> > > > > > > Sure. My point about the awkwardness is quite narrow: making the
> > > > > > > attribute take a `bool` argument is just a superior way of
> > > > > > > managing this over requiring a partial specialization. Several
> > > > > > > other language attributes have been heading in this same
> > > > > > > direction.
> > > > > > >
> > > > > > > > The libc++ implementation does not break ABI. The extra
> > > > > > > > template parameter is concealed in a private base class.
> > > > > > >
> > > > > > > Ah, apologies.
> > > > > > >
> > > > > > > > My intent with P1144 is that no industry programmer should
> > > > > > > > ever see this attribute; the right default for industry
> > > > > > > > programmers is to use the Rule of Zero. ... End-users
> > > > > > > > shouldn't be messing with attributes.
> > > > > > >
> > > > > > > Neither of these statements matches my experience. This is an
> > > > > > > "expert" feature to be sure, but the C++ community is full of
> > > > > > > experts who write their own rule-of-five types and who will
> > > > > > > happily use whatever attributes are available to them to make
> > > > > > > them faster.
> > > > > > >
> > > > > > > Also, I assume you are intending for this attribute to be
> > > > > > > standardized eventually, which will greatly expand its reach.
> > > > > > > Why would a class containing a member of type
> > > > > > > `boost::interprocess::offset_ptr` be trivially-relocatable? If
> > > > > > > you actually trivially relocate an object of the class, the
> > > > > > > pointer will not be rebased and so will be invalidated. It would
> > > > > > > have to be an offset_ptr where you happen to know that the
> > > > > > > referent will always be copied simultaneously, e.g. because it's
> > > > > > > a member of the object itself.
> > > > > >
> > > > > > Exactly! (And to preserve the class invariant, you'd have to add a
> > > > > > copy-constructor.)
> > > > > >
> > > > > > > Of course that's possible, but it's also such a corner case that
> > > > > > > we shouldn't balk at saying that the programmer ought to be more
> > > > > > > explicit about recognizing it.
> > > > > >
> > > > > > Exactly — and the way for the programmer to explicitly recognize
> > > > > > (or I say "warrant") that their class has the property is for them
> > > > > > to annotate it with `[[trivially_relocatable]]`. So I guess maybe I
> > > > > > don't understand what you mean by "more explicit"?
> > > > > >
> > > > > > > making the attribute take a `bool` argument is just a superior
> > > > > > > way of managing this
> > > > > >
> > > > > > That's possible, but it's also possible that it would increase the
> > > > > > complexity of parsing attributes for some implementations. I mean,
> > > > > > we're talking about something like the following, right? (Using the
> > > > > > libc++ patch as the example, but I've de-uglified some of the
> > > > > > names.) So I think it's a tradeoff and I'm ambivalent about it, so
> > > > > > far. (This is one of the [[
> > > > > > https://quuxplusone.github.io/blog/2018/11/11/trivially-relocatable-in-san-diego/#if-you-feel-comfortable-respondi
> > > > > > | straw poll questions in P1144R0 ]].)
> > > > > > ```
> > > > > > template <class T, class A = allocator<T>>
> > > > > > class [[trivially_relocatable(__deque_base<T,
> > > > > > A>::__allow_trivial_relocation::value)]] deque
> > > > > > : private __deque_base<T, A>
> > > > > > ```
> > > > > >
> > > > > > > This is an "expert" feature to be sure, but the C++ community is
> > > > > > > full of experts who write their own rule-of-five types and who
> > > > > > > will happily use whatever attributes are available to them to
> > > > > > > make them faster.
> > > > > >
> > > > > > Agreed. But the C++ community is //also// full of working
> > > > > > programmers who just write simple code with strings and vectors. :)
> > > > > > I want `[[trivially_relocatable]]` to be approximately as
> > > > > > frequently seen in real codebases as `[[no_unique_address]]` — i.e.
> > > > > > maybe a couple times in that smart-pointer library the contractor
> > > > > > wrote, but nowhere near the user code. If it's seen frequently in
> > > > > > user code, then we've failed those users.
> > > > > > Exactly! (And to preserve the class invariant, you'd have to add a
> > > > > > copy-constructor.)
> > > > >
> > > > > But then it still wouldn't be trivially relocatable, because there's
> > > > > user-defined code that has to run to copy it correctly. The only way
> > > > > such a type could ever be meaningfully trivially relocatable outside
> > > > > of obviously unknowable external conditions is if it has fields that
> > > > > it never uses after it's been relocated.
> > > > >
> > > > > > Exactly — and the way for the programmer to explicitly recognize
> > > > > > (or I say "warrant") that their class has the property is for them
> > > > > > to annotate it with [[trivially_relocatable]]. So I guess maybe I
> > > > > > don't understand what you mean by "more explicit"?
> > > > >
> > > > > I think it is far more likely that some well-intentioned library
> > > > > author will add `[[trivially_relocatable]]` incorrectly than that
> > > > > they'll actually intend to override the trivial relocatability of
> > > > > their subobjects.
> > > > >
> > > > > By "more explicit", I was suggesting that you add some kind of
> > > > > "force" syntax to the attribute (straw-man suggestion:
> > > > > `[[trivially_relocatable!]]`). Without the force, the attribute will
> > > > > negate non-triviality from special members in the class but won't
> > > > > override natural non-triviality from subobjects.
> > > > >
> > > > > > That's possible, but it's also possible that it would increase the
> > > > > > complexity of parsing attributes for some implementations.
> > > > >
> > > > > All conforming implementations have to do the work to support things
> > > > > like this already because of `alignas`, `noexcept`, etc.
> > > > >
> > > > > > Agreed. But the C++ community is also full of working programmers
> > > > > > who just write simple code with strings and vectors. :) I want
> > > > > > [[trivially_relocatable]] to be approximately as frequently seen in
> > > > > > real codebases as [[no_unique_address]] — i.e. maybe a couple times
> > > > > > in that smart-pointer library the contractor wrote, but nowhere
> > > > > > near the user code. If it's seen frequently in user code, then
> > > > > > we've failed those users.
> > > > >
> > > > > I think you are underestimating the sophistication of "working
> > > > > programmers" and overestimating the sophistication of library
> > > > > developers. A `[[trivially_relocatable]]` that doesn't override
> > > > > subobject triviality is far easier for library authors to use
> > > > > correctly and will avoid an endless cascade of oversights.
> > > > >
> > > > > Case in point, every single subtle thing that you had to anticipate
> > > > > and call out in your patch to `std::deque` would just go away if the
> > > > > language simply propagated non-triviality of subobjects.
> > > > > I think it is far more likely that some well-intentioned library
> > > > > author will add `[[trivially_relocatable]]` incorrectly than that
> > > > > they'll actually intend to override the trivial relocatability of
> > > > > their subobjects.
> > > >
> > > > But overriding "natural" non-trivial relocatability is precisely the
> > > > reason for P1144 `[[trivially_relocatable]]`! If you just have a plain
> > > > old Rule-of-Zero object with trivially relocatable subobjects, and you
> > > > want your object to be trivially relocatable as a result, the core
> > > > language takes care of that for you (just like with trivial
> > > > {con,de}structibility and trivial-abi-ness: a Rule-of-Zero composite of
> > > > trivial objects is itself trivial). The //only// use-case for the
> > > > `[[trivially_relocatable]]` attribute is when you are trying to tell
> > > > the compiler that you know exactly what you're doing. (Which is why
> > > > normal working programmers won't generally use it.)
> > > >
> > > > > By "more explicit", I was suggesting that you add some kind of
> > > > > "force" syntax to the attribute (straw-man suggestion:
> > > > > `[[trivially_relocatable!]]`). Without the force, the attribute will
> > > > > negate non-triviality from special members in the class but won't
> > > > > override natural non-triviality from subobjects.
> > > >
> > > > Can you give an example of how it would work with `deque`, for example?
> > > > What part of the `deque` implementation would become simpler, in
> > > > exchange for this added complexity of specification?
> > > >
> > > > You say:
> > > > > Case in point, every single subtle thing that you had to anticipate
> > > > > and call out in your patch to `std::deque` would just go away if the
> > > > > language simply propagated non-triviality of subobjects.
> > > >
> > > > But I don't understand what you mean by this. Are you saying that you
> > > > want to be able to write `class [[trivially_relocatable]] deque { ...
> > > > }` to mean that you want the trivially-relocatable-ness of `deque` to
> > > > match the trivially-relocatable-ness of its least relocatable
> > > > subobject? But then for example in the libc++ patch, I'd either have to
> > > > go out of my way to make sure that `__map` and `__deque_base` were
> > > > trivially relocatable (which would require metaprogramming similar to
> > > > what's there now, except one turtle lower down in the stack — or were
> > > > you thinking of adding `[[trivially_relocatable]]` to //all// the
> > > > turtles in the stack?), or else I'd have to use `class
> > > > [[trivially_relocatable!]] deque` to overrule the
> > > > non-trivial-relocatability of `deque`'s `__map` and `__deque_base`
> > > > subobjects, in which case I'd still need the four lines you were trying
> > > > to eliminate.
> > > > https://github.com/Quuxplusone/libcxx/commit/6524822c009e#diff-38adc80cec663f2f29c22e9ffc0de912R957
> > > >
> > > > My kneejerk reaction is that it is a bad idea to have two
> > > > similarly-named things, one of which has subtly correct semantics and
> > > > the other of which has subtly incorrect semantics, especially when it's
> > > > not obvious which one is correct in any specific situation. (OT: This
> > > > is basically the reason behind my [[
> > > > http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1155r0.html |
> > > > P1155 ]].) But even beyond that //general// reaction, I
> > > > //specifically// have not understood how `[[trivially_relocatable!]]`
> > > > would help the programmer of `deque`.
> > > >
> > > > > All conforming implementations have to do the work to support things
> > > > > like this already because of alignas, noexcept, etc.
> > > >
> > > > Yes, but those aren't attributes, grammatically... Well, I guess there
> > > > is already a standard grammar for parsing unknown attributes in terms
> > > > of `balanced-token-seq`, and I can't think of any boolean expression
> > > > that is not a `balanced-token-seq`, so okay, I'll retract my FUD over
> > > > the //technical// difficulties of `[[trivially_relocatable(bool)]]`. I
> > > > am still ambivalent as to whether it'd be a good tradeoff. (Complexity
> > > > of specification and arcane terseness of code, versus simplicity of
> > > > specification and boilerplate verbosity of code.)
> > > > But then for example in the libc++ patch, I'd either have to go out of
> > > > my way to make sure that __map and __deque_base were trivially
> > > > relocatable (which would require metaprogramming similar to what's
> > > > there now, except one turtle lower down in the stack — or were you
> > > > thinking of adding [[trivially_relocatable]] to all the turtles in the
> > > > stack?),
> > >
> > > You would need the attribute only at the level(s) that actually defined
> > > the special members; all the "rule of zero" levels would of course
> > > propagate trivial relocatability.
> > >
> > > > I specifically have not understood how [[trivially_relocatable!]] would
> > > > help the programmer of deque.
> > >
> > > It wouldn't. I don't think it's worth adding at all, actually. I'm just
> > > saying it's possible to add it if you really think that
> > > "trivially-relocatable type with a non-trivially-relocatable subobject"
> > > is a relevant use case.
> > >
> > > > Yes, but those aren't attributes, grammatically...
> > >
> > > True, `alignas` is not spelled as an attribute (which I constantly forget
> > > — it was a very odd decision).
> > >
> > > Recent drafts of the standard do include `expects` and `ensures`, which
> > > take arbitrary expressions as operands. The spelling's a bit different
> > > from the strawman I suggested, though: it's `[[expects: x > 0]]`.
> > >
> > > > I can't think of any boolean expression that is not a balanced-token-seq
> > >
> > > That's not a coincidence: all expressions are `balanced-token-seq`s, as
> > > is every other major production in the C++ grammar. The grammar allows a
> > > `balanced-token-seq` there precisely because the committee was
> > > anticipating attributes that take arbitrarily complex expressions as
> > > operands, which are quite common in all the vendor extensions they were
> > > looking at.
> > >
> > > > I am still ambivalent as to whether it'd be a good tradeoff.
> > > > (Complexity of specification and arcane terseness of code, versus
> > > > simplicity of specification and boilerplate verbosity of code.)
> > >
> > > My two pieces of feedback are separable. Even with your preferred
> > > semantics, your proposal would be much better if the attribute followed a
> > > `noexcept`-like design where it can optionally take a boolean argument.
> > > In fact, adding that is much more important with your semantics for the
> > > attribute since so many use sites will be conditional.
> > >
> > > Also, I don't think your semantics actually give rise to a simpler
> > > specification. The specification has to formalize "naturally trivially
> > > relocatable" in either case because it's important for rule-of-zero types
> > > to be trivially relocatable if all their subobjects are. My suggested
> > > semantics are basically just that the attribute doesn't override the
> > > natural relocatability of the type; it only prevents the presence of
> > > non-trivial special members from changing it. That's a short paragraph
> > > in the spec in exchange for an attribute that's much less prone to errors
> > > in corner cases (e.g. ignoring the possibility of non-trivial pointer
> > > types) and which rarely needs to be explicitly conditionalized. But it
> > > does make the (unsafe) override case impossible, although, again, I think
> > > the motivation for that is pretty weak.
> > > I don't think your semantics actually give rise to a simpler
> > > specification. The specification has to formalize "naturally trivially
> > > relocatable" in either case
> >
> > I think the spec is already simple: [[
> > http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1144r0.html#wording-inheritance
> > | P1144R0 section 4.4 ]]. The paper spec doesn't need to formalize
> > "naturally trivially relocatable"; that's an artifact of the Clang
> > implementation.
> >
> > > My suggested semantics are basically just that the attribute doesn't
> > > override the natural relocatability of the type; it only prevents the
> > > presence of non-trivial special members from changing it.
> >
> > In the Clang patch, I'm //currently// using "naturally trivially
> > relocatable" to mean "has defaulted special members //and// all subobjects
> > are trivially relocatable." (So when I say "Xly Yly Zable," it implies "Yly
> > Zable" as well as "Zable".) IIUC, you've been using it to mean "all
> > subobjects are trivially relocatable but maybe there are user-provided
> > special members"? I.e., if I had a class type with no data members, a
> > user-defined destructor, and no attribute, you would have considered that
> > class type to be "naturally trivially relocatable, but //not// trivially
> > relocatable"? ("Xly Yly Zable" wouldn't imply "Yly Zable"?)
> >
> > > if you really think that "trivially-relocatable type with a
> > > non-trivially-relocatable subobject" is a relevant use case
> >
> > Yes, that use-case is absolutely indispensable. For example, we must
> > provide a way for the programmer to express that `struct Widget {
> > boost::shared_ptr<Widget> sp; };` is trivially relocatable. (Without
> > upgrading our Boost distribution. ;)) In P1144R0, we can do that by putting
> > the attribute on `Widget`, or by putting the attribute on an "extremely
> > loud and incredibly narrow" wrapper class:
> > https://quuxplusone.github.io/blog/2018/10/04/trivially-relocatable-faq/#what-if-i-use-the-trivially_relo
> "Yes, that use-case is absolutely indispensable. For example, we must provide
> a way for the programmer to express that struct Widget {
> boost::shared_ptr<Widget> sp; }; is trivially relocatable. (Without upgrading
> our Boost distribution. ;)) "
>
> That's something I'd disagree with on first blush - why is this functionality
> absolutely indispensable?
>
> I'd be pretty concerned about people effectively annotating types they don't
> own with a guarantee they probably can't actually make - how's someone going
> to make the determination that a third party type is safe to (indirectyl) add
> this attribute to? & what if that type changes in the future (or has a debug
> mode, or something else the user hasn't anticipated) - and it only solves the
> cases where the third party type is embedded within a user type - which, for
> something like shared_ptr misses lots of cases where the shared_ptr itself is
> passed around.
>
> To me that doesn't seem like a "must have" feature - compared to the ability
> to annotate a type where I've written some user-defined special members, but
> I know/can guarantee (until I change the class in such a way that it doesn't
> have that feature - at which point I can remove the attribute too, because I
> control both implementation and attribution) that move+destroy is
> semantically equivalent to memcpy.
> > Yes, that use-case is absolutely indispensable. For example, we must
> > provide a way for the programmer to express that `struct Widget {
> > boost::shared_ptr<Widget> sp; };` is trivially relocatable. (Without
> > upgrading our Boost distribution. ;))
> That's something I'd disagree with on first blush - why is this functionality
> absolutely indispensable?
Because it's close to functionality that people are trying to use today (e.g.
Folly uses `FOLLY_ASSUME_RELOCATABLE` to apply trivial relocatability onto
non-annotated types such as `boost::shared_ptr` or `std::string`). Admittedly,
this is a source of bugs. But without it, I think people would just ask to have
it — because it's the thing that gives them the codegen they want.
Repository:
rC Clang
https://reviews.llvm.org/D50119
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