On Jan 27, 2014, at 3:22 PM, Richard Smith <[email protected]> wrote:
> On 27 January 2014 15:08, John McCall <[email protected]> wrote:
> On Jan 27, 2014, at 12:26 PM, Richard Smith <[email protected]> wrote:
>> On 27 January 2014 11:57, John McCall <[email protected]> wrote:
>> On Jan 24, 2014, at 7:30 PM, Nick Lewycky <[email protected]> wrote:
>>> On 24 January 2014 17:54, John McCall <[email protected]> wrote:
>>> On Jan 24, 2014, at 5:36 PM, Nick Lewycky <[email protected]> wrote:
>>>> On 23 January 2014 11:52, John McCall <[email protected]> wrote:
>>>> On Jan 21, 2014, at 2:01 PM, Richard Smith <[email protected]> wrote:
>>>>> On 21 January 2014 09:36, John McCall <[email protected]> wrote:
>>>>> On Jan 20, 2014, at 6:13 PM, Nick Lewycky <[email protected]> wrote:
>>>>> > I'm trying to mangle a vendor extension that encodes an expression
>>>>> > which applies to function overload candidates, with the goal that a
>>>>> > user running the demangler would see their expression demangled. While
>>>>> > there are various vendor extension points, none of them allow me to go
>>>>> > into encoding an expression, unless I stick a stray "decltype" in
>>>>> > there, or similar (expression in a template argument that doesn't
>>>>> > actually exist, etc.).
>>>>> >
>>>>> > The vendor extension is described in full here:
>>>>> > http://clang.llvm.org/docs/LanguageExtensions.html#controlling-overload-resolution
>>>>> > .
>>>>> >
>>>>> > I don't think I can do it without an extension to the mangling rules.
>>>>> > As a strawman proposal, I suggest this:
>>>>> >
>>>>> > <type> ::= Ue <expression> E # vendor extended type qualifier
>>>>>
>>>>> I think you mean
>>>>>
>>>>> <type> ::= Ue <source-name> <expression> E <type>
>>>>>
>>>>> And this would be valuable for mangling e.g. dependent address space
>>>>> qualifiers, if anybody ever wants to do those.
>>>>
>>>> Yep, that's what I meant. Thanks!
>>>>> We could generalize this slightly to
>>>>>
>>>>> <type> ::= U <source-name> <template-args> <type>
>>>>>
>>>>> to allow multiple arguments (as types or expressions), dependent pack
>>>>> expansions, and so on.
>>>>
>>>> That’s a bit more future-proof, I suppose, although I think it might
>>>> stretch the definition of a type-qualifier to embed anything other than a
>>>> value, and value pack-expansions are already part of the <expression>
>>>> grammar. You’re really asking for a “allow an arbitrarily complex type to
>>>> be manufactured here” mangling.
>>>>
>>>>> However, it feels really forced to add your feature, specifically, to
>>>>> <type>, since it can’t appear in an arbitrary position; it’s much closer
>>>>> to a qualified method name. But the ref/cv-qualifiers area is only
>>>>> allowed in a <nested-name>, and you need to be able to do this on a
>>>>> top-level function, so I suppose extending <type> in a known-useful
>>>>> direction and then abusing <type> might be the best thing.
>>>>>
>>>>> On the other hand, isn’t this a proposed direction for standardization?
>>>>> I would have no problem with giving this a proper, non-vendor mangling
>>>>> just in case.
>>>>>
>>>>> It's not proposed for standardization with this syntax, and it's likely
>>>>> that the final semantics will differ from the Clang extension in some
>>>>> ways (the proposed partial ordering rules for constraints are rather more
>>>>> complex, for instance), but it's close enough that it's an option worth
>>>>> considering.
>>>>
>>>> Unless the feature is likely to diverge so much that it won’t even be an
>>>> expression anymore, I don’t think this poses any problem for the ABI.
>>>>
>>>> Vendor hat on, I reserve the right to make my extension behave differently
>>>> from anything that's been standardized. As long as I can slip a vendor
>>>> extension particle into the mangled name I'll be happy to use otherwise
>>>> normal mangling. If it turns out I don't have to, all the better, but I'm
>>>> not banking on it.
>>>
>>> I completely agree that this is acceptable vendor-hat behavior and that the
>>> fake-qualifier idea isn’t a bad approach for it.
>>>
>>>> Do you want me to try to prepare a patch for template constraints? I think
>>>> it would look similar to my strawman proposal, except that I'd pick some
>>>> other available letter?
>>>
>>> Yes, except that grammatically you should make it part of the function
>>> <encoding> instead of adding it to <type>. It works out to the same basic
>>> position.
>>>
>>> Okay, first attempt at a patch attached. Please review.
>>>
>>> A couple things. I chose 'Q', short for 'requires' to indicate a
>>> constraint. I put the new part on all encodings, not just functions,
>>> because you may need to mangle a static data member inside a class that has
>>> a concept applied, and similarly for its vtable and special members.
>>
>> I’m confused about what you mean by ‘concept’ here. If it's just jargon for
>> the enable_if feature, it seems completely counterproductive.
>>
>> This is jargon for the proposed constraints feature, not for enable_if.
>>
>> We do not need to mangle enable_if conditions on class template patterns
>> because of the ODR (and related rules in the templates chapter).
>>
>> We don't even support enable_if conditions on class templates (+
>> specializations of them).
>
> Any particular reason?
>
> This is a feature for guiding overload resolution, not for anything directly
> template-related. We'd need to invent different semantics for it if we wanted
> to support it on class templates and partial specializations.
Okay.
> If you’ve got partial-ordering rules already, it seems like a very nice way
> to significantly improve the expressiveness of class template partial
> specialization. Right now, users who want to specialize a template on e.g.
> POD types have to factor that into the original template signature.
>
>> enable_if has to be mangled for a function template because it’s part of the
>> template signature. You can have two function template declarations in the
>> scope scope with the same name and with identical template parameter
>> signatures and type signatures, but if they have different enable_if
>> conditions, then they’re considered different templates. In typical use,
>> this isn’t actually important: two overlapping function templates are
>> probably defined in the same header file, and it’s likely that every call
>> site will see both of them, and so any given list of template arguments that
>> doesn’t lead to an ambiguity will probably pick the same template every
>> time. But that’s not actually *required*: it’s totally fine to have two
>> different function templates in two different translation units that both
>> accept the same template arguments, and we need to distinguish them.
>>
>> It's more complex than that. enable_if can look at the values of function
>> arguments at a call site (if they are constant expressions), not just at
>> template arguments, so we need to mangle it into the signature for all
>> functions and function templates, even in the single-TU case.
>
> What a majestic way of adding complexity without really making a significant
> improvement to expressiveness. Why not instead add a feature to generally
> infer a non-type template parameter from a constant argument?
>
> Primarily because it is intended to work in C as well as C++.
Ah, yes, I keep forgetting that, sorry.
John.
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