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.
John.
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