[patch] Don't drop attributes when checking explicit specializations.

[Reid Kleckner]

I think it's more interesting when the specialization lacks attributes:

    template <int>
    struct A { void __attribute__((visibility("hidden"))) foo() {} };
    template <> void A<0>::foo() {} // implicitly default visibility

    // compile with -fvisibility=hidden
    template <int>
    struct A { void __attribute__((visibility("default"))) foo() {} };
    template <> void A<0>::foo() {} // implicitly hidden visibility

I'm fine with this if there's no behavior change here.

On Thu, Dec 18, 2014 at 11:42 PM, Nico Weber <thakis at chromium.org> wrote:
>
> On Thu, Dec 18, 2014 at 9:58 PM, John McCall <rjmccall at apple.com> wrote:
>>
>> On Dec 18, 2014, at 6:18 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>> On Wed, Dec 17, 2014 at 6:58 PM, Nico Weber <thakis at chromium.org> wrote:
>>>
>>> Don't drop attributes when checking explicit specializations.
>>>
>>> Consider a template class with attributes on a method, and an explicit
>>> specialization of that method:
>>>
>>>     template <int>
>>>     struct A {
>>>       void foo() final;
>>>     };
>>>
>>>     template <>
>>>     void A<0>::foo() {}
>>>
>>> In this example, the attribute is `final`, but it might also be an
>>> __attribute__((visibility("foo"))), noreturn, inline, etc. clang's
>>> current
>>> behavior is to strip all attributes, which for some attributes is wrong
>>> (the snippet above allows a subclass of A<0> to override the final
>>> method, for
>>> example) and for others disagrees with gcc
>>> (__attribute__((visibility()))).
>>>
>>> So stop dropping attributes. r95845 added this code without a test case,
>>> and
>>> r176728 added the code for dropping attributes on parameters (with
>>> tests, but
>>> they still pass).
>>>
>>> As an additional wrinkle, do drop dllimport and dllexport, since that's
>>> how these two
>>> attributes work. (This is covered by existing tests.)
>>>
>>> Fixes PR21942.
>>>
>>> The approach is by Richard Smith, initial analysis and typing was done
>>> by me.
>>>
>>
>> Naturally, this makes sense to me =) It also matches GCC and EDG on all
>> attributes I tested.
>>
>> John, do you remember why you added this code (a long long time ago)?
>>
>>
>> The behavior on attribute((visibility)) is intentional, I think.  At the
>> very least, you have to allow for explicit specializations to override the
>> visibility of the template, using a general most-specific-rule-wins logic.
>>
>
> In practice, the attribute((visibility)) behavior seems unchanged,
> probably because it's propagated across redeclarations. I tried building
> the following program and compared `nm -m` output:
>
>     template <int>
>     struct A { void __attribute__((visibility("hidden"))) foo() {} };
>     template <> void __attribute__((visibility("default"))) A<0>::foo() {}
>
>     void f() {
>       A<0> a0; A<1> a1;
>       a0.foo(); a1.foo();
>     }
>
> I toggled each of the two attributes on and off, the nm output with and
> without the patch looked identical.
>
> I'm also doing a component build of Chromium with the patch, so far things
> look good.
>
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