Re: [boost] [winapi] Problem with the latest clang on Windows

Subject: Re: [boost] [winapi] Problem with the latest clang on Windows
From: Edward Diener (eldiener_at_[hidden])
Date: 2015-06-05 05:45:18

On 6/1/2015 6:57 PM, Andrey Semashev wrote:
> On Monday 01 June 2015 17:56:59 Edward Diener wrote:
>> On 6/1/2015 3:12 PM, Andrey Semashev wrote:
>>> On Monday 01 June 2015 11:23:36 Edward Diener wrote:
>>>> What does the C++ standard say about the extern "C" declarations
>>>> duplicating the same name as regular declarations but different types.
>>>> Is it an ODR violation ? Maybe that is what the clang problem is about
>>>> although gcc does not feel it is a problem seeing the exact same
>>>> preprocessed output.
>>>
>>> I could only find 7.5/6:
>>>     ...Two declarations for a function with C language linkage
>>>     with the same function name (ignoring the namespace names
>>>     that qualify it) that appear in different namespace scopes
>>>     refer to the same function....
>>>
>>> It doesn't say anything about argument types. ODR also doesn't apply here
>>> since we're talking about the function declaration, not its definition.
>>
>> OK, so just declaring a function with the same name but different
>> signature doesn't mean a compiler error. But when we invoke the function
>> with a signature that matches one of the declarations does the C++
>> standard say it is an error because another declaration with the same
>> name but a different signature exists ? Evidently clang thinks it is an
>> error but gcc does not.
>
> I didn't find any specifics on this (but I could be missing something). The
> extern "C" linkage affects overload resolution (13.3.2/3), so that if there
> are two viable extern "C" function declarations that are in different
> namespaces are visible at the call site, the two declarations do not conflict
> as they are considered to declare the one and only extern "C" function. But
> then again, the example does not consider the case when the two declarations
> differ in arguments. The standard also mentions that language-specific linkage
> may have other effects, such as special name mangling or calling conventions
> (7.5/1), but naturally, it doesn't go into specifics of these effects.
>
> Since other than the above there are no special treatment of extern "C"
> function calls from the language perspective, I would assume that as long as
> the call in unambiguous by the C++ overload resolution rules, the language
> should allow it.
>
> If clang folks say this should not compile then perhaps they could point to
> the relevant part of the standard.

The clang explanation, when asked to point to the C++ standard, is:

"[dcl.link]/6: "At most one function with a particular name can have C 
language linkage. Two declarations for a function with C language 
linkage with the same function name (ignoring the namespace names that 
qualify it) that appear in different namespace scopes refer to the same 
function."

[basic.link]/10: "the types specified by all declarations referring to a 
given variable or function shall be identical"

So the program is ill-formed because the same function is declared with 
two different types."