[cfe-dev] clang for Windows problem in Boost.TypeTraits

[Reid Kleckner]

On Tue, Oct 1, 2013 at 8:27 PM, Edward Diener <
eldlistmailingz at tropicsoft.com> wrote:

> On 10/1/2013 2:16 PM, Nico Rieck wrote:
>
>> On 30.09.2013 05:29, Edward Diener wrote:
>>
>>> I could not find anything in the C++11 standard about variadic functions
>>> and calling conventions. Is this a "bug" with clang under Windows or
>>> should Boost.TypeTraits not try to allow __fastcall in this situation
>>> when compiling with clang under Windows ?
>>>
>>
>> I think it's a defect in both. Boost should drop the overloaded vararg
>> is_function_ptr_tester specializations with non-standard calling
>> conventions as they are never used. When MSVC encounters a vararg
>> function with stdcall/fastcall it silently converts it to cdecl.
>>
>
> is there a way to test that this is what it actually does ?


First, compiling the code "void __fastcall foo() { }" produces the
symbol ?foo@@YAXZZ which demangles to 'void __cdecl foo(...)'.

Second, you can call it and verify that the assembly is adhering to the
usual C calling convention.


>  Though
>> for compatibility with (broken) MSVC code Clang should match that
>> behavior if desired.
>>
>
> The issue is that MSVC will accept the declaration without a compiler
> error as part of its extensions no matter what it may silently do. So if
> clang in Windows also defines the _MSC_EXTENSIONS macro it should ideally
> emulate MSVC in that regard.
>

I agree, this is a reasonable expectation, but I would classify this bug as
low priority, since variadic fastcall doesn't make sense.


> I will however mention in the Boost developers forum that it might be good
> to drop the overloaded vararg specializations with non-standard calling
> conventions for MSVC. Or Boost could decide to test for clang and not allow
> it when using clang.


Boost probably should drop this specialization.  It's actually declaring
the __cdecl specialization after the implicit adjustment.  When I attempt
to define a specialization for both 'R (__fastcall *)(...)' and 'R (__cdecl
*)(...)' in the same TU, I get a compiler error from MSVC:
t.cpp(7) : error C2995: 'void is_function_template(R (__cdecl *)(...))' :
function template has already been defined
        t.cpp(5) : see declaration of 'is_function_template'

extern "C" int printf(const char *fmt, ...);
template <typename T>
void is_function_template(T t) { printf("no\n"); }
template <typename R>
void is_function_template(R (__cdecl *fptr)(...)) { printf("cdecl\n"); }
template <typename R>
void is_function_template(R (__fastcall *fptr)(...)) {
printf("fastcall\n"); }
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