Template argument (pointer to member function) initialization with NULL

[vagran.ast]

On 11/14/2011 11:28 PM, Jonathan Wakely wrote:
> On 14 November 2011 20:22, vagran.ast wrote:
>> Hi,
>>
>> In the following code:
>>
>> class A {
>> public:
>>     void SomeMethod() { }
>> };
>>
>> template<class T, void (T::*SomeMethod)() = 0>
>> class B {
>>
>> };
>>
>> B<A>  b1; // error: could not convert template argument '0' to 'void
>> (A::*)()'
>>
>> B<A, 0>  b2; // error: could not convert template argument '0' to 'void
>> (A::*)()'
>>
>> void (A::*someMethod)() = 0; // OK
>>
>> there are two compilation errors. AFAIK per C++ standard 0 is a valid value
>> for a pointer to member
>> function. Variable of such type can be successfully initialized by 0 but
>> template arguments can not.
>> Is it desired behavior or a bug?
> I think G++ is correct, you need to cast the literal zero to the right
> type to prevent it being treated as an int in that context, or use
> C++11's nullptr.
I have tried both variants and still no luck:

B<A, static_cast<void (A::*)()>(0)> b2;
// error: '((void (A::*)())0)' is not a valid template argument for type 
'void (A::*)()'
// error: it must be a pointer-to-member of the form '&X::Y'
// error: could not convert template argument '((void (A::*)())0)' to 
'void (A::*)()'
// error: invalid type in declaration before ';' token

B<A, nullptr> b3; // error: could not convert template argument 
'nullptr' to 'void (A::*)()'

void (A::*someMethod)() = static_cast<void (A::*)()>(0);// OK
void (A::*someMethod2)() = nullptr;// OK

Seems the compiler wants only real method pointer for a template 
parameter however
the conversion from NULL to member function pointer is possible.