error "function address cannot be converted to pointer to class member"

[Baron-von-Riedesel]

I get this error when trying to compile DosBox's FM synthesizer with OW. I stripped down the code to a relatively simple test case:

#include <stdio.h>

typedef enum {
	sm2AM,
	sm2FM,
	sm3AM,
	sm3FM,
} SynthMode;

struct channel;

typedef void ( channel::*SynthHandler) ( int );

struct channel {
	int i;
	SynthHandler synthHandler;

	template<SynthMode mode>
		void BlockTemplate( int );

	channel( int );
};

channel::channel(int mode)
{
	i = mode;
	printf("channel::channel mode=%u\n", mode );
	synthHandler = &channel::BlockTemplate< sm3FM >;
}

template<SynthMode mode>
void channel::BlockTemplate( int i1 )
{
	printf("channel::BlockTemplate, mode=%u, i1=%u\n", mode, i1 );
	switch ( mode ) {
	case sm2AM: break;
	case sm2FM: break;
	}
	return;
}


int main ( int arcc, char * *argv )
{
	channel *myobj = new channel(1);
	printf("obj=%p\n", myobj );
	myobj->i = 3;
	( myobj ->* myobj->channel::synthHandler)(0);
	return( 0 );
}

Feeding OW's wcc386 with it emits:

tmpl.cpp(29): Error! E512: col(24) function address cannot be converted to pointer to class member
tmpl.cpp(29): Note! N630: col(24) source conversion type is 'void (channel::* )( int )'
tmpl.cpp(29): Note! N631: col(24) target conversion type is 'void (channel::* )( int )'

Which is a bit strange since source and targer don't differ. I'm no C++ expert, but I ran this test case with gcc, OW, Borland C++ v5.5.1 and the MS VC++ Toolkit 2003 - and it's just OW that chokes.

Attached is the test case, with .BATs how I ran the various compilers

tmpl.zip

[jmalak]

My knowledge for C++ is also limited. The OW C++ compiler is using very old C++ standard C++98 that some limitation can caused this message. I hope @pchapin will comment this issue.

[pchapin]

I think it's a compiler bug. Template members are one of the most recent additions to the Open Watcom C++ compiler and have some other limitations. Also, the compiler does not allow one to explicitly specialize function templates (contrary to C++ 98). Frankly, I'm surprised the example works as well as it does! The compiler seems to understand that channel::BlockTemplate<sm3FM> is an explicit specialization of a template member function. While that's legal C++, I wouldn't have expected Open Watcom to understand that.

So, yes, the compiler is confused... but not as confused as I would have thought!