Unable to pass a D function member to a C callback

[Luh]

Hello,

When trying to pass a D function to the C callback, the compiler 
says:

'Error: cannot implicitly convert expression &this.onProcessCb of 
type extern (C) bool delegate(const(short*) a, ulong b, void* c) 
to extern (C) bool function(const(short*), ulong, void*'

because my function is member of a class (compiles when the 
function is out of the class).

Is there any way to say to solve this ?
The wiki isn't very clear about the C callbacks:
https://dlang.org/spec/interfaceToC.html#callbacks

C code:
----
typedef bool (*onProcessCallback)(const short*, size_t, void*);
----


D Code:
-----
class Game
{
	onProcessCallback m_onProcessCb;
	
	this()
	{
		m_onProcessCb = &onProcessCb; // Error here
	}
	
	void onProcess()
	{
		// ...
	}

	extern(C) bool onProcessCb(const short* a, size_t b, void* c)
	{
		onProcess();
		return true;
	}
}

private extern(C)
{
	// Should call onProcess() when executed by the C lib
	alias onProcessCallback = bool function(const short*, size_t, 
void*);
}
-----