$ dmd -c memberfunptr.d 
memberfunptr.d(9): Error: cannot implicitly convert expression (__funcliteral1) of type void function(A) to invariant(void function(A))

$ cat memberfunptr.d 
==========================================
import std.stdio;

class A {
  void f() {writefln("f()");}
  void g() {writefln("g()");}
}
alias void function(A) FP;

invariant FP mfp1 = function void(A obj) {obj.f();};  // why this doesn't work?

// the following compiles, but ugly
static invariant FP mfp2;
static this() {
mfp2 = cast(invariant FP)(function void(A obj) {obj.g();});
}
==========================================

http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D&artnum=74330

Why a function literal is not a constant expression?

Especially after I defining a global top level wrapper function in this case.
Under the hood, it should just be a raw pointer to some memory address, why this
cannot be a constant?