#########################################################################


struct X {
  int a;
};

constexpr int X::* mem = &X::a;

void foo() 
{
  constexpr X x = {1};
  constexpr int k = x.a;          // ok

  constexpr int l = x.*mem;       // should be ok       <------------------

  //constexpr int const* m = &(x.a);  // legitimate error
}


#########################################################################

g++-4.7 -std=c++0x bug.cpp -o g47.out
bug.cpp: In function ‘void foo()’:
bug.cpp:29:24: error: ‘(const int*)(& x)’ is not a constant expression

#########################################################################


As far as I can see, a pointer to member access should be a constant expresssion, iff both the object and the pointer are constant expressions. 
This behaviour is shown by the clang++ compiler, which does not emit the above error.


It seems gcc evalutates the expression 

         x.*mem

as something like

         *((const int*)(& x) + mem)

In this implementation, the left (intermediate) operand to the + operator is not a constant expression, that's why gcc rejects it by mistake.