/*
* This returns a constant expressionn while determining if an argument is
* a constant expression, most importantly without evaluating the argument.
* Glory to Martin Uecker <Martin.Uecker@med.uni-goettingen.de>
*/
#define __is_constexpr(x) \
(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
I can try... In short, this is about how a C compiler is supposed to deduce the type of the ternary operation. So, if x is a constant expression, the compiler figures that the second operand must be NULL (because it is allowed to perform the multiplication, by 0l), whose type then is whatever the third operand says (a pointer to int). The comparison succeeds. Otherwise the compiler does not perform the multiplication, takes the type of the second operand at its “face value”, i.e. as a pointer to void, and converts the type of the third operand to it - which, of course, makes the comparison false in this case.
