Unexpected non-constant std::initializer_list

Question

I was toying a little bit with the indices trick to see where I could go to with and came across a strange error... First, the plain not-so-old indices:

temp         


        

Answer 1

From: the man himself http://www.stroustrup.com/sac10-constexpr.pdf

Specifically: its return type, and the types of its parameters (if any), are literal types (see x2.2). For concreteness, literal types include bool, int, or double; its body is a compound statement of the form { return expr; } where expr is such that if arbitrary constant expressions of appropriate types are substituted for the parameters in expr, then the resulting expression is a constant expression as defined in introductory paragraph of x2. The expression expr is called a potential constant expression.