Unable to create static constexpr

Question

I have the following code:

struct Literal { int val; constexpr Literal(int const& val) : val(val) {} constexpr Literal(Literal const& rhs) : val(rhs.val) {} }; struct Parent { struct StaticObject { Literal const zero; constexpr StaticObject() :zero(0) {} }; static constexpr StaticObject outer{}; }; 

String 'static constexpr StaticObject outer {};' gives me an error:

'expression was not evaluated by a constant'

Followed by

Note: failure caused by calling undefined function or not declared "constexpr"

Note: see "Parent :: StaticObject :: StaticObject"

As far as I can tell, all the functions used here are defined and declared as constexpr. Am I missing something, or is it a compiler error?