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I'm having trouble with the following code: template<typename T> constexpr int get(T vec) { return vec.get(); } struct coord { constexpr int get() const { return x; } int x; }; struct foo { struct coord2 { constexpr int get() const { return x; } int x; }; constexpr static coord f = { 5 }; constexpr static int g = get(f); // works constexpr static coord2 h = { 5 }; constexpr static int i = get(h); // doesn't work }; constexpr coord foo::f; constexpr foo::coord2 foo::h; int main(){} Essentially, get(f) is considered a constant expression, but get(h) is not. The only thing changed is that one uses a global struct coord , while the other uses a nested struct coord2 . The structs' bodies are identical. Why is this? GCC error: test.cpp:20:35: error: field initializer is not constant Clang error: test.cpp:20:26: error: constexpr variable 'i' must be initialized by a constant expression constexpr static int i = get(h); // doesn't work ^ ~~~~~~ test.cpp:8:10: note: undefined function 'get' cannot be used in a constant expression return vec.get(); ^ test.cpp:20:30: note: in call to 'get({5})' constexpr static int i = get(h); // doesn't work ^ test.cpp:13:21: note: declared here constexpr int get() const { return x; }

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