The trunk version of clang fails to compile the following test:

struct Empty {};

struct Empty2 : Empty {};

struct Test : Empty2 {
  constexpr Test() {}
};

void test() {
  constexpr Test t;	
}

> clang++ --std=c++1y test.cpp
test.cpp:10:18: error: constexpr variable 't' must be initialized by a constant expression
  constexpr Test t;
                 ^
test.cpp:10:18: note: subobject of type 'Empty' is not initialized
1 error generated.