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extern int const a;
 
struct A {
  constexpr A () {}
  int x { a };
};
 
int const a = 5;
 
int main () {
  static_assert (A {}.x == 5, "!!");
}

prog.cpp: In constructor ‘constexpr A::A()’:
prog.cpp:4:19: error: the value of ‘a’ is not usable in a constant expression
   constexpr A () {}
                   ^
prog.cpp:1:18: note: ‘a’ was not initialized with a constant expression
 extern int const a;
                  ^
prog.cpp: In function ‘int main()’:
prog.cpp:11:3: error: non-constant condition for static assertion
   static_assert (A {}.x == 5, "!!");
   ^
prog.cpp:11:3: error: ‘constexpr A::A()’ called in a constant expression

Standard output is empty