How to declare constexpr extern?

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no you can’t do it, here’s what the standard says (section 7.1.5):

1 The constexpr specifier shall be applied only to the definition of a
variable or variable template, the declaration of a function or
function template, or the declaration of a static data member of a
literal type (3.9). If any declaration of a function, function
template, or variable template has a constexpr specifier, then all its
declarations shall contain the constexpr specifier. [Note: An explicit
specialization can differ from the template declaration with respect
to the constexpr specifier. Function parameters cannot be declared
constexpr. — end note ]

some examples given by the standard:

  constexpr void square(int &x);  // OK: declaration
  constexpr int bufsz = 1024;  // OK: definition
  constexpr struct pixel {  // error: pixel is a type
    int x;
    int y;
    constexpr pixel(int);  // OK: declaration
  };

  extern constexpr int memsz; // error: not a definition

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