Is the constexpr specifier required on the declaration of a constexpr static member initialized outside of the class?

Question

C++17 §10.1.5/1 states:

The constexpr specifier shall be applied only to the definition of a variable or variable template

Answer 1

If I were to read this:

static S const ZERO; // not marked `constexpr`, but still `const`

S::ZERO will never change its value during run-time due to const.

However:

constexpr S S::ZERO{ 0 }; // implicitly `inline` (if C++17) and `const`

Constant Evaluation is done for S::ZERO which will have constant integral value 0 for _value.
This invokes your constexpr constructor:

constexpr S(int value = {}) : _value{ value } {}

As per basic.start.static - Constant Initialization:

A constant initializer for a variable or temporary object o is an initializer whose full-expression is a constant expression, except that if o is an object, such an initializer may also invoke constexpr constructors for o and its subobjects even if those objects are of non-literal class types.

AND expr.const/8.7 - Constant Evaluation:

a variable whose name appears as a potentially constant evaluated expression that is either a constexpr variable or is of non-volatile const-qualified integral type or of reference type.

Therefore:

Is it a violation to have non-matching use of the constexpr specifier across variable declarations and definitions?

I believe your code is fine.