constexpr data member defined in .h is unresolved in .cpp only on -std=c++14 and below [duplicate]

The following only links successfully with -std=c++17 or above. -std=c++14 or below gives an unresolved external error:

In function `C::f()':
C.cpp:(.text+0x19): undefined reference to `C::kConstant' 

Why doesn't this work with the c++14 standard and does work with the c++17 standard? (tested with both GCC and Clang)

And why does the commented-out workaround work even on c++14?

C.h

#pragma once

class C {
public:
    static constexpr int kConstant = 10;
    int f();
};

C.cpp

#include "C.h"

#include <algorithm>

int C::f()
{
    // This only works with -std=c++17 or above
    return std::min (1, kConstant); 

    // This works even with -std=c++14
//    return std::min (1, static_cast<int> (kConstant));
}

Main.cpp

#include "C.h"

int main()
{
    C c;
    c.f();
}

Wandbox

Update: This question isn't an exact duplicate, but has an answer which answers most of my question. It doesn't answer why the workaround works with -std=c++14, though, and the original doesn't. Is it perhaps because in the original the variable is odr-used, and in the workaround it is not?