关于声明和定义静态const数据成员的困惑

斯科特·迈耶斯（Scott Meyers）在《有效的现代C ++》第210页第30项中写道，

no need to define integral static const data members in classes; declarations alone suffice,

那么示例代码是

class Widget {
  public:
    static const std::size_t MinVals = 28; // MinVals' declaration;
    ...
};
...                                        // no defn. for MinVals
std::vector<int> widgetData;
widgetData.reserve(Widget::MinVals);       // use of MinVals

I was convinced that static const std::size_t MinVals = 28; is declaration and also a definition, as it is giving a value to MinVals, but the comments seems to claim that's only a declaration; the second comment actually claims there's no definition. The text after the code, indeed reads

MinVals lacks a definition.

Which confirms that static const std::size_t MinVals = 28; is not a definition, so I'm a bit confused.

cppreference doesn't help me much (my bold-italic):

If a static data member of integral or enumeration type is declared const (and not volatile), it can be initialized with an initializer in which every expression is a constant expression, right inside the class definition:

struct X
{
    const static int n = 1;
    const static int m{2}; // since C++11
    const static int k;
};
const int X::k = 3;

但是该课程的前两行对我来说是定义。

以下关于cppreference的示例也是如此：

struct X {
    static const int n = 1;
    static constexpr int m = 4;
};
const int *p = &X::n, *q = &X::m; // X::n and X::m are odr-used
const int X::n;             // … so a definition is necessary
constexpr int X::m;         // … (except for X::m in C++17)

where I'd have said static const int n = 1; is a definition, but it is not, based on the second to last comment.