SF.2 allow inline variables and static inline class member variables

[sgilbertson]

SF.2 has a list under "A header file must contain only" that does not include:

• static inline class member variables (not just functions)
• inline variables not in a class declaration (again, not just functions)

Both are supported since C++17, resulting in all translation units linking the entity to the same memory location, The first point above results in code simplification and allows implementation of header-only classes that would otherwise not be possible. In [https://en.cppreference.com/w/cpp/language/inline](inline specifier - cppreference.com):

An inline function or variable(since C++17) ... has the same address in every translation unit.

For example, if a header includes:

class MyClass
{
    public:
        static inline int staticInlineMember = 5;
};
inline int inlineVariable = 5;

The memory location of staticInlineMember is always the same, when that header is included in multiple source files. So is the location of inlineVariable.

[iglesias]

The last item of the list is ???, which I assume is a placeholder denoting that the list is not set in stone.