Prefer default member initializers also for non-constant initializers

[N-Dekker]

Removed the restriction "for constant initializers" from guideline C.48, Prefer default member initializers to member initializers in constructors for constant initializers

• The restriction appears unnecessary, as discussed by issue C.48: Prefer default member initializers, also for non-constant initializers #2332, showing the following example, having a non-constant (now()) as default member initializer, which also appears preferable:

    class MyContainer {
        time_point<system_clock> creationTime{ system_clock::now() };  // Preferable, right?
        vector<int> data{};
    public:
        MyContainer() = default;
        explicit MyContainer(size_t n) : data(n) {}
        MyContainer(initializer_list<int> values) : data{ values } {}
    };

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• Follow-up to pull request Use function call in Example of C.48: Prefer default member initializer #2330 commit b057f29 (which was merged by Sergey Zubkov, @cubbimew)

[hsutter]

Editors call: Thanks!

Regarding numeric_limits<T>::min, that is constexpr so it's not equivalent to now().

We do want to avoid dynamic initialization in general wherever possible.

We think the existing rule is right for the majority of code, and if you have a reason to initialize using something like system_clock::now() or thread::get_id() then we recommend putting it in the constructor which makes it more obvious that something dynamic is happening.