编译和工作的错误静态常量初始化

As far as I know, you can only initialize static const members in the same line of their declaration if they are integral types . However, I was still able to initialize and use some static const doubles:

// compiles and works, values are indeed doubles
struct Foo1{ 
    static const double A=2.5;
    static const double B=3.2;
    static const double C=1.7;
};

// compiles, but values are cast to int
struct Foo2{
    static const int A=2;
    static const int B=3;
    static const double C=B/A; //becomes 1
};

// does not compile, Foo3::B cannot appear in a constant-expression
struct Foo3{ 
    static const int A=2;
    static const double B=3;
    static const double C=A/B; 
};

// does not compile, a cast to a type other than an integral or enumeration
// cannot appear in a constant-expression
struct Foo4{ 
    static const int A=2;
    static const int B=3;
    static const double C=(double)A/B; 
};

Foo2 编译但 Foo2::C 变为 1，所以也许它被视为一个 int，因为它在数字上是 1.Foo3 和 Foo4 甚至没有按预期编译.但是，我不明白为什么 Foo1 既能编译又能正常工作.这种特定用法是否被接受?是不是因为一些优化?(我试过使用 -O1 和 -O0)

error: floating-point literal cannot appear in a constant-expression
 static const double A=2.5;
                       ^
warning: ISO C++ forbids initialization of member constant 'Foo1::A' of non-integral type 'const double' [-Wpedantic]

warning: in-class initializer for static data member of type 'const double' is a GNU extension [-Wgnu-static-float-init]
static const double A=2.5;
                    ^ ~~~