What is the reason for not being able to deduce array size from initializer-string in member variable?

Question

Consider the code:

struct Foo
{
    const char str[] = \"test\";
};

int main()
{
    Foo foo;
}

It fails to compile with both g++ and clan

Answer 1

If the compiler was allowed to support what you described, and the size of str was deduced to 5,

Foo foo = {{"This is not a test"}};

will lead to undefined behavior.