Why does gcc warn about large alignas value?

Question

When compiled with gcc 6.3, this example program

#include <array>

class alignas(4096) A {
    std::array<int, 256> array;
};

int main()
{
    A a;
}

gives the following warning:

3 : <source>:3:21: warning: requested alignment 4096 is larger than 128 [-Wattributes]
 class alignas(4096) A {
                     ^

There's no such error with gcc 7.2 though.

Example

1. Does it mean that the compiler just informs about alignment being too big? What's the purpose of such a warning?
2. The standard doesn't allow the compiler to ignore this alignment specifier (even in presence of the warning), does it?

Answer 1

The standard allows all sorts of things to break if you alignas to larger than that of std::max_align_t:

the largest fundamental alignment of any type is the alignment of std::max_align_t. If a type's alignment is made stricter (larger) than std::max_align_t using alignas, it is known as a type with extended alignment requirement. A type whose alignment is extended or a class type whose non-static data member has extended alignment is an over-aligned type. It is implementation-defined if new-expression, std::allocator::allocate, and std::get_temporary_buffer support over-aligned types. Allocators instantiated with over-aligned types are allowed to fail to instantiate at compile time, to throw std::bad_alloc at runtime, to silently ignore unsupported alignment requirement, or to handle them correctly.

Basically, you can declare highly aligned types, but some stuff might break when you use them; it's possible GCC6 is being conservative, or it might actually fail for some of the specified cases mentioned above and it's trying to warn you.