You quoted the answer yourself. In the SO answer that you linked, there is the following quote from the standard (section 6.8.6precisely):
If a program calls for the default initialization of an object of a
const-qualified type T, T shall be a class type with a user-provided
default constructor.
emphasis mine. The line
A() = default;
obviously does not provide a constructor, it does the opposite by telling the compiler that you don't want to provide one, thus your code doesn't compile. However, once you provide the constructor by uncommenting
A(){};
it works fine. So, to summarize, the error that clang shows is per standard, and the behaviour of gcc is probably a bug.
