What does the error about default member initializer needed means?

I have a code that works fine in Visual Studio:

struct A
{
    struct P{ P() = default; int x = 1; };
    void foo( const P & = {} );
};

However both gcc and clang refuse to compile it with the error:

<source>:4:28: error: default member initializer for 'A::P::x' required before the end of its enclosing class
    4 |     void foo( const P & = {} );

Could you please suggest what is wrong with the code?