Cannot declare anonymous union around generic type

[Thierry Delisle]

Minimal example:

forall(otype T)
struct result {
    #ifdef THE_ERROR
    union {
    #endif

    T value;

    #ifdef THE_ERROR
    };
    #endif
};

produces :

error: ‘_sizeof_Y1T’ undeclared here (not in a function)

Desired for:

forall(otype T, otype E)
struct result {
	bool has_value;
	union {
		T value;
		E error;
	};
};