Why can't I have flexible array members in union? Consider this:

    struct foo {
        enum { t_char, t_int, t_ptr, /* .. */ } type;
        int count;

        union {
            char c[];
            int i[];
            void *p[];
            /* .. */
        };
    };

This isn't allowed, since flexible array members are only allowed in structs (but the union here is exactly where you'd put a flexible array member if you had only one type to deal with).

Furthermore, you can't work around this by wrapping the union's members in a struct because they must have more than one named member:

    struct foo {
        enum { t_char, t_int, t_ptr } type;
        int count;

        union { /* not allowed! */
            struct { char c[]; };
            struct { int i[]; };
            struct { void *p[]; };
        };
    };

But it's all fine if we either add a useless dummy variable or move some prior member (such as count) into these structs:

    struct foo {
        enum { t_char, t_int, t_ptr } type;
        int count;

        union { /* this works but is silly and redundant */
            struct { int dumb1; char c[]; };
            struct { int dumb2; int i[]; };
            struct { int dumb3; void *p[]; };
        };
    };

Of course, you could have the last member be

    union { char c; int i; void *p; } u[];

but then each element of u is as large as the largest possible member which is wasteful, and u can't be passed to any function that expects to get a normal, tightly packed array of one specific type.