CTFE execution fails when nested enums or functions have a UDA on them:

    int f() {
        @("S")
        struct S {}

        @("U")
        union U {}

        @("C")
        class C {}

        // CTFE fails on this:
        @("E")
        enum E { X }

        // CTFE fails on this:
        @("f")
        void f() {}

        return 5;
    }

    static assert(f() == 5);

e.g the enum errors out with:

  Error: declaration `@("E")enum E : int { ... }` is not yet implemented in CTFE