farptr: add safety wrapper for dot barriers

It may be needed if the barriers are added by some stupid script,
rather than manually as now.
Unfortunately, writing such script is going to be quite a challenge.
So far we only have very few places where this fixup was needed,
so automating the task may not worth the trouble, but lets add the
wrapper nevertheless, then see.

fdpp/farptr.hpp

@@ -692,13 +692,15 @@ template<typename T, typename P, int (*M)(void), int O = 0>
 class SymMemb : public T, public MembBase<T, P, M, O> {
     template<const int *st>
     using wrp_type = typename WrpTypeS<T, st>::type;
-    using wrp_type_s = wrp_type<sym_store>;
 public:
+    using wrp_type_s = wrp_type<sym_store>;
     SymMemb() = default;
     SymMemb(const SymMemb&) = delete;
     T& operator =(const T& f) { *(T *)this = f; return *this; }
     FarPtr<T> operator &() const { return this->lookup_sym(); }
-    wrp_type_s& use() {
+    /* This dot_barrier is needed because operator dot can't save
+     * the lhs fptr. We add manual fixups whereever it crashes... */
+    wrp_type_s& dot_barrier() const {
         FarPtr<T> sym = this->lookup_sym();
         /* TODO: move store to SymWrp ctor. Currently impossible
          * because SymWrp needs to be trivially-constructible. */
@@ -707,6 +709,20 @@ class SymMemb : public T, public MembBase<T, P, M, O> {
     }
 } NONPOD_PACKED;
 
+/* Safety wrapper, in case the dot barriers are added by some stupid script,
+ * rather than manually, as now. */
+class DotBarrierWrap {
+public:
+    template<typename T0, typename T1, int (*M)(void), int O,
+        typename TN = SymMemb<T0, T1, M, O> >
+    static constexpr typename TN::wrp_type_s&
+            dot_barrier(SymMemb<T0, T1, M, O>& s) {
+        return s.dot_barrier();
+    }
+    template<typename T1>
+    static constexpr T1& dot_barrier(T1& s) { return s; }
+};
+
 template<typename T, typename P, int (*M)(void), int O = 0>
 class SymMembT : public MembBase<T, P, M, O> {
     T sym;
@@ -770,7 +786,7 @@ class SymMembT : public MembBase<T, P, M, O> {
 #define GET_FAR(f) (f).get_far()
 #define GET_PTR(f) (f).get_ptr()
 #define ADJUST_FAR(f) (f).adjust_far()
-#define _DOS_FP(p) p.use()
+#define _DOS_FP(p) DotBarrierWrap::dot_barrier(p)
 
 #undef NULL
 #define NULL           nullptr