Library: improve helper to deal with self-shadowing ctor (see: #963)

Whenever a class defines a single-arg templated constructor,
there is danger to shadow the auto-generated copy operations,
leading to insidious failures.

Some months ago, I did the ''obvious'' and added a tiny helper,
allowing to mask out the dangerous case when the ''single argument''
is actually the class itself (meaning, it is a copy invocation and
not meant to go through this templated ctor...


As this already turned out as tremendously helpful, I now extended
this helper to also cover cases where the problematic constructor
accepts variadic arguments, which is quite common with builder-helpers

src/lib/meta/util.hpp

@@ -136,9 +136,28 @@ namespace meta {
   
   
   
+  namespace {
+    template<typename...>
+    struct _ExtractFirst
+      {
+        using Type = void;
+      };
+    template<typename X, typename...XS>
+    struct _ExtractFirst<X,XS...>
+      {
+        using Type = X;
+      };
+  }
+  /** helper to extract the first argument from a variadic arg pack, if any */
+  template<typename...XS>
+  using extractFirst_t = typename _ExtractFirst<XS...>::Type;
+  
+  
+  
   /** helper to prevent a template constructor from shadowing inherited copy ctors */
-  template<typename ARG, class SELF>
-  using disable_if_self = disable_if<std::is_same<std::remove_reference_t<ARG>, SELF>>;
+  template<class SELF, typename ...ARGS>
+  using disable_if_self = disable_if<std::is_same<std::remove_cv_t<std::remove_reference_t<extractFirst_t<ARGS...>>>
+                                                 , SELF>>;
   
   
   

src/lib/random-draw.hpp

@@ -282,7 +282,7 @@ namespace lib {
        * - `void(RandomDraw&, ...)` : the function manipulates the current
        *   instance, to control parameters dynamically, based on input.
        */
-      template<class FUN,      typename =disable_if_self<FUN, RandomDraw>>
+      template<class FUN,      typename =disable_if_self<RandomDraw, FUN>>
       RandomDraw(FUN&& fun)
         : Lazy{Disabled()}
         , probability_{1.0}

src/vault/gear/special-job-fun.hpp

@@ -164,7 +164,7 @@ namespace gear {
        *          this reference is dangling and using it further will
        *          lead to SEGFAULT or memory corruption.
        */
-      template<class FUN,            typename =disable_if_self<FUN, SpecialJobFun>>
+      template<class FUN,            typename =disable_if_self<SpecialJobFun, FUN>>
       explicit
       SpecialJobFun (FUN&& someFun)
         : _Handle{selfAttached (new SpecialExecutor(forward<FUN> (someFun)))}

tests/library/lazy-init-test.cpp

@@ -357,7 +357,7 @@ namespace test{
               installInitialiser(fun, buildInit([](int){ return 0; }));
             }
                                                  // prevent this ctor from shadowing the copy ctors    //////TICKET #963
-          template<typename FUN,      typename =disable_if_self<FUN, LazyDemo>>
+          template<typename FUN,      typename =disable_if_self<LazyDemo, FUN>>
           LazyDemo (FUN&& someFun)
             : LazyInit{MarkDisabled()}
             , fun{}