Use C++ v.s. PP to handle libcpp in dynamic ctor

Summary: [Folly] Use C++ v.s. PP to handle libcpp in `dynamic` ctor taking `std::vector<bool>::const_reference`.

Reviewed By: shixiao

Differential Revision: D9027115

fbshipit-source-id: 94de4260d1d40ac450c37d3803994d436adcf06e

folly/dynamic-inl.h

@@ -364,10 +364,8 @@ struct dynamic::NumericTypeHelper<double> {
 
 inline dynamic::dynamic(std::vector<bool>::reference b)
     : dynamic(static_cast<bool>(b)) {}
-#if defined(_LIBCPP_VERSION)
-inline dynamic::dynamic(std::vector<bool>::const_reference b)
+inline dynamic::dynamic(VectorBoolConstRefCtorType b)
     : dynamic(static_cast<bool>(b)) {}
-#endif
 
 template <
     class T,

folly/dynamic.h

@@ -100,6 +100,19 @@ struct dynamic : private boost::operators<dynamic> {
  private:
   typedef std::vector<dynamic> Array;
 
+  /*
+   * Violating spec, std::vector<bool>::const_reference is not bool in libcpp:
+   * http://howardhinnant.github.io/onvectorbool.html
+   *
+   * This is used to add a public ctor which is only enabled under libcpp taking
+   * std::vector<bool>::const_reference without using the preprocessor.
+   */
+  struct VectorBoolConstRefFake : std::false_type {};
+  using VectorBoolConstRefCtorType = std::conditional_t<
+      std::is_same<std::vector<bool>::const_reference, bool>::value,
+      VectorBoolConstRefFake,
+      std::vector<bool>::const_reference>;
+
  public:
   typedef Array::iterator iterator;
   typedef Array::const_iterator const_iterator;
@@ -177,14 +190,9 @@ struct dynamic : private boost::operators<dynamic> {
    * bool. Calling a function f(dynamic) with f(v[idx]) would require a double
    * implicit conversion (reference -> bool -> dynamic) which is not allowed,
    * hence we explicitly accept the reference proxy.
-   *
-   * std::vector<bool>::const_reference is not bool in libcpp:
-   * http://howardhinnant.github.io/onvectorbool.html
    */
   /* implicit */ dynamic(std::vector<bool>::reference val);
-#if defined(_LIBCPP_VERSION)
-  /* implicit */ dynamic(std::vector<bool>::const_reference val);
-#endif
+  /* implicit */ dynamic(VectorBoolConstRefCtorType val);
 
   /*
    * Create a dynamic that is an array of the values from the supplied