Work around VS2015 bug with zero-arg default constructors.

c++/src/kj/common.h

@@ -303,6 +303,14 @@ KJ_NORETURN(void unreachable());
 #define KJ_CONSTEXPR(...) constexpr
 #endif
 
+#if defined(_MSC_VER) && _MSC_VER < 1910
+// TODO(msvc): Visual Studio 2015 mishandles declaring the no-arg constructor `= default` for
+//   certain template types -- it fails to call member constructors.
+#define KJ_DEFAULT_CONSTRUCTOR_VS2015_BUGGY {}
+#else
+#define KJ_DEFAULT_CONSTRUCTOR_VS2015_BUGGY = default;
+#endif
+
 // =======================================================================================
 // Template metaprogramming helpers.
 

c++/src/kj/table.h

@@ -761,7 +761,7 @@ kj::Array<HashBucket> rehash(kj::ArrayPtr<const HashBucket> oldBuckets, size_t t
 template <typename Callbacks>
 class HashIndex {
 public:
-  HashIndex() = default;
+  HashIndex() KJ_DEFAULT_CONSTRUCTOR_VS2015_BUGGY
   template <typename... Params>
   HashIndex(Params&&... params): cb(kj::fwd<Params>(params)...) {}
 
@@ -1299,7 +1299,7 @@ inline BTreeImpl::Iterator BTreeImpl::end() const {
 template <typename Callbacks>
 class TreeIndex {
 public:
-  TreeIndex() = default;
+  TreeIndex() KJ_DEFAULT_CONSTRUCTOR_VS2015_BUGGY
   template <typename... Params>
   TreeIndex(Params&&... params): cb(kj::fwd<Params>(params)...) {}
 

c++/src/kj/tuple.h

@@ -93,7 +93,7 @@ struct TupleElement {
   // from a TupleElement for each element, which is more efficient than a recursive definition.
 
   T value;
-  TupleElement() = default;
+  TupleElement() KJ_DEFAULT_CONSTRUCTOR_VS2015_BUGGY
   constexpr inline TupleElement(const T& value): value(value) {}
   constexpr inline TupleElement(T&& value): value(kj::mv(value)) {}
 };
@@ -123,7 +123,7 @@ struct TupleImpl<Indexes<indexes...>, Types...>
 
   static_assert(sizeof...(indexes) == sizeof...(Types), "Incorrect use of TupleImpl.");
 
-  TupleImpl() = default;
+  TupleImpl() KJ_DEFAULT_CONSTRUCTOR_VS2015_BUGGY
 
   template <typename... Params>
   inline TupleImpl(Params&&... params)
@@ -153,7 +153,7 @@ class Tuple {
   // The actual Tuple class (used for tuples of size other than 1).
 
 public:
-  Tuple() = default;
+  Tuple() KJ_DEFAULT_CONSTRUCTOR_VS2015_BUGGY
 
   template <typename... U>
   constexpr inline Tuple(Tuple<U...>&& other): impl(kj::mv(other)) {}