CXX-588 Cannot use kvp() with a std::string param

sub_document.append() specializations for std::string and
stdx::string_view keyed tuples fail to forward for the various cv and
ref qualified variants.  To keep up the perfect forwarding, these
functions need to take the key type as a template type parameter and
SFINAE themselves to disambiguate.

Closes #291, #281

Signed-off-by: Adam Midvidy <amidvidy@gmail.com>

Author: hanumantmk

src/bsoncxx/builder/basic/sub_document.hpp

@@ -34,6 +34,7 @@ void value_append(core* core, T&& t);
 /// Users should almost always construct a builder::basic::document instead.
 ///
 class BSONCXX_API sub_document {
+
    public:
     BSONCXX_INLINE sub_document(core* core) : _core(core) {
     }
@@ -51,31 +52,33 @@ class BSONCXX_API sub_document {
     ///
     /// Appends a basic::kvp where the key is a non-owning string view.
     ///
-    template <typename T>
+    template <typename K, typename V>
     BSONCXX_INLINE
-    void append(std::tuple<stdx::string_view, T>&& t) {
-        _core->key_view(std::get<0>(t));
-        impl::value_append(_core, std::forward<T>(std::get<1>(t)));
+    typename std::enable_if<std::is_same<typename std::decay<K>::type, stdx::string_view>::value>::type
+    append(std::tuple<K, V>&& t) {
+        _core->key_view(std::forward<K>(std::get<0>(t)));
+        impl::value_append(_core, std::forward<V>(std::get<1>(t)));
     }
 
     ///
     /// Appends a basic::kvp where the key is an owning STL string.
     ///
-    template <typename T>
+    template <typename K, typename V>
     BSONCXX_INLINE
-    void append(std::tuple<std::string, T>&& t) {
-        _core->key_owned(std::get<0>(t));
-        impl::value_append(_core, std::forward<T>(std::get<1>(t)));
+    typename std::enable_if<std::is_same<typename std::decay<K>::type, std::string>::value>::type
+    append(std::tuple<K, V>&& t) {
+        _core->key_owned(std::forward<K>(std::get<0>(t)));
+        impl::value_append(_core, std::forward<V>(std::get<1>(t)));
     }
 
     ///
     /// Appends a basic::kvp where the key is a string literal
     ///
-    template <std::size_t n, typename T>
+    template <std::size_t n, typename V>
     BSONCXX_INLINE
-    void append(std::tuple<const char (&)[n], T>&& t) {
+    void append(std::tuple<const char (&)[n], V>&& t) {
         _core->key_view(stdx::string_view{std::get<0>(t), n - 1});
-        impl::value_append(_core, std::forward<T>(std::get<1>(t)));
+        impl::value_append(_core, std::forward<V>(std::get<1>(t)));
     }
 
    private:

src/bsoncxx/test/bson_builder.cpp

@@ -682,6 +682,29 @@ TEST_CASE("basic document builder works", "[bsoncxx::builder::basic]") {
         viewable_eq_viewable(stream, basic);
     }
 
+    SECTION("kvp works with std::string key/value") {
+        {
+            using namespace builder::basic;
+
+            std::string a("hello");
+            std::string b("world");
+            basic.append(kvp(a, b));
+        }
+
+        viewable_eq_viewable(stream, basic);
+    }
+
+    SECTION("kvp works with stdx::string_view key/value") {
+        {
+            using namespace builder::basic;
+
+            stdx::string_view a("hello");
+            stdx::string_view b("world");
+            basic.append(kvp(a, b));
+        }
+
+        viewable_eq_viewable(stream, basic);
+    }
     SECTION("variadic works") {
         {
             using namespace builder::stream;