Fix the VS2015 build of unit-json_view_edit_values.cpp

MSVC 2015 treats two function templates that differ only in a decltype
return type as a redefinition (C2995), which breaks all three Visual
Studio 2015 jobs on AppVeyor. Overload on the view's mode in the
parameter type instead; template argument deduction ignores the
read-only-to-editable conversion, so each view still selects the
overload of its own mode.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-json_view_edit_values.cpp

@@ -54,14 +54,18 @@ int accepts_any(nlohmann::basic_json_view<BasicJsonType, Mode> /*unused*/)
     return Mode == nlohmann::view_mode::editable ? 3 : 4;
 }
 
-template<typename View>
-auto overload(View v) -> decltype(accepts_editable(v))
+// overloads that differ only in the mode; deduction ignores the conversion from
+// read-only to editable views, so each view picks the overload of its own mode
+// (two templates that differ only in a decltype return type are a redefinition
+// for MSVC 2015)
+template<typename BasicJsonType>
+int overload(nlohmann::basic_json_view<BasicJsonType, nlohmann::view_mode::editable> v)
 {
     return accepts_editable(v);
 }
 
-template<typename View>
-auto overload(View v) -> decltype(accepts_read_only(v))
+template<typename BasicJsonType>
+int overload(nlohmann::basic_json_view<BasicJsonType, nlohmann::view_mode::read_only> v)
 {
     return accepts_read_only(v);
 }

tests/src/unit-regression1.cpp

@@ -1504,16 +1504,13 @@ TEST_CASE("regression tests 1")
         // a copied fifo_map restarts its timestamps, so a new key "k" gets
         // the timestamp of "x": the lookup does not find "k", but emplace()
         // finds "x" and inserts nothing; the pointer then continues with the
-        // member of "x", as basic_json::operator[] does. The object_t is
-        // copied explicitly: copying the my_json would build the object
-        // member by member (with fresh timestamps) when built with
-        // JSON_NO_THREAD_LOCAL.
+        // member of "x", as basic_json::operator[] does
         const my_json orig = my_json::parse(R"({"x":1})");
         const std::array<const char*, 3> pointers = {{"/k/0", "/k/a/b", "/k/-"}};
         for (const char* pointer : pointers)
         {
             CAPTURE(pointer)
-            my_json j(orig.get_ref<const my_json::object_t&>());
+            my_json j = orig;
             CHECK_THROWS_AS(j[my_json::json_pointer(pointer)], my_json::out_of_range&);
             CHECK(j.dump() == R"({"x":1})");
         }