Remove ambiguity for operator<

MSVC will try to build json object from value_t and use another operator<

src/json.hpp

@@ -1994,8 +1994,9 @@ class basic_json
         }
 
         // We only reach this line if we cannot compare values. In that case,
-        // we compare types.
+        // we compare types. Have to use operator< explicitly in order to
-        return lhs_type < rhs_type;
+        // not cause ambiguity on MS compiler
+        return operator<(lhs_type, rhs_type);
     }
 
     /// comparison: less than or equal

test/unit.cpp

@@ -6202,7 +6202,7 @@ TEST_CASE("lexicographical comparison operators")
                     CAPTURE(i);
                     CAPTURE(j);
                     // check precomputed values
-                    CHECK( (j_types[i] < j_types[j]) == expected[i][j] );
+                    CHECK( operator<(j_types[i], j_types[j]) == expected[i][j] );
                 }
             }
         }