Remove ambiguity for operator<

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

src/json.hpp

@@ -4286,7 +4286,9 @@ class basic_json
 
         // We only reach this line if we cannot compare values. In that case,
         // we compare types.
-        return lhs_type < rhs_type;
+        // Have to use operator< explicitly in order to
+        // not cause ambiguity on MS compiler
+        return operator<(lhs_type, rhs_type);
     }
 
     /*!

test/unit.cpp

@@ -6939,7 +6939,7 @@ 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]);
                 }
             }
         }