If you try to use both of those in a Lua file you get the following runtime error:
CoreLibs/object.lua:73: assign to undeclared variable 'mt'
stack traceback:
[C]: in function 'error'
CoreLibs/strict.lua:27: in metamethod 'newindex'
CoreLibs/object.lua:73: in field 'extends'
main.lua:34: in main chunk