You shouldn't be able to call it as just _foo(1.2L); an operator name is
different from a normal function name.
According to 13.5.8/7 :
[ Note: literal operators and literal operator templates are usually invoked implicitly through user-deïned
literals (2.14.8). However, except for the constraints described above, they are ordinary namespace-scope
functions and function templates. In particular, they are looked up like ordinary functions and function tem-
plates and they follow the same overload resolution rules. Also, they can be declared inline or constexpr,
they may have internal or external linkage, they can be called explicitly, their addresses can be taken, etc.
â end note ]