[gram.over]

operator-function-id:
operator operator

operator: one of
new      delete   new[]    delete[] co_await ( )        [ ]        ->       ->*
~        !        +        -        *        /        %        ^        &
|        =        +=       -=       *=       /=       %=       ^=       &=
|=       ==       !=       <        >        <=       >=       <=>      &&
||       <<       >>       <<=      >>=      ++       --       ,

literal-operator-id:
operator unevaluated-string identifier
operator user-defined-string-literal

Annex A (informative) Grammar summary [gram]

A.10 Overloading [gram.over]