Summary:
Previously, all placeholders methods have return type `SomeType &` which is not correct because it allows the called to modify enclosed `static` value of the placeholders; the type should be `SomeType const &`.
Besides that this diff migrates some type aliases to the new style (which makes evething much prettier and readable).
Reviewed By: mdvacca
Differential Revision: D14819076
fbshipit-source-id: 87e68d546f16d7a9ad1fb65e1b238859f9381eb7