Summary:Instead of using preprocessor to disable SSO, use a default argument. Also
reimplement `insertImplDiscr` to mirror `insertImpl`.

Reviewed By: philippv

Differential Revision: D3130901

fb-gh-sync-id: a5b0ba97b3c7d91323be01ab617ca9b09dbb0fd2
fbshipit-source-id: a5b0ba97b3c7d91323be01ab617ca9b09dbb0fd2