ad--defalias-fset is a compiled Lisp function in `advice.el'.

(ad--defalias-fset FSETFUN FUNCTION NEWDEF)

Handle re/definition of an advised FUNCTION during de/activation.
If FUNCTION does not have an original definition associated with it and
the current definition is usable, then it will be stored as FUNCTION's
original definition. If no current definition is available (even in the
case of undefinition) nothing will be done. In the case of redefinition
the action taken depends on the value of `ad-redefinition-action' (which
see). Redefinition occurs when FUNCTION already has an original definition
associated with it but got redefined with a new definition and then
de/activated. If you do not like the current redefinition action change
the value of `ad-redefinition-action' and de/activate again.