Doubly-filled COMP filter
Doubly-filled COMP Filter is a filter which excludes the co-occurrence of a wh-phrase and a complementizer in a COMP-position, as in (i).
(i) I wonder who (*that/*whether) she saw
Examples like these have been accounted for by assuming that complementizer deletion must obtain so as to satisfy this filter. The introduction of the CP necessitates an alternative formulation of this filter. The problem of (i) is now the obligatory deletion of COMP in English.
Link
Utrecht Lexicon of Linguistics
References
- Riemsdijk, H. van and E. Williams 1986. Introduction to the theory of grammar, MIT Press, Cambridge, Mass.