This option specifies a function to be used for Insert mode completion with CTRL-X CTRL-U. i_CTRL-X_CTRL-U See complete-functions for an explanation of how the function is invoked and what it should return. The value can be the name of a function, a lambda or a Funcref. See option-value-function for more information. This option cannot be set from a modeline or in the sandbox, for security reasons.

(default: empty)

not available when compiled without the +eval feature