Proposed resolution:

This wording is relative to N4988.

1. In [meta.unary.prop] Table 51, change the Precondition text for is_assignable, is_trivially_assignable, and is_nothrow_assignable as follows:

   remove_cvref_t<T> and U shall be complete types, cv void, or arrays of unknown bound.

2. In [meta.unary.prop] Table 51, change the Precondition text for is_copy_assignable, is_move_assignable, is_trivially_copy_assignable, is_trivially_move_assignable, is_nothrow_copy_assignable, and is_nothrow_move_assignable as follows:

   remove_cvref_t<T> shall be a complete type, cv void, or an array of unknown bound.