Rationale (May, 2015):

The use of a qualified-id as a mem-initializer-id is sufficiently unusual that it is not worth changing the lookup rules to accommodate it.

The name lookup in a base-specifier and a mem-initializer differ in that the former ignores non-type names but the latter does not. When the mem-initializer-id is qualified, this can lead to surprising results:

  struct file_stat : ::stat {    // the class
    file_stat() : ::stat{} {}    // the function
  };