CLHS: Issue SPECIAL-TYPE-SHADOWING Writeup

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## Issue SPECIAL-TYPE-SHADOWING Writeup

    Issue:         SPECIAL-TYPE-SHADOWING

    References:    CLtL pages 156, 158

    Related issues: DECLARE-TYPE-FREE

    Category:      CLARIFICATION

    Edit history:  Version 1, 04-Nov-88 by David Gray

    Problem description:

      A Common Lisp user raised the question of whether something like the
      following is legal:

        (PROCLAIM '(TYPE NUMBER *X*))
        (DEFVAR *X*)
        (DEFUN FOO ()
          (LET ((*X* T))
            (DECLARE (TYPE SYMBOL *X*))
            (BAR)))

      Page 156 of CLtL says that a proclamation is "always in force unless
      locally shadowed" and page 158 says type declarations "only affect
      variable bindings", which might be interpreted to mean that the DECLARE
      locally shadows the PROCLAIM.  However, that interpretation would make
      the global type proclamation useless because it could not be relied on
      when compiling a function such as BAR.

    Proposal SPECIAL-TYPE-SHADOWING:CLARIFY

      Clarify that if there is a local type declaration for a special
      variable, and there is also a global type proclamation for that same
      variable, then the value of the variable within the scope of the local
      declaration must be a member of the intersection of the two declared
      types.

    Rationale:

      Some restriction on local type declarations for special variables is
      needed in order for type proclamations to be meaningful.  The wording
      used here was chosen for consistency with proposal DECLARE-TYPE-FREE.

    Current practice:

      The TI, Symbolics, and Lucid implementations do not report any error
      on the example above, but it isn't clear that they really do anything
      with type declarations for special variables anyway.

    Cost to Implementors:

      This is unlikely to require a change in any current implementation.

    Cost to Users:

      Anyone who has written code like the example above would have to
      modify it if compilers started enforcing this restriction.

    Cost of non-adoption:

      A minor ambiguity in the language specification that could confuse
      users.

    Performance impact:

      None.

    Benefits:

      A clearer definition of the meaning of type declarations for special
      variables.

    Discussion:

      This is obviously very closely related to issue DECLARE-TYPE-FREE, but
      this is an ambiguity in the existing language that should be resolved
      even if the language extension of proposal DECLARE-TYPE-FREE is not
      accepted.  Note also that DECLARE-TYPE-FREE makes no mention of type
      proclamations.

      Other possible resolutions of the ambiguity would be to either rule
      out use of local type declarations for special variables, or to say
      that the local type must be a subtype of the global type.

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