Issue SPECIAL-TYPE-SHADOWING (V1)

[David N Gray <Gray@DSG.csc.ti.com>]

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.