commit 84e00cbc1702be62eff1f83006603f02b24d93c3
Author: Alastair Bridgewater <nyef@lain.lisphacker.com>
Date:   Thu Oct 21 18:14:40 2010 -0400

    constraint: Check leaf visibility prior to changing REF LEAFs.
    
      * Define a new predicate, LEAF-VISIBLE-FROM-NODE to check if a
    NODE is in a lexical environment to be able to "see" a given LEAF.
    
      * Use the new predicate to prevent rewriting REFs to point to
    LEAFs that aren't in the REFs lexical environment (thus preventing
    problems with attempting to CLOSE-OVER a variable that isn't
    visible).
    
      * This fixes lp#551227 (error: the value NIL is not of type
    SB-C::NODE), but I'm not entirely convinced of its correctness.

diff --git a/src/compiler/constraint.lisp b/src/compiler/constraint.lisp
index eb936ba..da8e55e 100644
--- a/src/compiler/constraint.lisp
+++ b/src/compiler/constraint.lisp
@@ -604,6 +604,25 @@
           (modified-numeric-type x :low new-bound)
           (modified-numeric-type x :high new-bound)))))
 
+;;; Return true if LEAF is "visible" from NODE.
+(defun leaf-visible-from-node (leaf node)
+  (cond
+   ((lambda-var-p leaf)
+    ;; A LAMBDA-VAR is visible iif it is homed in a CLAMBDA in the
+    ;; allocator chain for NODE.
+    (let ((leaf-lambda (lambda-var-home leaf))
+          (node-lambda (node-home-lambda node)))
+      (loop for lambda = node-lambda then (awhen (functional-allocator lambda)
+                                                 (node-home-lambda it))
+            while lambda
+            when (eq lambda leaf-lambda)
+            return t)))
+   ;; FIXME: Check on FUNCTIONALs (CLAMBDAs and OPTIONAL-DISPATCHes),
+   ;; not just LAMBDA-VARs.
+   (t
+    ;; Assume everything else is globally visible.
+    t)))
+
 ;;; Given the set of CONSTRAINTS for a variable and the current set of
 ;;; restrictions from flow analysis IN, set the type for REF
 ;;; accordingly.
@@ -659,7 +678,11 @@
                               (and (leaf-refs other) ; protect from
                                         ; deleted vars
                                    (csubtypep other-type leaf-type)
-                                   (not (type= other-type leaf-type))))
+                                   (not (type= other-type leaf-type))
+                                   ;; KLUDGE: This "fixes" lp#551227,
+                                   ;; but I don't know that it's truly
+                                   ;; correct.  -- AB, 2010-Oct-21
+                                   (leaf-visible-from-node other ref)))
                           (change-ref-leaf ref other)
                           (when (constant-p other) (return)))
                          (t