Fix racket#1158, racket#848, racket#858: Add null checks in compute-defs

Added null checks in compute-defs to handle cases where identifier-binding
returns #f or when the definition lookup fails. This fixes internal errors
with recursive MListof types, recursive case-> types, and recursive
contract generation.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Closes racket#1158
Closes racket#848
Closes racket#858

Author: samth

typed-racket-lib/typed-racket/static-contracts/instantiate.rkt

@@ -96,12 +96,15 @@
          (set! bound (cons name* bound))
          ;; traverse what `name` refers to
          (define r (ref name*))
-         ;; ref returns a rib, get the one definition we want
-         (define target (for/first ([k (in-list (car r))]
-                                    [v (in-list (cdr r))]
-                                    #:when (free-identifier=? name* k))
-                          v))
-         (loop target #f))]
+         ;; r can be #f if the name is not in all-name-defs
+         (when r
+           ;; ref returns a rib, get the one definition we want
+           (define target (for/first ([k (in-list (car r))]
+                                      [v (in-list (cdr r))]
+                                      #:when (free-identifier=? name* k))
+                            v))
+           (when target
+             (loop target #f))))]
       [else (sc-traverse sc loop)]))
   (for*/hash ([b (in-list bound)]
               [v (in-value (ref b))]

typed-racket-test/succeed/gh-issue-1158.rkt

@@ -0,0 +1,15 @@
+#lang typed/racket/base
+
+;; Issue #1158: Regression test - previously caused car: contract violation
+;; during contract generation with recursive MListof.
+;; Fixed by adding null checks in compute-defs in static-contracts/instantiate.rkt
+
+(provide func-1 func-2)
+
+(define-type MNulls (MListof MNulls))
+
+(: func-1 [-> MNulls Any]) (define (func-1 ns) 1)
+(: func-2 [-> MNulls Any]) (define (func-2 ns) 2)
+
+(displayln (func-1 '()))
+(displayln (func-2 '()))

typed-racket-test/succeed/gh-issue-848.rkt

@@ -0,0 +1,20 @@
+#lang typed/racket/base
+
+;; Issue #848: Regression test - previously caused car: contract violation
+;; with recursive case-> types during contract generation.
+;; Fixed by adding null checks in compute-defs in static-contracts/instantiate.rkt
+
+(provide
+ new-T
+ consume-T)
+
+(define-type T
+  (case->
+   ['a -> Any]
+   ['b -> (-> T)]))
+
+(: new-T : -> T)
+(define (new-T) (new-T))
+
+(: consume-T : T -> Nothing)
+(define (consume-T t) (consume-T t))

typed-racket-test/succeed/gh-issue-858.rkt

@@ -0,0 +1,13 @@
+#lang racket
+
+;; Issue #858: Previously caused "recursive-contract: contract violation"
+;; with recursive types. Fixed by null checks in compute-defs.
+
+(module a typed/racket
+  (define-type T (Rec T (-> (U T String))))
+  (provide f)
+  (: f (-> T T))
+  (define (f x) x))
+
+(require 'a)
+(f (lambda () ""))