change +nan.0 and +nan.f, when viewed as contracts, to

be equal?-based contracts instead of = based contracts.

Before this change, the contract (or/c 1 2 +nan.0) was the same
contract as (or/c 1 2), because +nan.0 was the same contract as
the predicate (lambda (x) (= x +nan.0)), which is the same as
(lambda (x) #f). Now, +nan.0 and +nan.f are the only numbers
that are treated as equal?-based contracts, but this means that
(or/c 1 2 +nan.0) actually accepts +nan.0.

pkgs/racket-doc/scribblings/reference/contracts.scrbl

@@ -56,10 +56,12 @@ ordinary Racket values that double as contracts, including
 @racket[null], which are treated as contracts that recognize
 themselves, using @racket[eq?], }
 
-@item{@tech{strings} and @tech{byte strings}, which are treated as contracts
+@item{@tech{strings}, @tech{byte strings}, @racket[+nan.0], and
+       @racket[+nan.f], which are treated as contracts
 that recognize themselves using @racket[equal?], }
 
-@item{@tech{numbers}, which are treated as contracts
+@item{@tech{numbers} (except @racket[+nan.0] and
+       @racket[+nan.f]), which are treated as contracts
 that recognize themselves using @racket[=],}
 
 @item{@tech{regular expressions}, which are treated as contracts
@@ -114,6 +116,9 @@ For more about this hierarchy, see @tech{chaperones} and
 a research paper on chaperones, impersonators, and how they can be used to 
 implement contracts @cite{Strickland12}.
 
+@history[#:changed "6.1.1.8" @list{Changed @racket[+nan.0] and @racket[+nan.f] to
+                                           be @racket[equal?]-based contracts.}]
+
 @local-table-of-contents[]
 
 @; ----------------------------------------

pkgs/racket-test/tests/racket/contract-rand-test.rkt

@@ -27,6 +27,7 @@
 
 ;; test =, eq?, and equal? contract random generators
 (check-not-exn (λ () (test-contract-generation 0)))
+(check-not-exn (λ () (test-contract-generation +nan.0)))
 (check-not-exn (λ () (test-contract-generation 'x)))
 (check-not-exn (λ () (test-contract-generation "x")))
 

pkgs/racket-test/tests/racket/contract/flat-contracts.rkt

@@ -58,6 +58,8 @@
   (test-flat-contract "x" "x" 'x)
   (test-flat-contract 1 1 2)
   (test-flat-contract #e1 #i1.0 'x)
+  (test-flat-contract +nan.0 +nan.0 +nan.f)
+  (test-flat-contract +nan.f +nan.f +nan.0)
   (test-flat-contract #rx".x." "axq" "x")
   (test-flat-contract #rx#".x." #"axq" #"x")
   (test-flat-contract #rx".x." #"axq" #"x")

racket/collects/racket/contract/private/guts.rkt

@@ -251,7 +251,7 @@
                                  `',x
                                  x)
                              name))]
-      [(or (bytes? x) (string? x))
+      [(or (bytes? x) (string? x) (equal? +nan.0 x) (equal? +nan.f x))
        (make-equal-contract x (if (name-default? name) x name))]
       [(number? x)
        (make-=-contract x (if (name-default? name) x name))]

racket/collects/racket/contract/private/opt.rkt

@@ -85,7 +85,7 @@
       [(or (boolean? konst) (char? konst) (null? konst))
        (values #`(eq? #,konst #,v)
                "eq?")]
-      [(or (string? konst) (bytes? konst))
+      [(or (string? konst) (bytes? konst) (equal? konst +nan.0) (equal? konst +nan.f))
        (values #`(equal? #,konst #,v)
                "equal?")]
       [(number? konst)