Only generate poly contracts for functions

Closes PR 13815
(cherry picked from commit bafaf52056)

collects/tests/typed-racket/fail/pr13815.rkt

@@ -0,0 +1,6 @@
+#;
+(exn:pred #rx"Type (All (a) Flonum) could not be converted to a contract")
+#lang typed/racket
+(require/typed racket/base [list (All (a) Float)])
+(* 3.3 list)
+

collects/tests/typed-racket/unit-tests/contract-tests.rkt

@@ -21,7 +21,13 @@
               (t (-Number . -> . -Number))
               (t (-Promise -Number))
               (t (-set Univ)) 
-              ))
+              ;; Adapted from PR 13815
+              (t (-poly (a) (-> a a)))
+              (t (-poly (a) (-mu X (-> a X))))
+              (t (-poly (a) (-poly (b) (-> a a))))
+              (t (-poly (a) (-App (-poly (b) (-> a a)) (list -Number) #'#f)))
+              (t/fail (-poly (a) -Flonum))
+              (t/fail (-poly (a) (-set -Number)))))
 
 (define-go contract-tests)
 (provide contract-tests)

collects/typed-racket/private/type-contract.rkt

@@ -375,6 +375,17 @@
         [(F: v) (cond [(assoc v (vars)) => second]
                       [else (int-err "unknown var: ~a" v)])]
         [(Poly: vs b)
+         ;; Don't generate poly contracts for non-functions
+         (define function-type?
+           (let loop ([ty ty])
+             (match (resolve ty)
+               [(Function: _) #t]
+               [(Union: elems) (andmap loop elems)]
+               [(Poly: _ body) (loop body)]
+               [(PolyDots: _ body) (loop body)]
+               [_ #f])))
+         (unless function-type?
+           (exit (fail)))
          (if (not (from-untyped? typed-side))
              ;; in typed positions, no checking needed for the variables
              (parameterize ([vars (append (for/list ([v (in-list vs)]) (list v #'any/c)) (vars))])