erase object when inst is present (#572)

typed-racket-lib/typed-racket/typecheck/tc-expression.rkt

@@ -58,7 +58,11 @@
      number))
   (match tc-res
     [(tc-results: (list (tc-result: t ps o)) #f)
-     (ret (inst-type t inst) ps o)]
+     ;; we erase 'o' -- if they bothered to put an instantiation,
+     ;; odds are this is not something where 'o' matters, and leaving
+     ;; 'o' can cause complications (see TR gh issue 561) -- maybe there's
+     ;; a better way? this seems totally fine for now
+     (ret (inst-type t inst) ps -empty-obj)]
     [_ (error-case (if (and (tc-results? tc-res)
                             (null? (tc-results-ts tc-res)))
                        0

typed-racket-test/unit-tests/typecheck-tests.rkt

@@ -1188,7 +1188,7 @@
                                  ((-lst y) y)
                                  . ->... . (-lst z)))
                                : -true-propset
-                               : (-id-path #'map)))]
+                               : -empty-obj))]
 
         ;; error tests
         [tc-err (+ 3 #f)]
@@ -3966,6 +3966,10 @@
        ;; * Check that when passed as a #:∀ type, the inferred type is Char
        [tc-e #\b -Char]
        [tc-e (ann #\b Char) -Char]
+       ;; TR github issue 561
+       [tc-e (let ([a (inst cons Integer Integer)])
+               (cons #f #f))
+             (-pair -False -False)]
        )
 
   (test-suite