fix `read-language' exn construction in an EOF case

Closes PR 11683
 Merge to 5.1
(cherry picked from commit dd5f0dfc80)

collects/tests/racket/read.rktl

@@ -1103,6 +1103,13 @@
 ;; Check error-message formatting:
 (err/rt-test (read (open-input-string "#l"))
              (lambda (exn) (regexp-match? #rx"`#l'" (exn-message exn))))
+;; Make sure read-language error here is this can comes from read-language
+;; and not from an ill-formed srcloc construction:
+(let ()
+  (define p (open-input-string ";\n"))
+  (port-count-lines! p)
+  (err/rt-test (read-language p)
+               (lambda (exn) (regexp-match? #rx"read-language" (exn-message exn)))))
 
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 

src/racket/src/read.c

@@ -992,6 +992,13 @@ read_inner_inner(Scheme_Object *port, Scheme_Object *stxsrc, Scheme_Hash_Table *
     dispatch_ch = ch;
 
   if (get_info && (dispatch_ch != '#') && (dispatch_ch != ';')) {
+    /* If ch is EOF, then col or pos wasn't incremented by reading ch.
+       The col and pos might be used in an error message, which expects
+       to subtract one from each --- so counteract by adding one here. */
+    if (ch == EOF) {
+      if (pos >= 0) pos++;
+      if (col >= 0) col++;
+    }
     return expected_lang("", ch, port, stxsrc, line, col, pos, get_info);
   }