Conditional call in Macro with gensym binding

gistfile1.clj

(defmacro foo
  [& [fn]]
  `(let [env# 123]
     ~(when fn `(~fn env#))))

;; then expanded form of env does not match:
(macroexpand '(foo identity)) ;=> (let* [env__17699__auto__ 123] (identity env__17698__auto__))

The problem: auto gensyms only work in the form they were created in. To solve the issue, manual gensyms are required.