Fix letfn with duplicate function names

When letfn had duplicate names like (letfn [(a [] 1) (a [] 2)] (a)),
the inner let that calls var-get would try to deref the already-unwrapped
function value, causing a ClassCastException (fn cannot be cast to Future).

Fix: deduplicate syms in the inner var-get let block.

Author: borkdude

CHANGELOG.md

@@ -13,6 +13,7 @@ SCI is used in [babashka](https://github.com/babashka/babashka),
 ## Unreleased
 
 - Fix `read` with `nil` or `false` as eof-value throwing instead of returning the eof-value
+- Fix `letfn` with duplicate function names crashing with ClassCastException
 - `deftype` now macroexpands to `deftype*`, matching JVM Clojure, enabling code walkers like riddley
 - `case` now macroexpands to JVM-compatible `case*` format, enabling tools like riddley and cloverage to work with SCI
 - Fix `.method` on class objects (e.g. `(.getDeclaredField String "value")`) routing to static instead of instance method path

src/sci/impl/namespaces.cljc

@@ -314,14 +314,15 @@
              :cljs ::TODO)))
 
 (defn letfn* [_ _ fnspecs & body]
-  (let [syms (map first fnspecs)]
+  (let [syms (map first fnspecs)
+        unique-syms (distinct syms)]
     `(let ~(vec (interleave syms (repeat '(clojure.core/-new-var))))
        ~@(map (fn [sym fn-spec]
                 `(clojure.core/alter-var-root ~sym (constantly (fn ~sym ~@(rest fn-spec)))))
               syms fnspecs)
-       (let ~(vec (interleave syms (map (fn [sym]
-                                          `(clojure.core/var-get ~sym))
-                                        syms)))
+       (let ~(vec (interleave unique-syms (map (fn [sym]
+                                                 `(clojure.core/var-get ~sym))
+                                               unique-syms)))
          ~@body))))
 
 (defn with-local-vars* [form _ name-vals-vec & body]