defn rejects arity with args vector [&form]

Question

Here's the smallest case, but it seems to happen no matter how many other arities are provided or whether list or vector syntax is used for the arity.

user=> (clojure-version)
"1.12.0-rc1"
user=> (defn f [&form])
Unexpected error (IndexOutOfBoundsException) macroexpanding defn at (REPL:1:1).
null
user=> *e
#error {
 :cause nil
 :via
 [{:type clojure.lang.Compiler$CompilerException
   :message "Unexpected error macroexpanding defn at (1:1)."
   :data #:clojure.error{:phase :macroexpansion, :line 1, :column 1, :source "NO_SOURCE_PATH", :symbol defn}
   :at [clojure.lang.Compiler macroexpand1 "Compiler.java" 7551]}
  {:type java.lang.IndexOutOfBoundsException
   :message nil
   :at [clojure.lang.RT subvec "RT.java" 1634]}]
 :trace
 [[clojure.lang.RT subvec "RT.java" 1634]
  [clojure.core$sigs$asig__5505 invoke "core.clj" 235]
  [clojure.core$sigs invokeStatic "core.clj" 259]
  [clojure.core$defn__5514 invokeStatic "core.clj" 317]
  [clojure.core$defn__5514 doInvoke "core.clj" 294]
  [clojure.lang.RestFn applyTo "RestFn.java" 149]
  [clojure.lang.Var applyTo "Var.java" 707]
  [clojure.lang.Compiler macroexpand1 "Compiler.java" 7525]
  [clojure.lang.Compiler macroexpand "Compiler.java" 7598]
  [clojure.lang.Compiler eval "Compiler.java" 7684]
  [clojure.lang.Compiler eval "Compiler.java" 7655]
  [clojure.core$eval invokeStatic "core.clj" 3232]
  [clojure.core$eval invoke "core.clj" 3228]
  [clojure.main$repl$read_eval_print__9240$fn__9243 invoke "main.clj" 437]
  [clojure.main$repl$read_eval_print__9240 invoke "main.clj" 437]
  [clojure.main$repl$fn__9249 invoke "main.clj" 459]
  [clojure.main$repl invokeStatic "main.clj" 459]
  [clojure.main$repl_opt invokeStatic "main.clj" 523]
  [clojure.main$main invokeStatic "main.clj" 668]
  [clojure.main$main doInvoke "main.clj" 617]
  [clojure.lang.RestFn invoke "RestFn.java" 400]
  [clojure.lang.AFn applyToHelper "AFn.java" 152]
  [clojure.lang.RestFn applyTo "RestFn.java" 135]
  [clojure.lang.Var applyTo "Var.java" 707]
  [clojure.main main "main.java" 40]]}

Answer 1

Logged as https://clojure.atlassian.net/browse/CLJ-2874

Answer 2

Well, &form and &env are special arg names so ... don't do that?

https://clojure.org/reference/macros#_special_variables

Seems akin to anaphors in anaphoric macros.

Comments

They're special arg names in defmacro forms. Why would they be special in defn forms?

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Macros are functions too. In this case I think it actually comes up while building the arglists inside defn (a macro) during macroexpansion.

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I would expect that defn calls have different semantics from defmacro calls, even tho they rely on the same underlying machinery. Maybe "build the signatures of a macro (elide first two args)" can be performed in a different way than "build the signatures of a function (use all args)".