fix #18408, closure lowering of captured, never-assigned locals

src/julia-syntax.scm

@@ -2866,7 +2866,12 @@ f(x) = yt(x)
  (if (eqv? (string.char (string name) 0) #\@)
  (error "macro definition not allowed inside a local scope"))))
  (if lam2
- (lambda-optimize-vars! lam2))
+ (begin
+ ;; mark all non-arguments as assigned, since locals that are never assigned
+ ;; need to be handled the same as those that are (i.e., boxed).
+ (for-each (lambda (vi) (vinfo:set-asgn! vi #t))
+ (list-tail (car (lam:vinfo lam2)) (length (lam:args lam2))))
+ (lambda-optimize-vars! lam2)))
  (if (not local) ;; not a local function; will not be closure converted to a new type
  (cond (short e)
  ((null? cvs)
@@ -2992,6 +2997,8 @@ f(x) = yt(x)
  '(null)
  (convert-assignment name mk-closure fname lam interp)))))))
  ((lambda) ;; should only happen inside (thunk ...)
+ (for-each (lambda (vi) (vinfo:set-asgn! vi #t))
+ (car (lam:vinfo e)))
  `(lambda ,(cadr e)
  (,(clear-capture-bits (car (lam:vinfo e)))
  () ,@(cddr (lam:vinfo e)))

test/core.jl

@@ -478,6 +478,14 @@ function capt_before_def()
 end
 @test capt_before_def()() == 2
 
+function i18408()
+ local i
+ x->i
+end
+let f = i18408()
+ @test_throws UndefRefError f(0)
+end
+
 # variable scope, globals
 glob_x = 23
 function glotest()