patch 9.2.1172: Vim9: crash when lambda assigns to item of outer argument

Problem:  Assigning to an element of an outer function's argument inside
          a lambda causes a crash when the lambda is called.
Solution: When an outer function's argument is found on the left-hand
          side of an assignment, record that the outer scope is used and
          treat the lambda as a closure.

When an outer function's argument was used on the left-hand side of an
assignment, a LOADOUTER instruction was generated, but the lambda was
not treated as a closure. Consequently, the outer stack was not set for
the partial, leading to a NULL reference during the execution of
LOADOUTER.

fixes:  #21460
closes: #21465

Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_vim9_func.vim

@@ -3142,6 +3142,46 @@ def Test_closure_using_argument()
       assert_equal([3, 4, 5], Test(Inc))
   END
   v9.CheckScriptSuccess(lines)
+
+  # assign to an element of the outer function's argument
+  lines =<< trim END
+      vim9script
+      def MakeSetter(arg: dict<any>): func()
+        return () => {
+          arg.key = 1
+        }
+      enddef
+      def Run()
+        var Set = MakeSetter({})
+        Set()
+      enddef
+      Run()
+
+      var d: dict<any> = {}
+      MakeSetter(d)()
+      assert_equal({key: 1}, d)
+
+      def MakeListSetter(arg: list<number>): func()
+        return () => {
+          arg[0] = 2
+        }
+      enddef
+      var l = [0]
+      MakeListSetter(l)()
+      assert_equal([2], l)
+
+      def MakeNestedSetter(arg: dict<any>): func(): func()
+        return () => {
+          return () => {
+            arg.key = 3
+          }
+        }
+      enddef
+      var nd: dict<any> = {}
+      MakeNestedSetter(nd)()()
+      assert_equal({key: 3}, nd)
+  END
+  v9.CheckScriptSuccess(lines)
 enddef
 
 def s:MakeGetAndAppendRefs()

src/version.c

@@ -743,6 +743,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1172,
 /**/
     1171,
 /**/

src/vim9compile.c

@@ -183,7 +183,10 @@ arg_exists(
 									 == OK)
 	{
 	    if (gen_load_outer != NULL)
+	    {
 		++*gen_load_outer;
+		cctx->ctx_outer_used = TRUE;
+	    }
 	    return OK;
 	}
     }