patch 8.2.0483: Vim9: "let x = x + 1" does not give an error

Problem:    Vim9: "let x = x + 1" does not give an error.
Solution:   Hide the variable when compiling the expression.

src/testdir/test_vim9_expr.vim

@@ -738,6 +738,8 @@ def Test_expr7_dict()
   call CheckDefFailure("let x = {'a': xxx}", 'E1001:')
   call CheckDefFailure("let x = {xxx: 8}", 'E1001:')
   call CheckDefFailure("let x = #{a: 1, a: 2}", 'E721:')
+  call CheckDefFailure("let x += 1", 'E1020:')
+  call CheckDefFailure("let x = x + 1", 'E1001:')
   call CheckDefExecFailure("let x = g:anint.member", 'E715:')
   call CheckDefExecFailure("let x = g:dict_empty.member", 'E716:')
 enddef

src/version.c

@@ -738,6 +738,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    483,
 /**/
     482,
 /**/

src/vim9compile.c

@@ -3685,6 +3685,8 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
     }
     else if (oplen > 0)
     {
+	int r;
+
 	// for "+=", "*=", "..=" etc. first load the current value
 	if (*op != '=')
 	{
@@ -3717,10 +3719,16 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 	    }
 	}
 
-	// compile the expression
+	// Compile the expression.  Temporarily hide the new local variable
+	// here, it is not available to this expression.
+	if (idx >= 0)
+	    --cctx->ctx_locals.ga_len;
 	instr_count = instr->ga_len;
 	p = skipwhite(p + oplen);
-	if (compile_expr1(&p, cctx) == FAIL)
+	r = compile_expr1(&p, cctx);
+	if (idx >= 0)
+	    ++cctx->ctx_locals.ga_len;
+	if (r == FAIL)
 	    goto theend;
 
 	if (idx >= 0 && (is_decl || !has_type))