From 122616d9c1b57f1e1f29f1151f8e26d24df9092a Mon Sep 17 00:00:00 2001
From: Bram Moolenaar <Bram@vim.org>
Date: Fri, 21 Aug 2020 21:32:50 +0200
Subject: patch 8.2.1502: Vim9: can use += with a :let command at script level

Problem:    Vim9: can use += with a :let command at script level.
Solution:   Give an error.
---
 src/evalvars.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

(limited to 'src/evalvars.c')

diff --git a/src/evalvars.c b/src/evalvars.c
index bff6086e53..335f850bf7 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -786,7 +786,13 @@ ex_let(exarg_T *eap)
 	    op[1] = NUL;
 	    if (*expr != '=')
 	    {
-		if (vim_strchr((char_u *)"+-*/%.", *expr) != NULL)
+		if (vim9script && (flags & LET_NO_COMMAND) == 0)
+		{
+		    // +=, /=, etc. require an existing variable
+		    semsg(_(e_cannot_use_operator_on_new_variable), eap->arg);
+		    i = FAIL;
+		}
+		else if (vim_strchr((char_u *)"+-*/%.", *expr) != NULL)
 		{
 		    op[0] = *expr;   // +=, -=, *=, /=, %= or .=
 		    ++len;
-- 
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