patch 8.2.1731: Vim9: cannot use += to append to empty NULL list
Problem: Vim9: cannot use += to append to empty NULL list.
Solution: Copy the list instead of extending it. (closes #6998)
diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index 852db4a..4c825e8 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -223,6 +223,20 @@
endif
enddef
+def Test_extend_list()
+ let lines =<< trim END
+ vim9script
+ let l: list<number>
+ l += [123]
+ assert_equal([123], l)
+
+ let d: dict<number>
+ d['one'] = 1
+ assert_equal(#{one: 1}, d)
+ END
+ CheckScriptSuccess(lines)
+enddef
+
def Test_single_letter_vars()
# single letter variables
let a: number = 123