diff --git a/src/testdir/test_let.vim b/src/testdir/test_let.vim
index 24c6ef5..550655c 100644
--- a/src/testdir/test_let.vim
+++ b/src/testdir/test_let.vim
@@ -25,3 +25,124 @@
   let s = "\na                     #1\nb                     #2"
   call assert_equal(s, out)
 endfunc
+
+func s:set_arg1(a) abort
+  let a:a = 1
+endfunction
+
+func s:set_arg2(a) abort
+  let a:b = 1
+endfunction
+
+func s:set_arg3(a) abort
+  let b = a:
+  let b['a'] = 1
+endfunction
+
+func s:set_arg4(a) abort
+  let b = a:
+  let b['a'] = 1
+endfunction
+
+func s:set_arg5(a) abort
+  let b = a:
+  let b['a'][0] = 1
+endfunction
+
+func s:set_arg6(a) abort
+  let a:a[0] = 1
+endfunction
+
+func s:set_arg7(a) abort
+  call extend(a:, {'a': 1})
+endfunction
+
+func s:set_arg8(a) abort
+  call extend(a:, {'b': 1})
+endfunction
+
+func s:set_arg9(a) abort
+  let a:['b'] = 1
+endfunction
+
+func s:set_arg10(a) abort
+  let b = a:
+  call extend(b, {'a': 1})
+endfunction
+
+func s:set_arg11(a) abort
+  let b = a:
+  call extend(b, {'b': 1})
+endfunction
+
+func s:set_arg12(a) abort
+  let b = a:
+  let b['b'] = 1
+endfunction
+
+func Test_let_arg_fail()
+  call assert_fails('call s:set_arg1(1)', 'E46:')
+  call assert_fails('call s:set_arg2(1)', 'E461:')
+  call assert_fails('call s:set_arg3(1)', 'E46:')
+  call assert_fails('call s:set_arg4(1)', 'E46:')
+  call assert_fails('call s:set_arg5(1)', 'E46:')
+  call s:set_arg6([0])
+  call assert_fails('call s:set_arg7(1)', 'E742:')
+  call assert_fails('call s:set_arg8(1)', 'E742:')
+  call assert_fails('call s:set_arg9(1)', 'E461:')
+  call assert_fails('call s:set_arg10(1)', 'E742:')
+  call assert_fails('call s:set_arg11(1)', 'E742:')
+  call assert_fails('call s:set_arg12(1)', 'E461:')
+endfunction
+
+func s:set_varg1(...) abort
+  let a:000 = []
+endfunction
+
+func s:set_varg2(...) abort
+  let a:000[0] = 1
+endfunction
+
+func s:set_varg3(...) abort
+  let a:000 += [1]
+endfunction
+
+func s:set_varg4(...) abort
+  call add(a:000, 1)
+endfunction
+
+func s:set_varg5(...) abort
+  let a:000[0][0] = 1
+endfunction
+
+func s:set_varg6(...) abort
+  let b = a:000
+  let b[0] = 1
+endfunction
+
+func s:set_varg7(...) abort
+  let b = a:000
+  let b += [1]
+endfunction
+
+func s:set_varg8(...) abort
+  let b = a:000
+  call add(b, 1)
+endfunction
+
+func s:set_varg9(...) abort
+  let b = a:000
+  let b[0][0] = 1
+endfunction
+
+func Test_let_varg_fail()
+  call assert_fails('call s:set_varg1(1)', 'E46:')
+  call assert_fails('call s:set_varg2(1)', 'E742:')
+  call assert_fails('call s:set_varg3(1)', 'E46:')
+  call assert_fails('call s:set_varg4(1)', 'E742:')
+  call s:set_varg5([0])
+  call assert_fails('call s:set_varg6(1)', 'E742:')
+  " call assert_fails('call s:set_varg7(1)', 'E46:')
+  call assert_fails('call s:set_varg8(1)', 'E742:')
+  call s:set_varg9([0])
+endfunction