diff --git a/src/testdir/test_smartindent.vim b/src/testdir/test_smartindent.vim
index d00eac9..9e93a55 100644
--- a/src/testdir/test_smartindent.vim
+++ b/src/testdir/test_smartindent.vim
@@ -1,3 +1,4 @@
+" Tests for smartindent
 
 " Tests for not doing smart indenting when it isn't set.
 function! Test_nosmartindent()
@@ -12,3 +13,29 @@
   call assert_equal("		#test", getline(1))
   enew! | close
 endfunction
+
+function MyIndent()
+endfunction
+
+" When 'indentexpr' is set, setting 'si' has no effect.
+function Test_smartindent_has_no_effect()
+  new
+  exe "normal! i\<Tab>one\<Esc>"
+  set noautoindent
+  set smartindent
+  set indentexpr=
+  exe "normal! Gotwo\<Esc>"
+  call assert_equal("\ttwo", getline("$"))
+
+  set indentexpr=MyIndent
+  exe "normal! Gothree\<Esc>"
+  call assert_equal("three", getline("$"))
+
+  delfunction! MyIndent
+  set autoindent&
+  set smartindent&
+  set indentexpr&
+  bwipe!
+endfunction
+
+" vim: shiftwidth=2 sts=2 expandtab