patch 8.1.0648: custom operators can't act upon a forced motion

Problem:    Custom operators can't act upon a forced motion. (Christian
            Wellenbrock)
Solution:   Add the forced motion to the mode() result. (Christian Brabandt,
            closes #3490)

src/testdir/test_mapping.vim

@@ -230,3 +230,57 @@ func Test_cabbr_visual_mode()
   call assert_equal(expected, getreg(':'))
   cunabbr s
 endfunc
+
+func Test_motionforce_omap()
+  func GetCommand()
+    let g:m=mode(1)
+    let [g:lnum1, g:col1] = searchpos('-', 'Wb')
+    if g:lnum1 == 0
+        return "\<Esc>"
+    endif
+    let [g:lnum2, g:col2] = searchpos('-', 'W')
+    if g:lnum2 == 0
+        return "\<Esc>"
+    endif
+    return ":call Select()\<CR>"
+  endfunc
+  func Select()
+    call cursor([g:lnum1, g:col1])
+    exe "normal! 1 ". (strlen(g:m) == 2 ? 'v' : g:m[2])
+    call cursor([g:lnum2, g:col2])
+    execute "normal! \<BS>"
+  endfunc
+  new
+  onoremap <buffer><expr> i- GetCommand()
+  " 1) default omap mapping
+  %d_
+  call setline(1, ['aaa - bbb', 'x', 'ddd - eee'])
+  call cursor(2, 1)
+  norm di-
+  call assert_equal('no', g:m)
+  call assert_equal(['aaa -- eee'], getline(1, '$'))
+  " 2) forced characterwise operation
+  %d_
+  call setline(1, ['aaa - bbb', 'x', 'ddd - eee'])
+  call cursor(2, 1)
+  norm dvi-
+  call assert_equal('nov', g:m)
+  call assert_equal(['aaa -- eee'], getline(1, '$'))
+  " 3) forced linewise operation
+  %d_
+  call setline(1, ['aaa - bbb', 'x', 'ddd - eee'])
+  call cursor(2, 1)
+  norm dVi-
+  call assert_equal('noV', g:m)
+  call assert_equal([''], getline(1, '$'))
+  " 4) forced blockwise operation
+  %d_
+  call setline(1, ['aaa - bbb', 'x', 'ddd - eee'])
+  call cursor(2, 1)
+  exe "norm d\<C-V>i-"
+  call assert_equal("no\<C-V>", g:m)
+  call assert_equal(['aaabbb', 'x', 'dddeee'], getline(1, '$'))
+  bwipe!
+  delfunc Select
+  delfunc GetCommand
+endfunc