write the file before using gocode in module mode

autoload/go/complete.vim

@@ -2,6 +2,15 @@ function! s:gocodeCommand(cmd, args) abort
   let l:gocode_bin = "gocode"
   let l:gomod = go#util#gomod()
   if filereadable(l:gomod)
+    " Save the file when in module mode so that go list can read the
+    " imports. If the user doesn't have autowrite or autorwriteall enabled,
+    " they'll need to write the file manually to get reliable results.
+    " See https://github.com/fatih/vim-go/pull/1988#issuecomment-428576989.
+    "
+    " TODO(bc): don't save the file when in module mode once
+    " golang.org/x/tools/go/packages has support for an overlay and it's used
+    " by gocode.
+    call go#cmd#autowrite()
     let l:gocode_bin = "gocode-gomod"
   endif