Langmap breaks <Plug> Mappings

[SirVer]

This surfaced while investigating SirVer/ultisnips#1114.

Describe the bug
Using this minimal vimrc:

set nocompatible
inoremap <Plug>(Blub) <C-R>=Foobla()<cr>
imap <Tab> <Plug>(Blub)

" The next two lines are the interesting ones.
" set langmap=ln
" set langnoremap " <- This does not change anything.

And opening vim, hitting <tab> in insert mode:

$ vim -u vim.config
i<tab>

gives the expected error: Unknown function: Foobla. Commenting in the langmap line however changes the output: The literal string <Plug>(Blub) is inserted into the buffer.

Expected behavior
The langnoremap option does not change the behavior inside Vim, i.e. I'd still expect the Unknown function error in all cases for langmap. I am unsure about the job of langnoremap, but it does not affect this particular case.

Screenshots
Here is a screencap: https://asciinema.org/a/5UVsrorS8xsGbXmqRyprLVX2I, of course the pressing of <tab> is invisible here.

Environment (please complete the following information):
Output of vim --version:

VIM - Vi IMproved 8.1 (2018 May 18, compiled Oct 19 2019 12:50:19)
Included patches: 1
Compiled by root@390aeaaa713b
Huge version without GUI.  Features included (+) or not (-):
+acl               +farsi             +mouse_sgr         -tag_any_white
+arabic            +file_in_path      -mouse_sysmouse    -tcl
+autocmd           +find_in_path      +mouse_urxvt       +termguicolors
-autoservername    +float             +mouse_xterm       +terminal
-balloon_eval      +folding           +multi_byte        +terminfo
+balloon_eval_term -footer            +multi_lang        +termresponse
-browse            +fork()            -mzscheme          +textobjects
++builtin_terms    -gettext           +netbeans_intg     +timers
+byte_offset       -hangul_input      +num64             +title
+channel           +iconv             +packages          -toolbar
+cindent           +insert_expand     +path_extra        +user_commands
-clientserver      +job               -perl              +vertsplit
-clipboard         +jumplist          +persistent_undo   +virtualedit
+cmdline_compl     +keymap            +postscript        +visual
+cmdline_hist      +lambda            +printer           +visualextra
+cmdline_info      +langmap           +profile           +viminfo
+comments          +libcall           -python            +vreplace
+conceal           +linebreak         +python3           +wildignore
+cryptv            +lispindent        +quickfix          +wildmenu
+cscope            +listcmds          +reltime           +windows
+cursorbind        +localmap          +rightleft         +writebackup
+cursorshape       -lua               -ruby              -X11
+dialog_con        +menu              +scrollbind        -xfontset
+diff              +mksession         +signs             -xim
+digraphs          +modify_fname      +smartindent       -xpm
-dnd               +mouse             +startuptime       -xsmp
-ebcdic            -mouseshape        +statusline        -xterm_clipboard
+emacs_tags        +mouse_dec         -sun_workshop      -xterm_save
+eval              -mouse_gpm         +syntax            
+ex_extra          -mouse_jsbterm     +tag_binary        
+extra_search      +mouse_netterm     +tag_old_static    
   system vimrc file: "$VIM/vimrc"
     user vimrc file: "$HOME/.vimrc"
 2nd user vimrc file: "~/.vim/vimrc"
      user exrc file: "$HOME/.exrc"
       defaults file: "$VIMRUNTIME/defaults.vim"
  fall-back for $VIM: "/usr/local/share/vim"
Compilation: gcc -c -I. -Iproto -DHAVE_CONFIG_H     -I/usr/local/include/python3.6m -I/usr/local/include/python3.6m  -Wno-unused-result -Wsign-compare  -DNDEBUG -g -fwrapv -O3 -Wall -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1       
Linking: gcc   -L/usr/local/lib -Wl,--as-needed -o vim        -lm -lnsl   -lncurses -ldl     -L/usr/local/lib/python3.6/config-3.6m-x86_64-linux-gnu -lpython3.6m -lpthread -ldl -lutil -lm      

• OS: Ubuntu 18.04, but reproducible in Mac OS as well (see travis runs for the UltiSnips issue).
• Terminal: tested in MacVim & vim inside ITerm2 & vim inside tmux inside docker.

[SirVer]

/cc @blueyed

Daniel, this seems to be the root bug behind why the <Plug> mappings do not work as expected in UltiSnips.

[chrisbra]

I think this is essentially the same as #3018

The fix would need a bigger refactoring, thus I haven't tried it yet.

[SirVer]

@chrisbra The symptom seems very different (since there are only ASCII letters involved here), but the underlying cause might of course be the same.

[zeertzjq]

There seems to be a fix for this in neovim/neovim#7919 and Bram has been pinged in that PR.

[brammool]

There seems to be a fix for this in neovim/neovim#7919 and Bram has been pinged in that PR.

There have been some problems around 'langmap', it requires more investigation. You can find other issues for 'langmap', not sure if they are different or related. Perhaps someone can make a PR and then we can try out the reported problems and see what is fixed and what not. And check it doesn't create new problems or changes behavior.

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