Treat <expr> mapping as normal mapping

[linjiX]

• OS: Ubuntu 16.04

• Vim: Vi IMproved 8.1-1729

set nocompatible
let mapleader="\<Space>"

call plug#begin('~/.vim/plug')
Plug 'liuchengxu/vim-which-key'
call plug#end()

nnoremap <silent> <leader> :<C-u>WhichKey '<Space>'<CR>
let g:which_key_map = {}
call which_key#register('<Space>', "g:which_key_map")

function W() abort
    return ":echo 'test'\<CR>"
endfunction

nnoremap <expr> <leader>t W()

Problem Description

In the minimal vimrc above, a <expr> mapping is defined (<leader>t to echo 'test').
It works fine if you press <leader>t without a break.
But if you press <leader> and wait until which-key window appear, then press t, which-key will ignore <expr> and just execute W() in normal mode.