Substitution with an expression breaks messages

[chocolateboy]

Vim: 8.0
OS: Linux (Arch)
eregex: 950027d

Messages created with echom(sg) aren't displayed correctly or saved to the message history when using :%S/<pattern>/\=<expression>/.

echom-vimrc

call plug#begin()
Plug 'othree/eregex.vim'
call plug#end()

set nocompatible

" echo the supplied arg and return it unchanged
function! Echo(arg)
    echomsg 'arg: ' . a:arg
    return a:arg
endfunction

steps to reproduce

native (:substitute)

• open a file with vim -u echom-vimrc test.txt
• insert the word "foo", and replace it with: :%s/foo/\=Echo('bar')/
• undo the change, then re-run the substitution
• the message ("arg: foo") is displayed after each substitution, and appears twice in the output of :messages

eregex (:S)

• open a file with vim -u echom-vimrc test.txt
• insert the word "foo", and replace it with: :%S/foo/\=Echo('bar')/
• undo the change, then re-run the substitution
• the message ("arg: foo") is only displayed after the first substitution, and doesn't appear at all in the output of :messages

[othree]

I can reproduce and I got 0 message. Still looking.

[othree]

It use silent to execute substitute ref: http://vimdoc.sourceforge.net/htmldoc/various.html#:silent

[othree]

Not sure why original author try to manage error by itsself.
https://github.com/othree/eregex.vim/blob/master/plugin/eregex_e.vim#L4-L6
https://github.com/othree/eregex.vim/blob/master/plugin/eregex.vim#L844-L873