It seems to fix the issue for :s/{\n but unfortunately the following still locks for me:

echo -ne '{\n\n{' | build/bin/nvim -u NORC -c "set inccommand=nosplit | set gdefault" -
:%s/

Yeah, you're right, thanks for testing. I can reproduce, but things look a bit weird... I can actually type :%s/{\n until the lockup occurs. Then, if I press <C-C>, then replace the brackets with the letter a, no lockup occurs. If I then replace the letter a by the letterb, the lockup occurs again, only that I can just type :%s/ until the lockup. Undoing get me no lockup, but going back to the bracket makes it appear again...

Weird stuff! Might be connected to the order in which I'm doing things...

How did you come to the conclusion that the lines you added were needed? I took a quick look using gdb and it seems that neovim gets stuck in nfa_regmax (src/nvim/regexp_nfa.c:5027). But I know nothing about the code so I might be very wrong here.

(Note: Got a post notification via email, I can't see your answer here, so I'm quoting it).

When I ran your case through gdb, it was stuck in this loop: https://github.com/KillTheMule/neovim/blob/e72bb7ba6303c7b56e508c99056de66d706d19b4/src/nvim/ex_cmds.c#L3419. This also makes sense because the loop contains a breakcheck, that's why we can interrupt with <C-c>. Then I ran a nonblocking case through the debugger and noticed a different behavior here, which really fit well: Final newline in the match, no second delimiter, and this code isn't run in the "no second delimiter" case, so I just added it and tried, and voila :) I did not fully understand it, in particular wrt gdefault, but wanted to get it out to you to try fast. Too fast, indeed ;)

I'll dive in again tonight or so.

Closed in favor of #7231.