bash seems confused about it's state after unclosed single quotes in nested command substitution

[Martin Schulte]

Hello bash-bughunters,

please consider the following interactive lines:

$ echo $BASH_VERSION 
5.1.4(1)-release
$ uname -a
Linux t1 5.10.0-13-amd64 #1 SMP Debian 5.10.106-1 (2022-03-17) x86_64 GNU/Linux
$ echo $BASH_VERSION
5.1.4(1)-release
$ sleep $(expr 60 - $(date +%s')) ; date
bash: command substitution: line 417: unexpected EOF while looking for matching 
`''
bash: command substitution: line 418: syntax error: unexpected end of file
bash: unexpected EOF while looking for matching `)'
$ 

At this point it looks as if bash has encountered the problem and thus not 
executed the command line - fine.

Things get strange when you enter a command, e.g. echo:

$ echo
>

To get out of this you can enter the missing single quote followed by two 
closing braces:

> '))
expr: non-integer argument
sleep: missing operand
Try 'sleep --help' for more information
$

Now you can go on as expected, the last command in history is $'echo\n\'))' and 
the 'sleep ...' is not in the history at all.

I would either have expected to get PS2 and no error messages after entering 
the line starting with sleep or being able to just enter the next command.

Best regards

Martin