Bad substitution breaks conditional statement

[Martin Schulte]

Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -g -O2 -fdebug-prefix-map=/build/bash-2bxm7h/bash-5.0=. 
-fstack-protector-strong -Wformat -Werror=format-security -Wall 
-Wno-parentheses -Wno-format-sec$
uname output: Linux t1 4.19.0-10-amd64 #1 SMP Debian 4.19.132-1 (2020-07-24) 
x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.0
Patch Level: 3
Release Status: release

Description:

It looks as if a bad substitution detected inside a case of if continues the 
script flow immediately after the conditional statement skipping the remaining 
statements inside the conditional. Please take a look into section "Repeat-By:".

I detected this when running a script containing "${OPTARG@Q}" with a bash 
4.1.5 where @Q is not yet supported - the script continued to run in a bad 
manner so this bug might cause real troubles...

Repeat-By:

schulte@t1:~$ cat bad_substitutions_breaks_conditional 
#!/bin/bash

x=y
case $x in
  y)
    echo "x has value ${x@q} - stop now!"
    exit 1;;
esac

echo "running"

if [[ $x == y ]]; then
  echo "x has value ${x@q} - stop now!"
  exit 1
fi

echo "still running"

echo "x has value ${x@q} - stop now!"
exit 1

echo "and running"
schulte@t1:~$ ./bad_substitutions_breaks_conditional 
./bad_substitutions_breaks_conditional: line 6: x has value ${x@q} - stop now!: 
bad substitution
running
./bad_substitutions_breaks_conditional: line 11: x has value ${x@q} - stop 
now!: bad substitution
still running
./bad_substitutions_breaks_conditional: line 15: x has value ${x@q} - stop 
now!: bad substitution