bash 5.2 bad substitution for ${x:+${x}[}

[Martin von Gagern]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -g -O2
uname output: Linux … 5.18.16-…-amd64 #1 SMP PREEMPT_DYNAMIC Debian
5.18.16-… (2022-…) x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.2
Patch Level: !PATCHLEVEL!
Release Status: release

Description:

Some substitutions that used to work pre 5.2 are failing starting in
5.2. I did a git bisect on that, it's the initial 5.2 commit which
broke this, 
https://git.savannah.gnu.org/cgit/bash.git/commit/?h=bash-5.2&id=74091dd4e8086db518b30df7f222691524469998.

According to my reproducing example below, this appears to be due to a
combination of conditional substitution, a variable enclosed in curly
braces and an unbalanced opening square bracket that I think should
only be considered as text without any semantic relevance in this
position.

Repeat-By:

bash -c 'x=""; echo "a${x:+${x}[}b"'

Work-around:

Omit the inner curly braces, i.e. use "a${x:+$x[}b".