Regression in pattern substitution with compat42

[Tom Briden]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: x86_64-pc-linux-gnu-cc
Compilation CFLAGS: -march=native -O2 -pipe
uname output: Linux icarium.decompile.me.uk 6.2.0-rc7-18802-gadd2b733ea5d
#18 SMP PREEMPT_DYNAMIC Mon Feb  6 08:03:57 GMT 2023 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.2
Patch Level: 15
Release Status: release

Description:
  As of version 5.2-beta, replacing a single backslash with a double
backslash is no longer possible when using BASH_COMPAT=4.2.
  The issue appears to have been introduced in commit
https://git.savannah.gnu.org/cgit/bash.git/commit/?h=devel&id=2a1c81bf634d372fde5141d7e616042ca960d9b4
  which now returns `1` from `shouldexp_replacement`.

Repeat-By:
  BASH_COMPAT=4.2; a="backslash \\"; printf "%s\n" "${a//\\/\\\\}"

  Expected output: backslash \\
  Actual output: backslash \