NL character removed after \\ in command substitution

[Haojun Bao]

Configuration Information [Automatically generated, do not change]:
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-security
uname output: Linux bhj-pc1 5.10.0-0.bpo.5-amd64 #1 SMP Debian
5.10.24-1~bpo10+1 (2021-03-29) x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

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

Description:

A bug found in parse.y, that will treat reading of COMMAND and
$(COMMAND) differently, despite the info manual saying that:

> When the old-style backquote form of substitution is used, backslash
> retains its literal meaning except when followed by '$', '`', or '\'.
> The first backquote not preceded by a backslash terminates the command
> substitution.  When using the '$(COMMAND)' form, all characters between
> the parentheses make up the command; none are treated specially.

It seems the NL after \\ will be removed when run as $(COMMAND) in parse.y?

Repeat-By:

This command will output $'hello \\\nworld\n':

cat <<EOF
hello \\
world
EOF

This command will output $'hello \\world\n' (missing the \n after \\):
echo "$(
cat <<EOF
hello \\
world
EOF
)"