Bug with assignments in some commands

[Patrick Reader <zsh578@xxxxxxxxxx>]

Hi there,

I believe I have found a bug: in arithmetic $(( )) or $[ ] and variable 
expansions ${ } in some forms of command (pipelines, command 
substitutions, and inside here-documents and here-strings are the cases 
I know of so far), assignments to variables do not persist outside the 
command, almost as if it were in a subshell. I'm pretty sure this is a 
bug; if it's an intended behaviour, it's quite annoying and should be 
changed.

This does not occur in complex commands (if, for, while, etc.) except 
subshells.

This occurs on both zsh versions I have tested: 5.8 
(x86_64-pc-linux-gnu) and 5.6.2 (x86_64-redhat-linux-gnu).

Attached is a reproducible example, and the output of `set` and `set -o`.

Regards,

Patrick

# this increments x
x=1
echo "$((x++)), x is $x"
echo x is correct: $x

echo '========='

# but this doesn't change x outside the pipeline
x=1
echo "$((x++)), x is $x" | cat
echo x is still: $x

echo '========='

# same thing with non-arithmetic assignments ${name::=value}
# this works
x=1
echo "${x::=2}, x is $x"
echo x is correct: $x

echo '========='

# but this doesn't
x=1
echo "${x::=2}, x is $x" | cat
echo x is still: $x

echo '========='

x=1
echo $(echo "$((x++)), x is $x")
echo x is still: $x

echo '========='

x=1
<<<"$((x++)), x is $x"
echo x is still: $x

echo '========='

x=1
cat <<<"$((x++))"
echo x is still: $x

echo '========='

x=1
<<END
$((x++)), x is $x
END
echo x is still: $x

echo '========='

x=1
cat <<END
$((x++)), x is $x
END
echo x is still: $x

!=0
'#'=0
'$'=16129
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