Here-docs and $(...)

[Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>]

Discussion on the austin-group (POSIX) list indicates that this:

echo $(
cat <<\)
hello
)
)

Should be parsed as a here-document up through the second close paren, so
that the third close-paren matches the open paren to end the substitution.

Zsh gets this wrong, ending the command substitution at the second close.
There's probably no easy fix for this, as the here-document parser is not
invoked during scanning of the command substitution.