syntax: a quoted heredoc processes backslash escapes #1390

A heredoc with a quoted delimiter must be literal — POSIX: "if the delimiter is quoted, the here-document lines shall not be expanded". Backslash escapes are processed anyway.

$ gosh -c "cat <<'X'
a\b c\$d
X"
a� c$d
$ bash -c "cat <<'X'
a\b c\$d
X"
a\b c\$d

That is exactly the escape set an unquoted heredoc processes (\, \$, \`), so the quoted form appears to be taking the unquoted path for backslashes only. Everything else about quoting is correct: $HOME, $(cmd), `cmd` and $((1+1)) all stay literal under <<'X' and all expand under <<X, matching bash.

The consequence is that cat > file <<'EOF' — the idiom whose whole point is that the content is not interpreted — writes something other than what it was given, with no error and exit 0. It bites anything with a doubled backslash or an escaped $: shell and awk scripts, regexes whose source spelling is \d (JS, Java, JSON), Windows paths, LaTeX, printf formats, Makefile recipes.

Adjacent and closed: #515, #557.

Found by a bash-vs-gosh differential sweep (128 snippets, each run direct and through eval) while integrating interp as the substrate of a shell. Reproduced on gosh@v3.13.2-0.20260813221317-a04df9f0d4b8 against bash 5.3.15 on darwin/arm64. Happy to send a PR if the direction looks right.