[Bug?]: Portable shell mishandles `$@`

[gibson042]

Self-service

• I'd be willing to implement a fix

Describe the bug

From the yarn scripting documentation:

Of course, you can also use the $@ array to retrieve the list of all arguments, which will let you reuse them in multiple commands:

{
  "scripts": {
    "build-and-test": "yarn build \"$@\" && yarn test \"$@\""
  }
}

But I'm seeing improper handling of unquoted $@, in particular that separating whitespace is absent (and also that even when quoted, an immediately following adjacent string is incorrectly treated as a separate argument).

To reproduce

Comprehensive exploration:

$ for s in dollarat dollarat-quoted dollarat-adjacent dollarat-quoted-adjacent; do
    printf '# "%s": %s\n' "$s" "$(node -p "JSON.stringify(require('./package.json').scripts['$s'])")"
    yarn run "$s" arg1 'arg2  has  spaces' arg3
  done
# "dollarat": "node -p 'process.argv.slice(1)' -- $@"
[ 'arg1arg2', 'has', 'spacesarg3' ]
# "dollarat-quoted": "node -p 'process.argv.slice(1)' -- \"$@\""
[ 'arg1', 'arg2  has  spaces', 'arg3' ]
# "dollarat-adjacent": "node -p 'process.argv.slice(1)' -- ':'$@':'"
[ ':arg1arg2', 'has', 'spacesarg3:' ]
# "dollarat-quoted-adjacent": "node -p 'process.argv.slice(1)' -- ':'\"$@\"':'"
[ ':arg1', 'arg2  has  spaces', 'arg3', ':' ]

whereas correct behavior would look like this:

 # "dollarat": "node -p 'process.argv.slice(1)' -- $@"
-[ 'arg1arg2', 'has', 'spacesarg3' ]
+[ 'arg1', 'arg2', 'has', 'spaces', 'arg3' ]
 # "dollarat-quoted": "node -p 'process.argv.slice(1)' -- \"$@\""
 [ 'arg1', 'arg2  has  spaces', 'arg3' ]
 # "dollarat-adjacent": "node -p 'process.argv.slice(1)' -- ':'$@':'"
-[ ':arg1arg2', 'has', 'spacesarg3:' ]
+[ ':arg1', 'arg2', 'has', 'spaces', 'arg3:' ]
 # "dollarat-quoted-adjacent": "node -p 'process.argv.slice(1)' -- ':'\"$@\"':'"
-[ ':arg1', 'arg2  has  spaces', 'arg3', ':' ]
+[ ':arg1', 'arg2  has  spaces', 'arg3:' ]

Environment

System:
    OS: Linux 5.15 Debian GNU/Linux 12 (bookworm) 12 (bookworm)
    CPU: (4) x64 Intel(R) Core(TM) i5-4460  CPU @ 3.20GHz
  Binaries:
    Node: 22.20.0 - /tmp/xfs-7c7bcd5f/node
    Yarn: 4.10.3-dev - /tmp/xfs-7c7bcd5f/yarn
    npm: 10.9.3 - /usr/local/bin/npm

Additional context

No response