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[Bug] tree-sitter-bash parses `$_` / `${_}` as variable_name instead of special_variable_name

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Valutazione

Difficoltà
2/5
Tempo stimato
Meno di un'ora
Idoneità per principianti
85/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Attiva
Stack tecnologico
bash, typescript
Ambito
compilers

Direzione di ricerca

Inizia in src/parser.ts presso parseDollar() e parseExpansion(), dove 0 viene gestito come caso speciale; applica la diff di due righe per trattare anche _ come speciale. Aggiungi le righe del fixture a packages/tree-sitter-bash/test/fixtures/differential/expansions.txt. Esegui pnpm --filter @moonshot-ai/tree-sitter-bash test; fatto significa che la suite differenziale passa con special_variable_name per un _ isolato.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

What issue are you seeing?

packages/tree-sitter-bash parses the special parameter $_ as a variable_name, while the reference grammar ([email protected], the parser this package is differentially compared against) parses a lone _ as a special_variable_name. So ourDump(source) !== referenceDump(source) for input the package claims byte-identical tree parity on.

echo $_:

ours:
program [0,7] "echo $_"
  command [0,7] "echo $_"
    command_name [0,4] "echo"
      word [0,4] "echo"
    simple_expansion [5,7] "$_"
      ($) [5,6] "$"
      variable_name [6,7] "_"

reference:
program [0,7] "echo $_"
  command [0,7] "echo $_"
    command_name [0,4] "echo"
      word [0,4] "echo"
    simple_expansion [5,7] "$_"
      ($) [5,6] "$"
      special_variable_name [6,7] "_"

Same divergence for echo ${_}, echo ${_#x}, "${_}", ${_:-d}, ${_=x}: the _ child is variable_name here, special_variable_name in the reference.

Not affected (both sides agree): $?, $#, $-, $!, $@, $*, $$, $0, $1; and $_a, ${_a}, $__, which are variable_name on both sides (the \w+ rule wins once another word character follows).

What steps can reproduce the bug?
  1. Append a fixture to packages/tree-sitter-bash/test/fixtures/differential/expansions.txt:
@match: echo $_ and ${_} are special parameters, $_a and ${_a} are not
echo $_ ${_} ${_#x} $_a ${_a}
===
  1. pnpm --filter @moonshot-ai/tree-sitter-bash test

The differential suite fails on that fixture with the diff above (echo $_a and echo ${_a} still match).

What is the expected behavior?

A lone _ after $, and a lone _ as an expansion body, should produce special_variable_name "_" — matching _special_variable_name in the reference grammar:

_special_variable_name: $ => alias(choice('*', '@', '?', '!', '#', '-', '$', '0', '_'), $.special_variable_name),
_simple_variable_name: $ => alias(/\w+/, $.variable_name),
Additional information

Root cause: src/grammar.ts already lists _ in SPECIAL_VARIABLES (['@', '*', '#', '?', '-', '$', '!', '0', '_']), but SPECIAL_VARIABLE_CHARS filters word characters out of it (they are normally matched by the \w+ rule), so the only place a lone word-character special parameter is recognized explicitly is:

  • src/parser.ts, parseDollar() — if (j === i + 2 && next === '0')
  • src/parser.ts, parseExpansion() — const special = nameEnd === j + 1 && this.source[j] === '0';

Both special-case 0 but not _. Proposed fix (two lines, plus comment wording):

-      if (j === i + 2 && next === '0') {
+      if (j === i + 2 && (next === '0' || next === '_')) {
...
-      const special = nameEnd === j + 1 && this.source[j] === '0';
+      const special = nameEnd === j + 1 && (this.source[j] === '0' || this.source[j] === '_');

With that fix and the fixtures above: pnpm --filter @moonshot-ai/tree-sitter-bash test passes (6 files, 868 tests, 677 of them differential), typecheck is clean, oxlint --type-aware reports nothing.

Scope/impact: no user-visible behavior changes — nothing in packages/agent-core-v2 branches on special_variable_name vs variable_name (the permission policies key on command / command_name / string_content, bashTargets.ts on program / list / pipeline / redirected_statement). This is a fidelity bug against the package's stated contract of matching the reference tree byte for byte. @moonshot-ai/tree-sitter-bash is private, so no changeset is added.

Reproduced on main @ 7ccac2b on Windows 11 x64 with Node 24.21.0 / pnpm 10.33.0.

I'd like to submit the PR for this one — please /approve if that is welcome.

Contribution
  • I am willing to submit a PR for this bug fix myself (please wait for maintainer approval in this issue first)
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