Array-of-tables header accepted with a single closing bracket at EOF: [[a] parses as {a: [{}]} #65

Summary

An array-of-tables header closed by a single ] is accepted when it is the last
thing in the document. [[a] parses as { a: [ {} ] } instead of throwing. One
character of trailing content is also silently discarded, so [[a]x parses the
same way.

Version

• smol-toml 1.8.0 (also reproduces on 1.7.1)
• Node v22.17.0

Reproduction

import { parse } from 'smol-toml'

parse('[[a]')   // => { a: [ {} ] }   expected: throw
parse('[[a]x')  // => { a: [ {} ] }   expected: throw ('x' is dropped)

Why it is invalid

TOML 1.1.0's grammar requires two closing brackets:

array-table       = array-table-open key array-table-close
array-table-open  = %x5B.5B ws  ; [[ Double left square bracket
array-table-close = ws %x5D.5D  ; ]] Double right square bracket

array-table-close is %x5D.5D, so [[a] cannot derive from array-table.

Observed behaviour

Likely cause

In parse.js:

let k = parseKey(ctx, ']');           // consumes through the first ']'
if (isTableArray) {
    if (toml.charCodeAt(ctx.p - 1) !== 0x5d /* ] */) {
        throw new TomlError('expected end of table declaration', { ... });
    }
    ctx.p++;
}

The guard tests ctx.p - 1, which is the ] that parseKey has already
consumed and so is always 0x5d — the condition can never fire. The second
bracket is therefore never required, and ctx.p++ advances past whatever
happens to be there.

That accounts for the whole table: at EOF the increment is a no-op ([[a]), with
exactly one trailing character it swallows it ([[a]x), and with more the
leftovers produce an unrelated error ([[a]xyz).

Testing charCodeAt(ctx.p) for the closing bracket before advancing looks like
the intended check.

Why the test suite doesn't catch it

smol-toml passes all five existing toml-test cases for unclosed table headers
(invalid/table/array-no-close-01..04 and nested-brackets-close). Every one
of them has a line following the header, so none exercises the end-of-input path.
I've opened toml-lang/toml-test#205 with the two EOF cases ([[a] and [[a]x).

Other implementations

All reject [[a]:

• BurntSushi/toml (Go) v1.6.0 — expected end of table array name delimiter ']'
• CPython tomllib (3.14.7) — raises
• Rust toml 1.1.4+spec-1.1.0 — rejects

How this was found

Generated by a conformance-suite generator that derives one test per named error
branch of an executable TOML 1.1.0 semantics.