v2 grammar: an admit_callers list may end in a comma (27 of 31 measured modules stop refusing whole)

[gunbai-bot]

XL-2, per quiet-seal-543's ruling: v2 parse refusals, one class per PR. This one is class C2.

The defect

v2.extdeps.languages.dag dag_grammar_admit_callers_clause_expr read admit_callers: [expr (, expr)* ] with no trailing comma. Every other comma list in the grammar (dag_grammar_comma_list_expr) admits one, and v1 reads the list with one. A multi-line caller list written with a trailing comma, which is the corpus's usual style, refused its whole module as parse_g0_tokens_remain at the ].

The change

The list's closing becomes seq(opt(comma), rbracket). The first element is still required, so admit_callers: [] still refuses, as v1 does, and a list that is only a comma refuses too. The clause's only v2 consumer treats it as metadata (body_lowering_fold lists dag_surface_admit_callers_clause among preserved shells; gunbc.rung_drop admit_callers_discarded_on_the_native_route), so no positional reader depends on the spine.

Controls (v2.test.claim.parse.admit_callers_trailing_comma, one parse per claim)

• a_trailing_comma_in_admit_callers_parses_holds. Red on main, executed: on main with only this test added it FAILS, and the two controls beside it pass.
• an_admit_callers_list_without_a_trailing_comma_parses_holds: the unchanged form.
• an_admit_callers_list_of_only_a_comma_refuses_holds: the first element is still required.

Parse count before and after

Population: the 46 .dag files whose admit_callers list ends in a comma, all of which refuse on main. That includes src/v2/std/compilers/cli_surface.dag, a v2 source.

Instrument: a v2 parse (conservation_subject_of_text .parsed).

Coverage: 31 of the 46 were measured in both arms. The other 15 were in batches the BuildBuddy runner's one-hour limit cut off, in both arms.

over the 31 measured	main	this branch
parse	0	27
refuse	31	4

The 4 that still refuse are the next classes, now exposed:

• D: a named uses clause, uses net: Network, in gunbc.auth.approval_ntfy_access_readback (line 299) and gunbc.auth.approval_device_routes (line 771).
• E: a line that begins with &&, continuing the expression on the line before, in test.claim.dispatch_preflight_witness_test (line 163) and test.claim.approval_device_redemption_witness_test (line 493).

No emitted Rust changes: the v2 grammar has no stage0 mirror.

Land only via the merge queue.

🤖 Generated with Claude Code

[briansrls]

APPROVE-MERGE at exact head 9b97527.

No findings. The grammar admits exactly one optional trailing comma after the required first admit_callers element, preserving the unchanged non-trailing form and continuing to refuse an empty or comma-only list. The change is parse-only at a clause already carried as preserved metadata, and the warm-enrolled controls discriminate the main failure without widening the element grammar.

Merge queue only. The actual merge_group candidate must pass against then-current main; no direct merge and no check bypass.