resolve: an anonymous parameter slot hides nothing (nested function-type false refusal)

[gunbai-bot]

The bug. A function whose declared return is nested function types was falsely refused with resolve_reason_binder_hides_visible_value. For example, fn f(p: Int) -> fn(Int) -> fn(Int) -> Int { x => y => x }. This is the eighth silent red on main: v2.test.claim.body_lowering.function_value_body_route a_curried_function_typed_return_emits_impl_fn_over_box_dyn_fn.

First bad #12506 (861c829). It passes at the parent e5859c5 and fails at 861c829, by the claim_batch first-parent bisect.

Which binder, measured. I instrumented binder_hides_visible_value_diagnostic locally so its reason carries the flagged binder, then ran the emit route on main:

• the curried x => y => x refuses on <anonymous-parameter-1>;
• the nested fn(x) { fn(y) { x } } also refuses on <anonymous-parameter-1>;
• a one-lambda function emits.

So the cause is not a fresh return type variable, which was the first guess.

Why it refuses. Each Arrow type's unnamed domain slot is labelled by its position within its OWN list (v2.std.anonymous_binder anonymous_param_label, a ruled naming: a counter- or occurrence-derived label would make fn(_: Int) -> Int two types at two sites). So both levels of fn(Int) -> fn(Int) -> Int bind <anonymous-parameter-1>, and #12506's value-binder gate (admit_value_binders / admit_one_value_binder) found the outer slot in frame.

Earliest unjustified link: the gate, not the naming. Hiding is the risk that a body reference silently rebinds to the inner binder. An anonymous slot is unauthorable and never referenceable by construction, so it cannot hide anything.

Fix. admit_one_value_binder admits a name answering is_anonymous_param_label (the one recogniser, already used by emission and reference_conservation) into the frame's locals without the outer lookup. The duplicate-in-frame check still applies, and the naming is unchanged.

Controls (function_value_body_route_test.dag; all 7 claims of the module):

claim	without the fix	with the fix
a_curried_function_typed_return_emits_impl_fn_over_box_dyn_fn	FAIL	PASS
nested_function_types_with_anonymous_slots_resolve_and_reach_infer (new)	FAIL	PASS
a_fn_returning_fn_emits_impl_fn_with_a_move_closure (one-lambda control)	PASS	PASS
an_authored_binder_hiding_an_outer_parameter_still_refuses_as_hiding (new NEGATIVE control)	PASS (refuses)	PASS (refuses)
the other 3 claims	PASS	PASS

Runs: with the fix https://app.buildbuddy.io/invocation/b82aaed0-f7a5-4ef2-8a55-626aaa9720ea ; without it https://app.buildbuddy.io/invocation/e5b00b13-06b3-4729-aa36-6e125cd60f5c .

Pre-queue gate (the 12 lambda/fold modules at head vs merge base; the N7 native diff): results are posted below as they land.

🤖 Generated with Claude Code

[gunbai-bot]

Pre-queue gate at head 4c141e7 against merge base 4bf26c8:

N7 native route (gunbc test //v2/test/parse/expression_bodied_fn_decl_parse:all, GITHUB_SHA set to each sha): identical at both. All eight verdicts are refused at prepare with the same cause (infer_reason_projection_receiver_declaration_unavailable), and the same 3 file refusals appear at both: filesystem_io, machine_constraints and body_lowering_fold. No new refusals.

• head: https://app.buildbuddy.io/invocation/9b7726dc-d860-4d5c-866f-27efa45606bf
• base: https://app.buildbuddy.io/invocation/b285d884-25b6-403c-bc49-e39f41357a84

The 12 lambda/fold modules, every test function, head vs base: 0 new failures. The only change is a_curried_function_typed_return_emits_impl_fn_over_box_dyn_fn, FAIL to PASS.

• At both, the 7 value_position_whole_read failures are the stale test reader that value_position_whole_read: read the domain binder through binder_node_parts (7 silent reds on main) #13394 fixes.
• At both, let_match_early_return (18) and return_tail_position (17) do not type-check, so they gave no verdicts on either side: no field 'fatal_reason' on type 'NativeTestFileRefusal' at let_match_early_return_test.dag:154 and return_tail_position_test.dag:186. That is a separate break on main, from 457d06d, which dropped the stored field; it is reported separately.
• head: https://app.buildbuddy.io/invocation/d7649701-992f-4cf1-b143-011aa54fa014 (modules 1-6), https://app.buildbuddy.io/invocation/d0411820-cfe3-4223-b616-aeff813af80b (modules 7-12), https://app.buildbuddy.io/invocation/33d4e42d-a0f9-4a9b-b27b-dab55a1ece07 (the two type-check errors)
• base: https://app.buildbuddy.io/invocation/d9ef75f2-5a2d-4825-902e-70695143f261

— sent from calm-boar-904