XL-2: two more silent drops rostered (match-arm statement body; data-initializer match scrutinee), residual attributed by sampling

dag/gunbc/recurring_failure_mode/data_initializer_match_reads_its_scrutinee_from_an_arm_at_v2_body_lowering.dag

@@ -0,0 +1,20 @@
+module gunbc.recurring_failure_mode.data_initializer_match_reads_its_scrutinee_from_an_arm_at_v2_body_lowering
+
+import std.types { NonEmptyStr }
+import std.decl_ref { DeclarationRef, WholeDeclaration }
+import gunbc.recurring_failure_mode { RecurringFailureMode }
+
+data data_initializer_match_reads_its_scrutinee_from_an_arm_at_v2_body_lowering: RecurringFailureMode = RecurringFailureMode {
+  identity: "data_initializer_match_reads_its_scrutinee_from_an_arm_at_v2_body_lowering" as NonEmptyStr,
+
+  receipts: [
+    "**a data initializer that is a match reads its scrutinee from its first arm, and normalize ACCEPTS the module** (INVALID STATE: `data d: Bool = match h(a: zq) { X => true  Y => false }` lowers to a match whose scrutinee is `true`, the first arm's body. The call scrutinee, callee and arguments are gone, and there is no diagnostic.) The declaration matches a constant instead of what the source computes, so the answer is wrong whenever the arms depend on the scrutinee. A name in the scrutinee never reaches resolve. The same match as a fn body keeps its scrutinee.",
+    "WHERE, cited by symbol, traced (a normalized-tree dump of the fixture shows the scrutinee edge holding `dag_token_kw_true` at the first arm body's occurrence). `v2.compiler.body_lowering_fold` `body_lower_match_scrutinee_optional` finds the scrutinee by searching the WHOLE match subtree for the first `dag_surface_binary_expr`, then `dag_surface_postfix_expr` (`body_lower_find_captured`). A data initializer is folded bottom-up before the declaration is dispatched (`body_lower_data_initializer_body`'s own note), so by then the scrutinee's shell has been reduced and is no longer that production. The search therefore finds the next one, inside an arm body. That is a positional fact obtained by searching for its shape (`a_positional_fact_is_obtained_by_searching_for_its_shape`'s class), at a new site.",
+    "RECEIPTS. Census rerun on the #12313 lexer (exact loci): 13 of the 3,472 atoms absent in accepted modules have a production chain `data_decl/expr/match_expr/binary_expr...` (the scrutinee position), which is exact at chain grain, all in `test.claim.manual.outcome_diagnostics_eq` `witness_diagnostics_merge_some`. Fixtures: a call scrutinee with record arguments and one with plain arguments both drop; the fn-body form drops only the argument label, which is a separate row.",
+    "RUNG FOUND AT: below the floor (silent wrongness). CEILING: structurally guaranteed. The scrutinee is read from its grammar position, the child after `match`, or the match refuses. NEXT-RUNG TRIGGER, NAMING THE CAPABILITY: v2 body lowering reads a match's scrutinee positionally on every route, data initializers included, never by searching the match subtree. Owner: bright-boar-848's fold queue. The pinned drop claim holds while the drop happens and reds when the repair lands.",
+  ],
+
+  evidence: [
+    DeclarationRef { module_path: "v2.test.claim.namespace_xl0.reference_conservation_accepted_drops", decl_name: "a_data_initializer_match_scrutinee_is_reported_dropped_holds", field: WholeDeclaration },
+  ],
+}

dag/gunbc/recurring_failure_mode/match_arm_statement_body_lowers_to_its_first_expression_at_v2_body_lowering.dag

@@ -0,0 +1,20 @@
+module gunbc.recurring_failure_mode.match_arm_statement_body_lowers_to_its_first_expression_at_v2_body_lowering
+
+import std.types { NonEmptyStr }
+import std.decl_ref { DeclarationRef, WholeDeclaration }
+import gunbc.recurring_failure_mode { RecurringFailureMode }
+
+data match_arm_statement_body_lowers_to_its_first_expression_at_v2_body_lowering: RecurringFailureMode = RecurringFailureMode {
+  identity: "match_arm_statement_body_lowers_to_its_first_expression_at_v2_body_lowering" as NonEmptyStr,
+
+  receipts: [
+    "**a match arm whose body is a statement sequence lowers to its FIRST expression, and normalize ACCEPTS the module** (INVALID STATE: the arm body `let t = v` followed by `e` lowers to `v`. The binder `t` and every following statement, including a nested match, are gone, and there is no diagnostic.) The arm answers the let's bound value instead of what its last statement computes, and a name in any later statement never reaches resolve.",
+    "WHERE, cited by symbol. `v2.compiler.body_lowering_fold` `body_lower_match_arm_body_capture_optional` answers the arm body with the first `dag_surface_expr` it finds in the subtree (`body_lower_find_captured`, a pre-order first match). For a `dag_surface_match_arm_stmt_body`, the first expr is the let's value. The `let` and the continuation are never read. This is one of the first-match readers in bright-boar-848's fold-cleanup lane.",
+    "RECEIPTS. Fixture traced by the census at the #12313 head: `X => let t = zq / g(a: zr)` drops `t`, `g` and `zr`; with a nested match as the continuation, its scrutinee and arms are dropped too. Census: `v2.compiler.reference_conservation_census` `reference_conservation_census_for_paths` over `reference_conservation_stratified_sample_paths`, rerun on the #12313 lexer (exact loci). 29 of the 3,472 atoms absent in accepted modules have a production chain through `match_arm/stmt`, which is exact at chain grain; one of them is `test.claim.sql_create_table_fold_test` `sql_fold_fold_serialize`. DISTINGUISHING FACTS: not `lowering_rebuilds_an_authored_atom_without_its_occurrence`'s let binder, where the binder is kept at the let shell's occurrence; here the continuation is lost too. Not `match_arms_after_the_second_are_dropped_at_v2_body_lowering`: the arm is present, but its body is truncated.",
+    "RUNG FOUND AT: below the floor (silent wrongness). CEILING: structurally guaranteed. An arm body lowers to the bind-and-continue form its statements denote, or refuses at the statement it could not read. NEXT-RUNG TRIGGER, NAMING THE CAPABILITY: v2 body lowering reads a match arm's statement body through the statement authority that fn bodies use, not by searching for an expression. Owner: bright-boar-848's fold queue. The pinned drop claim holds while the drop happens and reds when the repair lands.",
+  ],
+
+  evidence: [
+    DeclarationRef { module_path: "v2.test.claim.namespace_xl0.reference_conservation_accepted_drops", decl_name: "a_statement_after_a_let_in_a_match_arm_is_reported_dropped_holds", field: WholeDeclaration },
+  ],
+}

src/v2/test/claim/namespace_xl0/reference_conservation_accepted_drops_test.dag

@@ -87,3 +87,30 @@ fn qualified_constructor_value_subject() -> ReferenceConservationSubject {
 test fn a_qualified_constructor_literal_value_is_reported_dropped_holds() -> Bool {
   report_drop_count(report: conservation_report_of(subject: qualified_constructor_value_subject()), wanted: ^rc_value) == 1
 }
+
+// A match-arm body that is a statement sequence -- `let t = v` then an expression -- lowers to `v`:
+// the binder and every following statement are gone, a nested match included. Rostered as
+// gunbc.recurring_failure_mode match_arm_statement_body_lowers_to_its_first_expression_at_v2_body_lowering.
+data match_arm_statement_body_source: String = "module v2.test.reference_conservation_arm_stmt\n\ntype RcT = RcA | RcB\n\ndata rc_bound: Int = 1\n\ndata rc_after: Int = 2\n\nfn rc_id(a: Int) -> Int { a }\n\nfn rc_arm(t: RcT) -> Int {\n  match t {\n    RcA =>\n      let rc_local = rc_bound\n      rc_id(rc_after)\n    RcB => 0\n  }\n}\n"
+
+fn match_arm_statement_body_subject() -> ReferenceConservationSubject {
+  conservation_subject(id_tail: "match_arm_statement_body", source: match_arm_statement_body_source)
+}
+
+test fn a_statement_after_a_let_in_a_match_arm_is_reported_dropped_holds() -> Bool {
+  report_drop_count(report: conservation_report_of(subject: match_arm_statement_body_subject()), wanted: ^rc_after) == 1
+}
+
+// A data initializer that is a match reads its scrutinee from an arm: the call scrutinee -- callee
+// and arguments -- is gone and the first arm's body stands in its place. The same match as a fn body
+// keeps its scrutinee. Rostered as gunbc.recurring_failure_mode
+// data_initializer_match_reads_its_scrutinee_from_an_arm_at_v2_body_lowering.
+data data_initializer_match_scrutinee_source: String = "module v2.test.reference_conservation_data_match\n\ntype RcT = RcA | RcB\n\ndata rc_arg: Int = 1\n\nfn rc_pick(a: Int) -> RcT { RcA }\n\ndata rc_matched: Bool = match rc_pick(rc_arg) {\n  RcA => true\n  RcB => false\n}\n"
+
+fn data_initializer_match_scrutinee_subject() -> ReferenceConservationSubject {
+  conservation_subject(id_tail: "data_initializer_match_scrutinee", source: data_initializer_match_scrutinee_source)
+}
+
+test fn a_data_initializer_match_scrutinee_is_reported_dropped_holds() -> Bool {
+  report_drop_count(report: conservation_report_of(subject: data_initializer_match_scrutinee_subject()), wanted: ^rc_arg) == 1
+}

src/v2/workflow/floor_pure_producer_share.dag

@@ -863,6 +863,8 @@ data floor_cross_claim_pure_producers_warm: List<String> = [
   "v2.test.claim.namespace_xl0.reference_conservation_accepted_drops.call_argument_beside_match_subject",
   "v2.test.claim.namespace_xl0.reference_conservation_accepted_drops.map_literal_value_subject",
   "v2.test.claim.namespace_xl0.reference_conservation_accepted_drops.qualified_constructor_value_subject",
+  "v2.test.claim.namespace_xl0.reference_conservation_accepted_drops.match_arm_statement_body_subject",
+  "v2.test.claim.namespace_xl0.reference_conservation_accepted_drops.data_initializer_match_scrutinee_subject",
   "v2.test.claim.namespace_xl0.reference_conservation.anonymous_slots_subject",
   "v2.test.claim.namespace_xl0.reference_conservation_admission.two_same_spelled_labels_subject",
   "v2.test.claim.namespace_xl0.reference_conservation_admission.operator_call_argument_subject",