XL-2 LoweringOccurrenceProjection 1/3: constructor tag atoms lowered from the tag token

src/v2/compiler/body_lowering_fold.dag

@@ -4801,13 +4801,6 @@ fn body_lower_match_arm_body_optional(body_capture: Node) -> Optional<Node> {
 // resolver instead of vanishing (the loss gunbc#11998 turned into
 // body_lowering_reason_positional_pattern_binder_unlowered, retired here). Positional arity is
 // one, as the seed's parse admits; more refuses with a located cause.
-fn body_lower_pattern_head_symbol_optional(head: Node) -> Optional<Symbol> {
-  match body_lower_pattern_leaf_atom_optional(node: head) {
-    Absent => Absent
-    Present { value: atom } => node_atom_identity_optional(node: atom)
-  }
-}
-
 fn body_lower_pattern_reject<T>(pattern_capture: Node) -> Outcome<T> {
   outcome_rejected(
     d: body_lower_diagnostic(reason: ^body_lowering_reason_pattern_unlowered, n: pattern_capture)
@@ -4889,9 +4882,12 @@ fn body_lower_pattern_suffix_lowered(head: Node, suffix: Node, pattern_capture:
                 match body_lower_comma_list_items_optional(list_capture: inner.left) {
                   Absent => body_lower_pattern_reject(pattern_capture: pattern_capture)
                   Present { value: items } =>
-                    match body_lower_pattern_head_symbol_optional(head: head) {
+                    match body_lower_pattern_leaf_atom_optional(node: head) {
                       Absent => body_lower_pattern_reject(pattern_capture: pattern_capture)
-                      Present { value: tag } =>
+                      Present { value: tag_atom } =>
+                        match node_atom_identity_optional(node: tag_atom) {
+                          Absent => body_lower_pattern_reject(pattern_capture: pattern_capture)
+                          Present { value: tag } =>
                         bind_outcome(
                           o: body_lower_pattern_field_edges(
                             items: items,
@@ -4900,10 +4896,11 @@ fn body_lower_pattern_suffix_lowered(head: Node, suffix: Node, pattern_capture:
                           ),
                           f: fn(edges) {
                             outcome_accepted(
-                              value: construct_node(tag: tag, field_edges: edges, source: pattern_capture)
+                              value: construct_node(tag: tag, tag_source: tag_atom, field_edges: edges, source: pattern_capture)
                             )
                           }
                         )
+                        }
                     }
                 }
             }
@@ -7099,15 +7096,22 @@ fn body_lower_record_literal_field_items_optional(captured: Node) -> Optional<Li
   }
 }
 
-fn body_lower_record_literal_tag_optional(captured: Node) -> Optional<Symbol> {
+// The tag is read with the atom it was read from, so the construct's tag atom can be lowered from
+// that token rather than from the whole capture (v2.std.node_query construct_tag_edge).
+type RecordLiteralTag {
+  tag: Symbol
+  at: Node
+}
+
+fn body_lower_record_literal_tag_optional(captured: Node) -> Optional<RecordLiteralTag> {
   match sugar_sequence_pair_optional(node: captured) {
     Absent => Absent
     Present { value: pair } =>
       match dag_binding_branded_atom_identity(node: pair.left) {
         Present { value: tag } =>
           match dag_lex_token_symbol(sym: tag) {
             true => optional_absent()
-            false => optional_present(value: tag)
+            false => optional_present(value: RecordLiteralTag { tag: tag, at: pair.left })
           }
         Absent => Absent
       }
@@ -7117,7 +7121,7 @@ fn body_lower_record_literal_tag_optional(captured: Node) -> Optional<Symbol> {
 fn body_lower_try_record_literal(captured: Node) -> Outcome<Optional<Node>> {
   match body_lower_record_literal_tag_optional(captured: captured) {
     Absent => outcome_accepted(value: optional_absent())
-    Present { value: tag } =>
+    Present { value: read } =>
       match body_lower_record_literal_field_items_optional(captured: captured) {
         Absent => outcome_accepted(value: optional_absent())
         Present { value: items } =>
@@ -7131,7 +7135,7 @@ fn body_lower_try_record_literal(captured: Node) -> Outcome<Optional<Node>> {
             Rejected { diagnostics: r } => Rejected { diagnostics: r }
             Accepted { value: edges, diagnostics: d } =>
               Accepted {
-                value: optional_present(value: construct_node(tag: tag, field_edges: edges, source: captured)),
+                value: optional_present(value: construct_node(tag: read.tag, tag_source: read.at, field_edges: edges, source: captured)),
                 diagnostics: d
               }
           }

src/v2/std/node_query.dag

@@ -41,6 +41,7 @@ import v2.std.node {
   arrow_body_target_lookup,
   coproduct_nullary_payload_type_name,
   named_edge_sort_key,
+  node_lowered_from,
   node_with_occurrence_id
 }
 import std.constructors { Cardinality, Required, Optional }
@@ -260,22 +261,27 @@ fn declared_field_named(payload: Node, name: Symbol) -> Optional<DeclaredField>
 // (v2.extdeps.languages.dag dag_rec_construct_body_node, dag_variant_holds_construct_node). A
 // match-arm PATTERN over a fielded variant is the same construct whose field targets are the
 // binders, so construction and elimination read one shape.
-fn construct_tag_edge(tag: Symbol, source: Node) -> Edge {
+// THE TAG ATOM STANDS FOR THE TAG TOKEN, so it is lowered from that token, not from the whole
+// construct: `tag_source` is the authored terminal the constructor name was read from, and the
+// Conj keeps the construct's own occurrence. Lowering the atom from the enclosing capture gave it
+// the shell's occurrence, so the tag's locus was a coarser, different span
+// (docs/plans/lowering-occurrence-projection-design.md, rank 3).
+fn construct_tag_edge(tag: Symbol, tag_source: Node) -> Edge {
   Edge {
     label: Named { name: tag },
-    target: node_with_occurrence_id(
+    target: node_lowered_from(
       kind: TypeNode { connective: Atom { identity: tag } },
       children: [],
-      occurrence_id: source.occurrence_id
+      source: tag_source
     )
   }
 }
 
-fn construct_node(tag: Symbol, field_edges: List<Edge>, source: Node) -> Node {
+fn construct_node(tag: Symbol, tag_source: Node, field_edges: List<Edge>, source: Node) -> Node {
   node_with_occurrence_id(
     kind: TypeNode { connective: Conj },
     children: list_append(
-      left: [construct_tag_edge(tag: tag, source: source)],
+      left: [construct_tag_edge(tag: tag, tag_source: tag_source)],
       right: field_edges
     ),
     occurrence_id: source.occurrence_id

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

@@ -303,6 +303,25 @@ fn statement_let_binder_subject() -> ReferenceConservationSubject {
   conservation_subject(id_tail: "statement_let_binder", source: statement_let_binder_source)
 }
 
+// ---- constructor tags keep their own occurrence (XL-2 LoweringOccurrenceProjection, rank 3) -----
+
+// A record-literal tag (`RcRow { v: n }`) and a constructor pattern (`RcRow { v: x } =>`) each
+// lower to a construct whose tag atom v2.std.node_query construct_tag_edge builds. Before the
+// repair that atom was lowered from the enclosing capture, so it carried the shell's occurrence and
+// both mentions were counted locus-erased. MEASURED on this fixture: 9 conserved / 13 locus-erased
+// before, 11 / 11 after -- the two tags and nothing else moved. Every other atom here is a binder,
+// a field name or a type position, so the counts are the fixture's own, not a corpus literal.
+data construct_tags_source: String = "module v2.test.reference_conservation_construct_tags\n\ntype RcRow {\n  v: Int\n}\n\nfn rc_make(n: Int) -> RcRow { RcRow { v: n } }\n\nfn rc_read(r: RcRow) -> Int {\n  match r {\n    RcRow { v: x } => x\n  }\n}\n"
+
+fn construct_tags_subject() -> ReferenceConservationSubject {
+  conservation_subject(id_tail: "construct_tags", source: construct_tags_source)
+}
+
+test fn constructor_tags_keep_their_own_occurrence_holds() -> Bool {
+  let r = conservation_report_of(subject: construct_tags_subject())
+  r.authored == 22 && r.conserved == 11 && r.locus_erased == 11 && length(xs: r.dropped) == 0
+}
+
 // ---- anonymous parameter slots are conserved by their minted identity -------------------------
 
 // Each `_` parameter lowers to its own minted binder (v2.std.anonymous_binder), so the label is

src/v2/workflow/floor_pure_producer_share.dag

@@ -852,6 +852,7 @@ data floor_cross_claim_pure_producers_warm: List<String> = [
   "v2.test.claim.namespace_xl0.reference_conservation.list_literal_argument_subject",
   "v2.test.claim.namespace_xl0.reference_conservation.where_refinement_predicate_subject",
   "v2.test.claim.namespace_xl0.reference_conservation.match_later_arm_subject",
+  "v2.test.claim.namespace_xl0.reference_conservation.construct_tags_subject",
   "v2.test.claim.namespace_xl0.reference_conservation.statement_let_binder_subject",
   "v2.test.claim.namespace_xl0.reference_conservation.anonymous_slots_subject",
   "v2.test.claim.namespace_xl0.reference_conservation_admission.two_same_spelled_labels_subject",