feat: project class inheritance as EXTENDS/IMPLEMENTS neo4j relationships

Resolves each type's heritage signatures (base_classes/implements_types)
through idBySig to can:// ids in the v2 emitter, dropping unresolved
external/library supertypes rather than nulling them. Exposes the result
as extends_ids/implements_ids on the V2 type node (JSON props unchanged)
and projects them as EXTENDS/IMPLEMENTS relationships in Neo4j via the
previously-dead RowBuilder.edgeToSymbol deferred gate, which was also
re-pointed at the correct CanNode/id addressing (it had been hardcoded to
a Symbol/signature pair no node in the v2 schema actually carries).

Adds a first-party class hierarchy fixture to dataflow-app and extends
both the schema conformance test and the issue #27 Neo4j<->JSON parity
gate (the latter now also has an exhaustive total-edge accounting test).

Author: rahlk

schema.neo4j.json

@@ -372,6 +372,29 @@
       "properties": {
         "var": "string"
       }
+    },
+    {
+      "type": "EXTENDS",
+      "from": [
+        "Class",
+        "Interface"
+      ],
+      "to": [
+        "Class",
+        "Interface"
+      ],
+      "properties": {}
+    },
+    {
+      "type": "IMPLEMENTS",
+      "from": [
+        "Class"
+      ],
+      "to": [
+        "Interface",
+        "Class"
+      ],
+      "properties": {}
     }
   ],
   "constraints": [

src/build/neo4j/project.ts

@@ -96,6 +96,10 @@ function projectType(b: RowBuilder, t: V2Type, parent: NodeRef, fileKey: string)
   const label = KIND_LABEL[t.kind] ?? "Class";
   const node = b.node([CAN, label], "id", t.id, typeProps(t, fileKey));
   b.edge("DECLARES", parent, node);
+  // Inheritance overlay — resolved-only (emit.ts already dropped unresolved/external supertypes);
+  // the deferred gate is defense-in-depth against a resolved id that never materialized as a node.
+  for (const eid of t.extends_ids ?? []) b.edgeToSymbol("EXTENDS", node, eid);
+  for (const iid of t.implements_ids ?? []) b.edgeToSymbol("IMPLEMENTS", node, iid);
   if (t.kind === "namespace") {
     projectScope(b, t, node, fileKey); // a namespace nests types/functions/fields
     return;

src/build/neo4j/rows.ts

@@ -150,6 +150,12 @@ export const REL_TYPES: RelType[] = [
   { type: "SUMMARY", from: ["BodyNode"], to: ["BodyNode"], properties: { var: "string" } },
   { type: "PARAM_IN", from: ["BodyNode"], to: ["BodyNode"], properties: { var: "string" } },
   { type: "PARAM_OUT", from: ["BodyNode"], to: ["BodyNode"], properties: { var: "string" } },
+  // Inheritance, projected from the `extends_ids`/`implements_ids` node props (schema/v2/emit.ts) —
+  // resolved-only: an unresolved (external/library) supertype never reaches here. A `to` of `Class`
+  // covers TS's `implements SomeClass` (structural, not just interfaces); an interface may itself
+  // `extends` a class's instance type, hence `EXTENDS` also allows an `Interface` source.
+  { type: "EXTENDS", from: ["Class", "Interface"], to: ["Class", "Interface"], properties: {} },
+  { type: "IMPLEMENTS", from: ["Class"], to: ["Interface", "Class"], properties: {} },
 ];
 
 // ----------------------------------------------------------------------------------------------

src/build/neo4j/schema.ts

@@ -111,11 +111,19 @@ function memberKey(sig: string, accessorKind?: string | null): string {
   return seg;
 }
 
+/** A type's heritage signatures, resolved to `can://` ids once the whole tree walk registers them. */
+interface PendingHeritage {
+  node: V2Type;
+  extendsSigs: string[]; // class: the extended base class; interface: extended interface(s)
+  implementsSigs: string[]; // class only: implemented interfaces
+}
+
 /** State shared across the whole tree walk (edge-rewriting + gating). */
 interface SharedState {
   idBySig: Map<string, string>;
   collisions: string[];
   pendingCallees: Array<{ node: V2BodyNode; calleeSig: string | null }>; // backfilled at L2
+  pendingHeritage: PendingHeritage[]; // resolved sig→id once the whole tree walk completes
   callableBySig: Map<string, V2Callable>; // locates each callable's node for the L3/L4 dataflow pass
   level: number;
 }
@@ -201,7 +209,14 @@ function toClass(c: TSClass, ctx: Ctx): V2Type {
   const fields: Record<string, V2Field> = {};
   for (const [name, a] of Object.entries(c.attributes ?? {}))
     fields[name] = fieldNode(id, name, (a as TSClassAttribute).span, a as unknown as Record<string, unknown>);
-  return { ...carry(c as unknown as Record<string, unknown>), id, kind: "class", signature: c.signature, span: c.span, callables, fields };
+  const node: V2Type = { ...carry(c as unknown as Record<string, unknown>), id, kind: "class", signature: c.signature, span: c.span, callables, fields };
+  // `base_classes` is the union of extends + implements (schema.ts:231); subtract implements_types
+  // to recover just the extended base class (0 or 1 — TS classes extend at most one class).
+  if (c.base_classes.length) {
+    const extendsSigs = c.base_classes.filter((s) => !c.implements_types.includes(s));
+    ctx.pendingHeritage.push({ node, extendsSigs, implementsSigs: c.implements_types });
+  }
+  return node;
 }
 
 function toInterface(i: TSInterface, ctx: Ctx): V2Type {
@@ -212,7 +227,11 @@ function toInterface(i: TSInterface, ctx: Ctx): V2Type {
   const fields: Record<string, V2Field> = {};
   for (const [name, p] of Object.entries(i.properties ?? {}))
     fields[name] = fieldNode(id, name, (p as TSClassAttribute).span, p as unknown as Record<string, unknown>);
-  return { ...carry(i as unknown as Record<string, unknown>), id, kind: "interface", signature: i.signature, span: i.span, callables, fields };
+  const node: V2Type = { ...carry(i as unknown as Record<string, unknown>), id, kind: "interface", signature: i.signature, span: i.span, callables, fields };
+  // Interface heritage is extends-only (schema.ts:255) — an interface can extend other interfaces
+  // (or, rarely, a class's instance type), but never "implements".
+  if (i.base_classes.length) ctx.pendingHeritage.push({ node, extendsSigs: i.base_classes, implementsSigs: [] });
+  return node;
 }
 
 function toEnum(e: TSEnum, ctx: Ctx): V2Type {
@@ -332,8 +351,9 @@ export function toV2Detailed(app: TSApplication, opts: AnalysisOptions): ToV2Res
   const idBySig = new Map<string, string>();
   const collisions: string[] = [];
   const pendingCallees: Array<{ node: V2BodyNode; calleeSig: string | null }> = [];
+  const pendingHeritage: PendingHeritage[] = [];
   const callableBySig = new Map<string, V2Callable>();
-  const shared: SharedState = { idBySig, collisions, pendingCallees, callableBySig, level };
+  const shared: SharedState = { idBySig, collisions, pendingCallees, pendingHeritage, callableBySig, level };
 
   // L1 — the containment tree (registers every real callable/type id in idBySig).
   const symbol_table: Record<string, V2Module> = {};
@@ -342,6 +362,16 @@ export function toV2Detailed(app: TSApplication, opts: AnalysisOptions): ToV2Res
   }
   const root: V2Root = { id: appId, kind: "application", symbol_table, call_graph: [], param_in: [], param_out: [] };
 
+  // Resolve heritage sig → can:// id now that every first-party type is registered in idBySig
+  // (independent of level: types are homed during the unconditional L1 walk above). Unresolved
+  // (external/library) supertypes are dropped, never nulled — the "resolved-only" rule.
+  for (const { node, extendsSigs, implementsSigs } of pendingHeritage) {
+    const extendsIds = extendsSigs.map((s) => idBySig.get(s)).filter((x): x is string => x !== undefined);
+    const implementsIds = implementsSigs.map((s) => idBySig.get(s)).filter((x): x is string => x !== undefined);
+    if (extendsIds.length) node.extends_ids = extendsIds;
+    if (implementsIds.length) node.implements_ids = implementsIds;
+  }
+
   // L2 — home the off-tree edge endpoints, backfill `callee`, rewrite the call graph.
   const dangling: string[] = [];
   if (level >= 2) {

src/schema/v2/emit.ts

@@ -102,6 +102,11 @@ export interface V2Type extends V2Node {
   fields?: Record<string, V2Field>; // class attributes / interface properties / enum members
   types?: Record<string, V2Type>; // namespace: nested types
   functions?: Record<string, V2Callable>; // namespace: nested functions
+  // Heritage: `base_classes`/`implements_types` (carried from v1) stay signature strings — the
+  // resolved-id projection lives here, additively, for the Neo4j EXTENDS/IMPLEMENTS overlay.
+  // External/library supertypes that never resolve to a first-party id are dropped, not nulled.
+  extends_ids?: string[]; // resolved `can://` id(s) of the extended class/interface(s)
+  implements_ids?: string[]; // resolved `can://` id(s) of implemented interfaces (classes only)
 }
 
 export interface V2Callable extends V2Node {

src/schema/v2/model.ts

@@ -0,0 +1,28 @@
+/** A minimal first-party class hierarchy — exercises the Neo4j EXTENDS/IMPLEMENTS projection. */
+
+export interface Shape {
+  area(): number;
+}
+
+export interface Labeled {
+  readonly label: string;
+}
+
+export class Rectangle implements Shape {
+  constructor(
+    protected readonly width: number,
+    protected readonly height: number,
+  ) {}
+
+  area(): number {
+    return this.width * this.height;
+  }
+}
+
+export class Square extends Rectangle implements Labeled {
+  readonly label = "square";
+
+  constructor(side: number) {
+    super(side, side);
+  }
+}

test/fixtures/dataflow-app/src/hierarchy.ts

@@ -91,3 +91,44 @@ describe("neo4j schema conformance", () => {
     expect(onDisk).toBe(fresh);
   });
 });
+
+// ---- Class inheritance: EXTENDS/IMPLEMENTS (issue #33) ------------------------------------------
+// dataflow-app's src/hierarchy.ts is a minimal, first-party heritage fixture: `Rectangle implements
+// Shape`, `Square extends Rectangle implements Labeled`.
+
+describe("neo4j inheritance edges (issue #33)", () => {
+  test("EXTENDS and IMPLEMENTS are declared in the schema catalog", () => {
+    expect(relByType.has("EXTENDS")).toBe(true);
+    expect(relByType.has("IMPLEMENTS")).toBe(true);
+  });
+
+  function nodeBySignature(signature: string) {
+    return rows.nodes.find((n) => n.props.signature === signature);
+  }
+
+  test("hierarchy.ts's first-party heritage projects the expected, non-dangling EXTENDS/IMPLEMENTS edges", () => {
+    const square = nodeBySignature("src/hierarchy.Square");
+    const rectangle = nodeBySignature("src/hierarchy.Rectangle");
+    const shape = nodeBySignature("src/hierarchy.Shape");
+    const labeled = nodeBySignature("src/hierarchy.Labeled");
+    expect(square, "Square node").toBeDefined();
+    expect(rectangle, "Rectangle node").toBeDefined();
+    expect(shape, "Shape node").toBeDefined();
+    expect(labeled, "Labeled node").toBeDefined();
+
+    const ext = rows.edges.filter((e) => e.type === "EXTENDS");
+    const impl = rows.edges.filter((e) => e.type === "IMPLEMENTS");
+    expect(ext.length).toBeGreaterThan(0);
+    expect(impl.length).toBeGreaterThan(0);
+
+    expect(ext.some((e) => e.from.value === square!.value && e.to.value === rectangle!.value)).toBe(true);
+    expect(impl.some((e) => e.from.value === rectangle!.value && e.to.value === shape!.value)).toBe(true);
+    expect(impl.some((e) => e.from.value === square!.value && e.to.value === labeled!.value)).toBe(true);
+
+    const nodeValues = new Set(rows.nodes.map((n) => n.value));
+    for (const e of [...ext, ...impl]) {
+      expect(nodeValues.has(e.from.value), `dangling EXTENDS/IMPLEMENTS source ${e.from.value}`).toBe(true);
+      expect(nodeValues.has(e.to.value), `dangling EXTENDS/IMPLEMENTS target ${e.to.value}`).toBe(true);
+    }
+  });
+});

test/neo4j-schema.test.ts

@@ -170,6 +170,58 @@ describe("schema v2 — L1 tree shape", () => {
   });
 });
 
+// ---- Inheritance: heritage signatures resolve to can:// ids (issue #33) ------------------------
+// sample-app's models.ts is first-party heritage: Entity implements Identifiable; User extends
+// Entity implements Named; Robot implements Named (a second, unrelated implementer of Named).
+// This resolution is unconditional (independent of `-a` level) — it only needs idBySig, which the
+// unconditional L1 tree walk already populates — so it's exercised here even though `st` is L1.
+describe("schema v2 — inheritance resolves heritage signatures to can:// ids (issue #33)", () => {
+  test("a class's extends/implements signatures resolve to their declaring node's id", () => {
+    const types = st["src/models.ts"].types;
+    const entity = types.Entity;
+    const user = types.User;
+    const robot = types.Robot;
+    const identifiable = types.Identifiable;
+    const named = types.Named;
+
+    expect(entity.extends_ids).toBeUndefined(); // no base class, only `implements`
+    expect(entity.implements_ids).toEqual([identifiable.id]);
+
+    expect(user.extends_ids).toEqual([entity.id]);
+    expect(user.implements_ids).toEqual([named.id]);
+
+    expect(robot.extends_ids).toBeUndefined();
+    expect(robot.implements_ids).toEqual([named.id]);
+  });
+
+  test("base_classes/implements_types (signature strings) are unchanged — additive, not replaced", () => {
+    const entity = st["src/models.ts"].types.Entity as unknown as { base_classes: string[]; implements_types: string[] };
+    const user = st["src/models.ts"].types.User as unknown as { base_classes: string[]; implements_types: string[] };
+    const identifiableSig = st["src/models.ts"].types.Identifiable.signature;
+    const entitySig = st["src/models.ts"].types.Entity.signature;
+    const namedSig = st["src/models.ts"].types.Named.signature;
+
+    expect(entity.base_classes).toEqual([identifiableSig]);
+    expect(entity.implements_types).toEqual([identifiableSig]);
+    expect(user.base_classes).toEqual([entitySig, namedSig]);
+    expect(user.implements_types).toEqual([namedSig]);
+  });
+
+  test("an unresolved (external/library) supertype is dropped, not nulled", () => {
+    // Every type in sample-app either has no heritage or fully first-party heritage, so there is no
+    // dropped entry to observe directly here — instead assert the shape invariant the resolver
+    // relies on: extends_ids/implements_ids, when present, are always non-empty arrays of can://
+    // ids (never contain undefined/null placeholders for an unresolved signature).
+    for (const t of Object.values(st["src/models.ts"].types)) {
+      for (const ids of [t.extends_ids, t.implements_ids]) {
+        if (ids === undefined) continue;
+        expect(ids.length).toBeGreaterThan(0);
+        for (const id of ids) expect(id.startsWith("can://")).toBe(true);
+      }
+    }
+  });
+});
+
 describe("schema v2 — L1 superset", () => {
   test("every v1 callable/type signature has a v2 id", () => {
     const v1sigs = new Set<string>();
@@ -511,6 +563,26 @@ function symbolIds(app: V2Application): Set<string> {
   return out;
 }
 
+/** Every type node — classes/interfaces/enums/aliases/namespaces — recursing through nested scopes. */
+function allTypes(app: V2Application): V2Type[] {
+  const out: V2Type[] = [];
+  const walkCallable = (c: V2Callable): void => {
+    for (const cc of Object.values(c.callables ?? {})) walkCallable(cc);
+    for (const t of Object.values(c.types ?? {})) walkType(t);
+  };
+  const walkType = (t: V2Type): void => {
+    out.push(t);
+    for (const c of Object.values(t.callables ?? {})) walkCallable(c);
+    for (const nested of Object.values(t.types ?? {})) walkType(nested);
+    for (const fn of Object.values(t.functions ?? {})) walkCallable(fn);
+  };
+  for (const m of Object.values(app.application.symbol_table)) {
+    for (const t of Object.values(m.types)) walkType(t);
+    for (const c of Object.values(m.functions)) walkCallable(c);
+  }
+  return out;
+}
+
 /** Every body-node key, namespaced by its owning callable id so bare local keys never collide. */
 function bodyKeys(app: V2Application): Set<string> {
   const out = new Set<string>();
@@ -602,6 +674,15 @@ function canNodeIds(app: V2Application): Set<string> {
   return ids;
 }
 
+/** Every `call` body node whose `callee` resolved to an id — the JSON-side source of RESOLVES_TO. */
+function resolvesToCount(app: V2Application): number {
+  let n = 0;
+  for (const c of allCallables(app.application)) {
+    for (const bn of Object.values(c.body)) if (typeof bn.callee === "string") n++;
+  }
+  return n;
+}
+
 describe("neo4j ↔ json count parity — full depth (issue #27)", () => {
   test("node count: 1 :Application row + every :CanNode id", () => {
     expect(monoRows.nodes.length).toBe(1 + canNodeIds(monoApp4).size);
@@ -622,11 +703,50 @@ describe("neo4j ↔ json count parity — full depth (issue #27)", () => {
     // HAS_MODULE/DECLARES/HAS_METHOD/HAS_FIELD/HAS_BODY_NODE have no JSON edge-list counterpart —
     // they ARE the containment tree. Every node except :Application and the off-tree External/
     // AnonymousCallable homes gets exactly one incoming containment edge from its tree parent, so
-    // their total must equal every other CanNode id.
+    // their total must equal every other CanNode id. (RESOLVES_TO and EXTENDS/IMPLEMENTS ALSO have
+    // no top-level JSON edge-list — they're sourced from a body node's `callee` and a type node's
+    // `extends_ids`/`implements_ids` props respectively — but they are not containment either, so
+    // they're deliberately excluded from this invariant too; see the dedicated tests below and the
+    // exhaustive edge-accounting test that folds every relationship family back into one total.)
     const containment = ["HAS_MODULE", "DECLARES", "HAS_METHOD", "HAS_FIELD", "HAS_BODY_NODE"];
     const containmentEdges = containment.reduce((n, t) => n + relCount(monoRows, t), 0);
     const externalCount = Object.keys(monoApp4.application.external_symbols ?? {}).length;
     const synthCount = Object.keys(monoApp4.application.synthesized_callables ?? {}).length;
     expect(containmentEdges).toBe(canNodeIds(monoApp4).size - externalCount - synthCount);
   });
+
+  test("EXTENDS/IMPLEMENTS have no JSON edge-list (extends_ids/implements_ids node props are the source of truth); counts still match 1:1", () => {
+    const types = allTypes(monoApp4);
+    const extendsCount = types.reduce((n, t) => n + (t.extends_ids?.length ?? 0), 0);
+    const implementsCount = types.reduce((n, t) => n + (t.implements_ids?.length ?? 0), 0);
+    // sanity: dataflow-app's first-party hierarchy (src/hierarchy.ts) exercises both relationship
+    // kinds — guards against a vacuous 0 === 0 pass if the fixture ever loses its heritage.
+    expect(extendsCount).toBeGreaterThan(0);
+    expect(implementsCount).toBeGreaterThan(0);
+    expect(relCount(monoRows, "EXTENDS")).toBe(extendsCount);
+    expect(relCount(monoRows, "IMPLEMENTS")).toBe(implementsCount);
+  });
+
+  test("every projected edge is accounted for: typed overlay + containment + RESOLVES_TO + EXTENDS/IMPLEMENTS sums to the total", () => {
+    // The exhaustive form of the parity gate: unlike the per-family tests above (which only prove
+    // each family individually matches its JSON source), this proves nothing is left over — if
+    // project.ts ever grows a new relationship family without this test learning about it, the two
+    // sides diverge and the gate fails, instead of silently passing on an incomplete accounting.
+    const typedOverlay =
+      relCount(monoRows, "CALLS") +
+      relCount(monoRows, "PARAM_IN") +
+      relCount(monoRows, "PARAM_OUT") +
+      relCount(monoRows, "CFG_NEXT") +
+      relCount(monoRows, "CDG") +
+      relCount(monoRows, "DDG") +
+      relCount(monoRows, "SUMMARY");
+    const containment = ["HAS_MODULE", "DECLARES", "HAS_METHOD", "HAS_FIELD", "HAS_BODY_NODE"].reduce(
+      (n, t) => n + relCount(monoRows, t),
+      0,
+    );
+    const resolvesTo = relCount(monoRows, "RESOLVES_TO");
+    const heritage = relCount(monoRows, "EXTENDS") + relCount(monoRows, "IMPLEMENTS");
+    expect(resolvesTo).toBe(resolvesToCount(monoApp4));
+    expect(typedOverlay + containment + resolvesTo + heritage).toBe(monoRows.edges.length);
+  });
 });