Read an Error subclass's own code through NodeJS.ErrnoException (#1471)

packages/compiler/src/frontend/lowering/builtins/errors.ts

@@ -1,7 +1,16 @@
 import * as ts from "../../ts7/adapter.js";
 import { type Lowerer } from "../lowerer.js";
 import { locOf } from "../../program.js";
-import { BOOL, CAUGHT, DYN, F64, type IrExpr, type IrType, STRING } from "../../../ir/ir.js";
+import {
+  BOOL,
+  CAUGHT,
+  DYN,
+  DYN_CLASS_PROPERTIES,
+  F64,
+  type IrExpr,
+  type IrType,
+  STRING,
+} from "../../../ir/ir.js";
 
 const ASSERTION_ERROR_PROPERTIES: ReadonlySet<string> = new Set([
   "actual",
@@ -126,7 +135,13 @@ export function lowerErrorCodeProperty(
       declared,
     );
   }
-  const systemField = ["errno", "syscall", "path", "dest"].includes(expr.name.text);
+  // A program class under the receiver's class can carry its own `code`
+  // (a declared field or a property added through an untyped reference),
+  // which Node reads instead of the errno code: read through the error's
+  // dynamic view, which answers both.
+  const systemField =
+    ["errno", "syscall", "path", "dest"].includes(expr.name.text) ||
+    (expr.name.text === "code" && programClassMayOwnCode(lowerer, recvT.className));
   if (
     !systemField &&
     expr.name.text !== "code" &&
@@ -162,7 +177,9 @@ export function lowerErrorCodeProperty(
         type: DYN,
         loc,
       },
-      lowerer.withUndefinedArm(expr.name.text === "errno" ? F64 : STRING),
+      expr.name.text === "code"
+        ? lowerer.envValueType()
+        : lowerer.withUndefinedArm(expr.name.text === "errno" ? F64 : STRING),
     );
   }
   return {
@@ -184,6 +201,19 @@ export function lowerErrorCodeProperty(
   };
 }
 
+/** Whether a class declared in the program, `className` or one of its
+ * subclasses, has its own `code` field or a dynamic property bag. The
+ * runtime error classes keep `code` in the errno slot. */
+function programClassMayOwnCode(lowerer: Lowerer, className: string): boolean {
+  for (const [name, info] of lowerer.classes) {
+    if (name.startsWith("%")) continue;
+    if (name !== className && !lowerer.isSubclassOf(name, className)) continue;
+    if (info.def.fields.some((f) => f.name === "code" || f.name === DYN_CLASS_PROPERTIES))
+      return true;
+  }
+  return false;
+}
+
 /** Whether values of `type` are errors: class instances whose class is
  * %Error or extends it. A subclass of a class that does not extend Error
  * cannot be an error either, so the answer depends on the static class

packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6348-errno-code-own-field.ts.json

@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/6348-errno-code-own-field.ts"
+ ],
+ "diags": []
+}

tests/corpus/6348-errno-code-own-field.ts

@@ -0,0 +1,46 @@
+// `(err as NodeJS.ErrnoException).code` reads an Error subclass's own
+// `code` field, as Node reads the own property, and still reads the errno
+// code of runtime errors and undefined for plain errors.
+import { readFileSync } from "node:fs";
+
+class AppError extends Error {
+  code: string;
+  constructor(message: string, code = "E_APP") {
+    super(message);
+    this.name = "AppError";
+    this.code = code;
+  }
+}
+class DbError extends AppError {
+  constructor() {
+    super("db", "E_DB");
+  }
+}
+class Later extends Error {
+  code?: string;
+}
+
+function codeOf(err: unknown): string {
+  if (err instanceof Error) return (err as NodeJS.ErrnoException).code ?? "(none)";
+  return "(not an error)";
+}
+
+console.log(codeOf(new AppError("direct")));
+console.log(codeOf(new DbError()));
+const wrapped = new TypeError("wrapped", { cause: new AppError("cause", "E_CAUSE") });
+console.log(codeOf(wrapped.cause), codeOf(wrapped));
+try {
+  throw new DbError();
+} catch (err) {
+  console.log(codeOf(err));
+}
+const later = new Later("later");
+console.log(codeOf(later));
+later.code = "E_LATER";
+console.log(codeOf(later));
+try {
+  readFileSync("/nonexistent/scriptc-corpus-6348");
+} catch (err) {
+  console.log(codeOf(err));
+}
+console.log(codeOf(new Error("plain")), codeOf(new RangeError("range")), codeOf("text"));

tests/harness/corpus-costs/6348-errno-code-own-field.ts.json

@@ -0,0 +1 @@
+{ "plain": 1200, "san": 1500 }