From cd85070d5bc10c420183fb04acbfd2b04845d5b8 Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Fri, 25 Sep 2026 10:01:40 -0500
Subject: [PATCH] Handle optional string-search needles with positions

---
 .../lowering/containers/string-and-regexp.ts    | 17 ++++++++++-------
 .../test/ts7/baselines/order-parity.json        |  6 ++++++
 tests/corpus/3029-string-search-union-needle.js | 17 +++++++++++++++++
 3 files changed, 33 insertions(+), 7 deletions(-)
 create mode 100644 tests/corpus/3029-string-search-union-needle.js

diff --git a/packages/compiler/src/frontend/lowering/containers/string-and-regexp.ts b/packages/compiler/src/frontend/lowering/containers/string-and-regexp.ts
index 8a1d5fd6..a3b370ca 100644
--- a/packages/compiler/src/frontend/lowering/containers/string-and-regexp.ts
+++ b/packages/compiler/src/frontend/lowering/containers/string-and-regexp.ts
@@ -274,7 +274,8 @@ export function lowerStringMethodCall(lowerer: Lowerer, call: ts.CallExpression,
   if ((entry.method === "indexOf" || entry.method === "includes" ||
     entry.method === "startsWith" || entry.method === "endsWith") && call.arguments.length === 2) {
     const needle = lowerer.lowerExpr(call.arguments[0]!);
-    if (needle.type.kind !== "string" && needle.type.kind !== "dyn") {
+    const optionalNeedle = needle.type.kind === "union" && lowerer.runtimeOptionalWidening(needle.type, STRING) !== null;
+    if (needle.type.kind !== "string" && needle.type.kind !== "dyn" && !optionalNeedle) {
       lowerer.noLowering(`.${entry.method} with '${lowerer.fmt(needle.type)}' search values`, call);
     }
     const defaultPosition: IrExpr = entry.method === "endsWith"
@@ -289,8 +290,8 @@ export function lowerStringMethodCall(lowerer: Lowerer, call: ts.CallExpression,
     if (!helper) {
       helper = `%str.positions.${lowerer.widthHelpers.size}`;
       const params = [receiver, needle, position].map((arg, index) => ({ localId: `arg.${index}`, name: `arg${index}`, type: arg.type }));
-      const dynamicNeedle = needle.type.kind === "dyn";
-      const search: IrExpr = dynamicNeedle ? varRef("search.0", STRING, loc) : varRef("arg.1", STRING, loc);
+      const coerceNeedle = needle.type.kind !== "string";
+      const search: IrExpr = coerceNeedle ? varRef("search.0", STRING, loc) : varRef("arg.1", STRING, loc);
       const result: IrExpr = {
         kind: "strIntrinsic", method: entry.method, receiver: varRef("arg.0", STRING, loc),
         args: [
@@ -301,11 +302,13 @@ export function lowerStringMethodCall(lowerer: Lowerer, call: ts.CallExpression,
       };
       const locals = params.map(param => ({ id: param.localId, name: param.name, type: param.type, mutable: false }));
       const body: IrStmt[] = [];
-      if (dynamicNeedle) {
+      if (coerceNeedle) {
+        const value = varRef("arg.1", needle.type, loc);
+        const init: IrExpr = needle.type.kind === "dyn"
+          ? { kind: "libCall", fn: "dyn.toStringCoerce", args: [value], type: STRING, loc }
+          : lowerer.ensureString(value, call.arguments[0]!);
         locals.push({ id: "search.0", name: "search", type: STRING, mutable: false });
-        body.push({ kind: "varDecl", localId: "search.0", init: {
-          kind: "libCall", fn: "dyn.toStringCoerce", args: [varRef("arg.1", needle.type, loc)], type: STRING, loc,
-        }, loc });
+        body.push({ kind: "varDecl", localId: "search.0", init, loc });
       }
       body.push({ kind: "return", value: result, loc });
       lowerer.widthHelpers.set(key, helper);
diff --git a/packages/compiler/test/ts7/baselines/order-parity.json b/packages/compiler/test/ts7/baselines/order-parity.json
index 22b4a9d2..bdb78512 100644
--- a/packages/compiler/test/ts7/baselines/order-parity.json
+++ b/packages/compiler/test/ts7/baselines/order-parity.json
@@ -6943,6 +6943,12 @@
    ],
    "diags": []
   },
+  "<repo>/tests/corpus/3029-string-search-union-needle.js": {
+   "order": [
+    "<repo>/tests/corpus/3029-string-search-union-needle.js"
+   ],
+   "diags": []
+  },
   "<repo>/tests/corpus/303-break-continue.ts": {
    "order": [
     "<repo>/tests/corpus/303-break-continue.ts"
diff --git a/tests/corpus/3029-string-search-union-needle.js b/tests/corpus/3029-string-search-union-needle.js
new file mode 100644
index 00000000..dd7e1ebb
--- /dev/null
+++ b/tests/corpus/3029-string-search-union-needle.js
@@ -0,0 +1,17 @@
+/** @param {string | undefined} needle */
+function probe(needle) {
+  const value = "ababa";
+  console.log(
+    value.indexOf(needle),
+    value.indexOf(needle, 1),
+    value.includes(needle),
+    value.includes(needle, 1),
+    value.startsWith(needle),
+    value.startsWith(needle, 1),
+    value.endsWith(needle),
+    value.endsWith(needle, 2),
+  );
+}
+
+probe("ba");
+probe(undefined);