From c2963628343abb47685249cedaa2613bb612c888 Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Fri, 23 Oct 2026 14:21:21 -0500
Subject: [PATCH] Park typed collection iterators on the entry they read

An early exit from a loop over an iterator-typed Map or Set method parks
the iterator with a second yield of the current entry. That yield read the
entry again by its index, which aborted with a map iteration error when
the loop had deleted the entry. Each step now reads the entry once into a
local that both yields answer.

`entries()` takes the typed walk only for a slot whose yield type is the
key and value pair record. Other yield types, such as
`IterableIterator<any>` or `<unknown>`, keep the checked adapter instead of
failing to compile.
---
 .../frontend/lowering/iterator-adapters.ts    | 28 +++++++++-
 .../7746-key-order-iterable-iterator.ts       | 51 +++++++++++++++++++
 2 files changed, 78 insertions(+), 1 deletion(-)

diff --git a/packages/compiler/src/frontend/lowering/iterator-adapters.ts b/packages/compiler/src/frontend/lowering/iterator-adapters.ts
index a8b4ea496..9f2b1e31f 100644
--- a/packages/compiler/src/frontend/lowering/iterator-adapters.ts
+++ b/packages/compiler/src/frontend/lowering/iterator-adapters.ts
@@ -84,6 +84,26 @@ function collectionIteratorSlotValue(
       collT.kind === "map"
         ? { kind: "mapIntrinsic", method: m, receiver: coll, args: [index], type: t, loc }
         : { kind: "setIntrinsic", method: "iterKey", receiver: coll, args: [index], type: t, loc };
+    // entries() answers [key, value] pairs: only a slot whose yield type is
+    // that pair as a record of the two fields takes the typed walk; a tuple
+    // or other pair type keeps the checked adapter.
+    if (method === "entries") {
+      const pair =
+        expected.yieldT.kind === "record" ? lowerer.shapes.get(expected.yieldT.shapeId) : undefined;
+      const field = (name: string): IrType | undefined =>
+        pair?.fields.find((f) => f.name === name)?.type;
+      const first = field("0");
+      const second = field("1");
+      if (
+        pair === undefined ||
+        pair.fields.length !== 2 ||
+        first === undefined ||
+        second === undefined ||
+        !typeEquals(first, keyT) ||
+        !typeEquals(second, valueT)
+      )
+        return null;
+    }
     const entry = (): IrExpr =>
       method === "keys" || (collT.kind === "set" && method === "values")
         ? read("iterKey", keyT)
@@ -99,9 +119,13 @@ function collectionIteratorSlotValue(
               loc,
             };
     if (!typeEquals(entry().type, expected.yieldT)) return null;
+    // The entry is read once per step and kept: the parking yield of an
+    // early exit answers it again rather than rereading a slot the loop may
+    // have deleted.
+    const current = varRef("current.0", expected.yieldT, loc);
     const suspend = (): IrStmt => ({
       kind: "exprStmt",
-      expr: { kind: "yieldExpr", value: entry(), type: resumeType(expected.nextT), loc },
+      expr: { kind: "yieldExpr", value: current, type: resumeType(expected.nextT), loc },
       loc,
     });
     const finish: IrStmt = {
@@ -120,6 +144,7 @@ function collectionIteratorSlotValue(
         { id: "native.0", name: "native", type: DYN, mutable: false },
         { id: "index.0", name: "index", type: F64, mutable: true },
         { id: "resumed.0", name: "resumed", type: BOOL, mutable: true },
+        { id: "current.0", name: "current", type: expected.yieldT, mutable: false },
       ],
       body: [
         {
@@ -173,6 +198,7 @@ function collectionIteratorSlotValue(
               loc,
             },
             setResumed(false, loc),
+            { kind: "varDecl", localId: "current.0", init: entry(), loc },
             {
               kind: "tryCatch",
               tryBody: [suspend(), setResumed(true, loc)],
diff --git a/tests/corpus/7746-key-order-iterable-iterator.ts b/tests/corpus/7746-key-order-iterable-iterator.ts
index 13f27ee83..d9d4ea15a 100644
--- a/tests/corpus/7746-key-order-iterable-iterator.ts
+++ b/tests/corpus/7746-key-order-iterable-iterator.ts
@@ -56,3 +56,54 @@ const ordered = [...newest(listed)];
 console.log(JSON.stringify(ordered), ordered[0] === listed[1]);
 const step = newest(listed).next();
 console.log(step.done, JSON.stringify(step.value));
+
+// Deleting the current entry before leaving a loop over an iterator-typed
+// method: the iterator parks after that entry and continues with the next.
+class Store {
+  readonly rows = new Map<string, number>();
+  readonly tags = new Set<string>();
+  values(): IterableIterator<number> {
+    return this.rows.values();
+  }
+  keys(): IterableIterator<string> {
+    return this.rows.keys();
+  }
+  tagValues(): IterableIterator<string> {
+    return this.tags.values();
+  }
+  anyPairs(): IterableIterator<any> {
+    return this.rows.entries();
+  }
+  unknownPairs(): IterableIterator<unknown> {
+    return this.rows.entries();
+  }
+  tagAny(): IterableIterator<any> {
+    return this.tags.entries();
+  }
+}
+const store = new Store();
+for (const [k, v] of [["a", 1], ["b", 2], ["c", 3]] as const) store.rows.set(k, v);
+for (const t of ["x", "y", "z"]) store.tags.add(t);
+console.log(JSON.stringify([...store.anyPairs()]), JSON.stringify([...store.unknownPairs()]));
+console.log(JSON.stringify([...store.tagAny()]));
+const values = store.values();
+for (const v of values) {
+  if (v === 2) {
+    store.rows.delete("b");
+    break;
+  }
+}
+console.log([...values].join(), [...store.rows.keys()].join());
+for (const k of store.keys()) {
+  store.rows.delete(k);
+  break;
+}
+console.log([...store.rows.keys()].join());
+const tags = store.tagValues();
+for (const t of tags) {
+  if (t === "y") {
+    store.tags.delete("y");
+    break;
+  }
+}
+console.log([...tags].join(), [...store.tags].join());