From c21fc0120180fb89bf6ff2251bba30d6739f3c7b Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Sun, 25 Oct 2026 14:49:55 -0500
Subject: [PATCH] Key bound alias instances by the declared alias

The compiler compiles itself, and a Map keyed by another Map is outside
the key domains it compiles (SC2009 in the self-hosted frontend). The
Lowerer's table is now keyed by the declared alias reference: each entry
lists the instances that instantiate it with their ts-level bindings maps,
and the resolver picks the current instance's by identity.
---
 .../frontend/lowering/generic-functions.ts    |  7 ++--
 .../compiler/src/frontend/lowering/lowerer.ts | 32 ++++++++++++-------
 2 files changed, 24 insertions(+), 15 deletions(-)

diff --git a/packages/compiler/src/frontend/lowering/generic-functions.ts b/packages/compiler/src/frontend/lowering/generic-functions.ts
index a2eda839a..1769e8562 100644
--- a/packages/compiler/src/frontend/lowering/generic-functions.ts
+++ b/packages/compiler/src/frontend/lowering/generic-functions.ts
@@ -1041,11 +1041,12 @@ function unifySignatureBindings(
       iAliasArgs.length === dAliasArgs.length
     ) {
       if (tsBindings && declared !== inst) {
-        let pairs = lowerer.boundAliasInstances.get(tsBindings);
-        if (!pairs) lowerer.boundAliasInstances.set(tsBindings, (pairs = new Map()));
+        let entries = lowerer.boundAliasInstances.get(declared);
+        if (!entries) lowerer.boundAliasInstances.set(declared, (entries = []));
         // A body forwarding its own alias (`inner(g)` returning `Catalog<G,
         // "x">` inside an instance) resolves through the enclosing one.
-        if (!pairs.has(declared)) pairs.set(declared, lowerer.boundAliasTsResolver(inst) ?? inst);
+        if (!entries.some((entry) => entry.tsBindings === tsBindings))
+          entries.push({ tsBindings, instance: lowerer.boundAliasTsResolver(inst) ?? inst });
       }
       dAliasArgs.forEach((da, i) => {
         const ia = iAliasArgs[i];
diff --git a/packages/compiler/src/frontend/lowering/lowerer.ts b/packages/compiler/src/frontend/lowering/lowerer.ts
index 216851c9c..c77741997 100644
--- a/packages/compiler/src/frontend/lowering/lowerer.ts
+++ b/packages/compiler/src/frontend/lowering/lowerer.ts
@@ -2233,16 +2233,19 @@ export class Lowerer {
    * the body needs — indexed accesses (`T[K]` needs K's literal key) and
    * keyed record reads (`o[k]` where k's type is a literal-bound K). */
   typeParamTsBindings: Map<ts.Symbol, ts.Type> | null = null;
-  /** Generic alias references of a call-keyed instance's declared signature
-   * (`Catalog<G, P>`), each paired with the same alias as the call's
-   * resolved signature instantiates it (`Catalog<{ g1: … }, "acme">`),
-   * keyed by the instance's ts-level bindings (typeParamTsBindings). The
-   * checker keeps the alias symbolic inside the generic body and offers no
-   * way to instantiate it with the bound arguments, but alias
-   * instantiations are interned: the body's `x as Catalog<G, P>` is the
-   * very type the signature declares, so the pair recorded during binding
-   * unification is that re-instantiation. */
-  readonly boundAliasInstances = new Map<Map<ts.Symbol, ts.Type>, Map<ts.Type, ts.Type>>();
+  /** Generic alias references of call-keyed instances' declared signatures
+   * (`Catalog<G, P>`), each with the same alias as every call's resolved
+   * signature instantiates it (`Catalog<{ g1: … }, "acme">`), together with
+   * that instance's ts-level bindings map (typeParamTsBindings), which every
+   * scope that lowers the instance installs. The checker keeps the alias
+   * symbolic inside the generic body and offers no way to instantiate it
+   * with the bound arguments, but alias instantiations are interned: the
+   * body's `x as Catalog<G, P>` is the very type the signature declares, so
+   * the pair recorded during unification is that re-instantiation. */
+  readonly boundAliasInstances = new Map<
+    ts.Type,
+    { tsBindings: Map<ts.Symbol, ts.Type>; instance: ts.Type }[]
+  >();
   /** Non-null while an IMPLICIT-ANY instance body lowers (npm-static JS;
    * see generic-functions.ts): bound param symbol →
    * the call site's checker type, consulted by typeOf for identifier
@@ -9563,10 +9566,15 @@ export class Lowerer {
 
   /** The concrete instantiation of a generic alias reference (`Catalog<G,
    * P>`) the current call-keyed instance's signature declares
-   * (boundAliasInstances). Inert outside generic function instantiation. */
+   * (boundAliasInstances), or null when its signature never instantiated
+   * that reference. Inert outside generic function instantiation. */
   readonly boundAliasTsResolver = (t: ts.Type): ts.Type | null => {
     if (!this.typeParamTsBindings || t.getAliasTypeArguments().length === 0) return null;
-    return this.boundAliasInstances.get(this.typeParamTsBindings)?.get(t) ?? null;
+    const tsBindings = this.typeParamTsBindings;
+    return (
+      this.boundAliasInstances.get(t)?.find((entry) => entry.tsBindings === tsBindings)?.instance ??
+      null
+    );
   };
 
   irTypeOf(node: ts.Node): IrType {