From 327a9385edbdec572c2bb320530f5984c863ad35 Mon Sep 17 00:00:00 2001
From: Glen Whitney <glen@studioinfinity.org>
Date: Wed, 23 Aug 2023 16:52:16 +0000
Subject: [PATCH] fix: Use intersection of matching types in AssociatedTypes<>
 lookup (#14)

Resolves #11.
Reviewed-on: https://code.studioinfinity.org/glen/typocomath/pulls/14
Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Co-committed-by: Glen Whitney <glen@studioinfinity.org>
---
 src/interfaces/type.ts | 10 +++++++---
 src/numbers/type.ts    |  2 +-
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/src/interfaces/type.ts b/src/interfaces/type.ts
index 9e6870d..9fb1163 100644
--- a/src/interfaces/type.ts
+++ b/src/interfaces/type.ts
@@ -13,6 +13,10 @@
  * but that's OK, the generic parameter doesn't hurt in those cases.
  ****/
 
+type ValueIntersectionByKeyUnion<T,  TKey extends keyof T> = {
+  [P in TKey]: (k: T[P])=>void
+} [TKey] extends ((k: infer I)=>void) ? I : never
+
 export interface AssociatedTypes<T> {
    undefined: {
       type: undefined
@@ -24,10 +28,10 @@ export interface AssociatedTypes<T> {
 }
 
 type AssociatedTypeNames = keyof AssociatedTypes<unknown>['undefined']
-type ALookup<T, Name extends AssociatedTypeNames> = {
+type ALookup<T, Name extends AssociatedTypeNames> = ValueIntersectionByKeyUnion<{
    [K in keyof AssociatedTypes<T>]:
-   T extends AssociatedTypes<T>[K]['type'] ? AssociatedTypes<T>[K][Name] : never
-}[keyof AssociatedTypes<T>]
+   T extends AssociatedTypes<T>[K]['type'] ? AssociatedTypes<T>[K][Name] : unknown},
+   keyof AssociatedTypes<T>>
 
 // For everything to compile, zero and one must be subtypes of T:
 export type ZeroType<T> = ALookup<T, 'zero'> & T
diff --git a/src/numbers/type.ts b/src/numbers/type.ts
index 11e5d1c..48fb95e 100644
--- a/src/numbers/type.ts
+++ b/src/numbers/type.ts
@@ -8,7 +8,7 @@ export const number_type = {
 
 declare module "../interfaces/type" {
    interface AssociatedTypes<T> {
-      numbers: {
+      number: {
          type: number
          zero: 0
          one: 1