feat: add element-wise intersection

In typescript 3.9.0, the intersection between two objects will be
`never` if the two objects have incompatible private members. This broke
an implicit behavior that `CombineObjects` was taking advantage of;
element-wise intersection. Because `CombineObjects` is intended to be a
thin wrapper over the raw intersection type, I don't want its contract
to deviate from the intersection. So instead I added the
`ElementwiseIntersect` utility; which relies on the newly added `TryKey`
utility.

`TryKey` is just like `GetKey`, except it fails "silently" when the key
does not exist. Specifically, `GetKey` returns `never` so that the
resultant type is unusable and `TryKey` returns `unknown` which can be
eliminated via intersection.

Relevant PR from typescript which changed behavior of intersections (and
broke the future-proofing test cases):
microsoft/TypeScript#37762

src/types/objects.ts

@@ -34,6 +34,23 @@ export type CombineObjects<T extends object, U extends object> = ObjectType<T &
  * @returns `T[K]` if the key exists, `never` otherwise
  */
 export type GetKey<T, K extends keyof any> = K extends keyof T ? T[K] : never;
+/**
+ * Like `GetKey`, but returns `unknown` if the key is not present on the object.
+ * @param T Object to get values from
+ * @param K Key to query object for value
+ * @returns `T[K]` if the key exists, `unknown` otherwise
+ */
+export type TryKey<T, K extends keyof any> = K extends keyof T ? T[K]: unknown;
+/**
+ * Takes two objects and returns their element-wise intersection.
+ * *Note*: this removes any key-level information, such as optional or readonly keys.
+ * @param T First object to be intersected
+ * @param U Second object to be intersected
+ * @returns element-wise `T` & `U` cleaned up to look like flat object to VSCode
+ */
+export type ElementwiseIntersect<T extends object, U extends object> = {
+    [k in (keyof T | keyof U)]: TryKey<T, k> & TryKey<U, k>;
+};
 
 // ----
 // Keys

test/objects/CombineObjects.test.ts

@@ -5,12 +5,12 @@ import { CombineObjects } from '../../src';
 
 test('Can combine two objects (without pesky & in vscode)', t => {
     type a = { x: number, y: 'hi' };
-    type b = { z: number, y: 'there' };
+    type b = { z: number };
 
     type got = CombineObjects<a, b>;
     type expected = {
         x: number,
-        y: 'hi' & 'there',
+        y: 'hi',
         z: number
     };
 

test/objects/ElementwiseIntersect.test.ts

@@ -0,0 +1,44 @@
+import test from 'ava';
+import { assert } from '../helpers/assert';
+
+import { ElementwiseIntersect } from '../../src';
+
+test('Can combine two objects elementwise', t => {
+    type a = { x: number, y: 'hi' };
+    type b = { z: number, y: 'there' };
+
+    type got = ElementwiseIntersect<a, b>;
+    type expected = {
+        x: number,
+        y: 'hi' & 'there',
+        z: number,
+    };
+
+    assert<got, expected>(t);
+    assert<expected, got>(t);
+});
+
+test('Can combine two objects with private members elementwise', t => {
+    class A {
+        a: number = 1;
+        private x: number = 2;
+        y: 'hi' = 'hi';
+        private z: 'hey' = 'hey';
+    }
+
+    class B {
+        a: 22 = 22;
+        private x: number = 2;
+        y: 'there' = 'there';
+        private z: 'friend' = 'friend';
+    }
+
+    type got = ElementwiseIntersect<A, B>;
+    type expected = {
+        a: 22,
+        y: 'hi' & 'there',
+    };
+
+    assert<got, expected>(t);
+    assert<expected, got>(t);
+});