Fix type narrowing of branded string types

src/types/helpers.ts

@@ -121,7 +121,10 @@ type BuiltInObjects =
   | { readonly [Symbol.toStringTag]: string };
 
 export type IsPlainObject<o> = o extends object
-  ? o extends BuiltInObjects
+  ? // to excluded branded string types,
+    // like `string & { __brand: "id" }`
+    // and built-in objects
+    o extends string | BuiltInObjects
     ? false
     : true
   : false;

tests/branded-nominal-types.test.ts

@@ -0,0 +1,26 @@
+import { match, when } from '../src';
+
+describe('Branded strings', () => {
+  type BrandedId = string & { __brand: 'brandId' };
+  type FooBar = { type: 'foo'; id: BrandedId; value: string } | { type: 'bar' };
+  type State = {
+    fooBar: FooBar;
+    fooBarId: BrandedId;
+  };
+
+  it('should treat branded strings as regular string, and not as objects', () => {
+    const state: State = {
+      fooBar: { type: 'foo', id: '' as BrandedId, value: 'value' },
+      fooBarId: '' as BrandedId,
+    };
+
+    expect(
+      match(state)
+        .with(
+          { fooBar: { type: 'foo' }, fooBarId: when((id) => id === '') },
+          (x) => `Match: ${x.fooBar.value}`
+        )
+        .otherwise(() => 'nope')
+    ).toEqual('Match: value');
+  });
+});

tests/extract-precise-value.test.ts

@@ -222,4 +222,42 @@ describe('ExtractPreciseValue', () => {
       ];
     });
   });
+
+  describe('Branded strings', () => {
+    it('Type narrowing should correctly work on branded strings', () => {
+      // Branded strings is a commonly used way of implementing
+      // nominal types in typescript.
+
+      type BrandedId = string & { __brand: 'brandId' };
+
+      type FooBar =
+        | { type: 'foo'; id: BrandedId; value: string }
+        | { type: 'bar' };
+
+      type cases = [
+        Expect<
+          Equal<
+            ExtractPreciseValue<
+              {
+                fooBar: FooBar;
+                fooBarId: BrandedId;
+              },
+              {
+                fooBar: { type: 'foo' };
+                fooBarId: BrandedId;
+              }
+            >,
+            {
+              fooBar: {
+                type: 'foo';
+                id: BrandedId;
+                value: string;
+              };
+              fooBarId: BrandedId;
+            }
+          >
+        >
+      ];
+    });
+  });
 });