✅ Pin the duplicate-tag union collapse

src/tagged-union.test.ts

@@ -1244,6 +1244,82 @@ test("property union: a collision is rejected beside legal members", () => {
     getTaggedUnionMatcher<Colliding>()("kind")({ true: () => 1, a: () => 2 });
 });
 
+// ============================================================================
+// Duplicate tags
+// ============================================================================
+
+// A tag is a discriminant only while it is unique across the union. Two members
+// may still share one: the tag set `Tags<T, K>` dedupes, so a single handler is
+// exhaustive for both and receives their union. This is the documented known
+// issue (development/library.md § Tagged-union matcher); the tests below pin the
+// behavior so it cannot change silently.
+
+test("duplicate tag: members sharing a tag collapse to one handler", () => {
+    // Arrange — `kind` is not a true discriminant: both members carry `"a"`.
+    interface First {
+        readonly kind: "a";
+        readonly first: number;
+    }
+    interface Second {
+        readonly kind: "a";
+        readonly second: string;
+    }
+    type Clashing = First | Second;
+    const factory = getTaggedUnionMatcher<Clashing>()("kind");
+
+    // Act
+    const pick = factory({
+        a: (s) => {
+            // The shared tag cannot be split, so the handler sees both members.
+            expectTypeOf(s).toEqualTypeOf<First | Second>();
+            assert.equal(s.kind, "a");
+            return "first" in s ? s.first : s.second.length;
+        },
+    });
+
+    // Assert — the one `a` key is exhaustive and both members reach it.
+    expectTypeOf(pick).toEqualTypeOf<(shape: Clashing) => number>();
+    assert.equal(pick({ kind: "a", first: 1 }), 1);
+    assert.equal(pick({ kind: "a", second: "abc" }), 3);
+});
+
+test("duplicate tag: handling one tag consumes every member that shares it", () => {
+    // Arrange — `"a"` selects two members; `"c"` selects one.
+    type Mixed =
+        | { readonly kind: "a"; readonly a: number }
+        | { readonly kind: "a"; readonly b: string }
+        | { readonly kind: "c"; readonly c: boolean };
+    const factory = getTaggedUnionMatcher<Mixed>()("kind");
+
+    // Act
+    const describe = factory(
+        {
+            a: (s) => {
+                expectTypeOf(s).toEqualTypeOf<
+                    | { readonly kind: "a"; readonly a: number }
+                    | { readonly kind: "a"; readonly b: string }
+                >();
+                return 1 as const;
+            },
+        },
+        (s) => {
+            // Handling `"a"` removes both of its members, not just one.
+            expectTypeOf(s).toEqualTypeOf<{
+                readonly kind: "c";
+                readonly c: boolean;
+            }>();
+            assert.equal(s.kind, "c");
+            return 2 as const;
+        },
+    );
+
+    // Assert
+    expectTypeOf(describe).toEqualTypeOf<(shape: Mixed) => 1 | 2>();
+    assert.equal(describe({ kind: "a", a: 1 }), 1);
+    assert.equal(describe({ kind: "a", b: "x" }), 1);
+    assert.equal(describe({ kind: "c", c: true }), 2);
+});
+
 // ============================================================================
 // Dispatch — runtime behavior
 // ============================================================================