feat: ✨ Add DeepReturnType

.changeset/wet-pens-worry.md

@@ -0,0 +1,5 @@
+---
+"@crbroughton/ts-test-utils": minor
+---
+
+Add DeepReturnType

index.ts

@@ -60,3 +60,10 @@ export type NonVoid<Actual> = Actual extends void ? never : Actual
 export type GetParameters<T> = T extends (...args: infer P) => any ? P : never
 
 export type GetArrayUnion<T> = T extends readonly (infer U)[] ? U : never
+
+export type DeepReturnType<T> =
+  T extends (...args: unknown[]) => infer ReturnType
+    ? ReturnType extends (...args: unknown[]) => unknown
+      ? DeepReturnType<ReturnType>
+      : ReturnType
+    : never

tests/DeepReturnType.test.ts

@@ -0,0 +1,22 @@
+/* eslint-disable unused-imports/no-unused-vars */
+import type { DeepReturnType, Equals, Expect } from '..'
+
+function multiLevelFunction() {
+  return () => {
+    return () => {
+      return () => {
+        return 42
+      }
+      return false
+    }
+    return 'hello'
+  }
+}
+
+type Result = Expect<Equals<DeepReturnType<typeof multiLevelFunction>, number | false | 'hello'>>
+//   ^?
+
+// Failed the DeepReturnType test when the resulting type does equal the expectation
+// @ts-expect-error - GetParameters failing the equality checker
+type ResultFailure = Expect<Equals<DeepReturnType<typeof multiLevelFunction>, number | false | string>>
+//   ^?