Preserve parameter inference

src/types.ts

@@ -3,8 +3,8 @@ export type NoInfer<T> = [T][T extends any ? 0 : never];
 /**
  * Adapted from type-fest's PartialDeep
  */
-export type PartialDeep<T> = T extends (...args: any[]) => any
-  ? PartialDeepObject<T> | undefined
+export type PartialDeep<T> = T extends (...args: infer P) => infer R
+  ? ((...args: P) => PartialDeep<R> | void) & PartialDeepObject<T> | PartialDeepObject<T> | undefined
   : T extends object
   ? T extends ReadonlyArray<infer ItemType> // Test for arrays/tuples, per https://github.com/microsoft/TypeScript/issues/35156
     ? ItemType[] extends T // Test for arrays (non-tuples) specifically

tests/fromPartial.test.ts

@@ -69,6 +69,10 @@ describe("fromPartial", () => {
   it("Should accept functions", () => {
     accept<() => void>(fromPartial({}));
   });
+
+  it("Should deeply infer types from functions", () => {
+    accept<(foo: string) => (bar: number) => {baz: string}>(fromPartial((foo) => (bar) => {}));
+  });
 
   it("Should accept interfaces which combine object properties and a call signature", () => {
     accept<{
@@ -89,4 +93,17 @@ describe("fromPartial", () => {
       }),
     );
   });
+
+  it("Should handle deeply recursive partial types", () => {
+    interface RecursiveInterface {
+      (bar: string) : RecursiveInterface;
+      foo: {
+        deepProperty: string
+      },
+    }
+    accept<RecursiveInterface>(
+        fromPartial(bar => (bar2)=> ({})),
+    );
+  });
+
 });