minor implementation cleanups

Author: michaelficarra

src/index.ts

@@ -1,10 +1,5 @@
 function concatImpl(): Iterator<never>
-function concatImpl<A>(...iterables: Array<Iterable<A>>): Iterator<A>
-function concatImpl<A>(iterableA: Iterable<A>): Iterator<A>
-function concatImpl<A, B>(iterableA: Iterable<A>, iterableB: Iterable<B>): Iterator<A | B>
-function concatImpl<A, B, C>(iterableA: Iterable<A>, iterableB: Iterable<B>, iterableC: Iterable<C>): Iterator<A | B | C>
-function concatImpl<A, B, C, D>(iterableA: Iterable<A>, iterableB: Iterable<B>, iterableC: Iterable<C>, iterableD: Iterable<D>): Iterator<A | B | C | D>
-function concatImpl<A, B, C, D, E>(iterableA: Iterable<A>, iterableB: Iterable<B>, iterableC: Iterable<C>, iterableD: Iterable<D>, iterableE: Iterable<E>): Iterator<A | B | C | D | E>
+function concatImpl<T extends Iterable<unknown>[]>(...iterables: T): Iterator<T[number] extends Iterable<infer U> ? U : never, undefined, void>;
 function concatImpl(...iterables: Array<Iterable<unknown>>): Iterator<unknown>
 function concatImpl(...iterables: Array<unknown>): Iterator<unknown> {
   const openMethods: Array<{ openMethod: () => Iterator<unknown>, iterable: Iterable<unknown>}> = [];
@@ -21,8 +16,8 @@ function concatImpl(...iterables: Array<unknown>): Iterator<unknown> {
   return Iterator.from({
     next() {
       while (!done) {
-        if (iterator != null && nextMethod != null) {
-          let iterResult = nextMethod.call(iterator);
+        if (iterator != null) {
+          let iterResult = nextMethod!.call(iterator);
           if (!iterResult.done) {
             return iterResult;
           }
@@ -48,7 +43,7 @@ function concatImpl(...iterables: Array<unknown>): Iterator<unknown> {
         }
         done = true;
       }
-      return { done: true, value: void 0, };
+      return { done: true, value: void 0 };
     },
   });
 }