Add a regression test for issue-70703

Author: JohnTitor

src/test/ui/inference/issue-70703.rs

@@ -0,0 +1,26 @@
+// check-pass
+
+trait Factory {
+    type Product;
+}
+
+impl Factory for () {
+    type Product = ();
+}
+
+trait ProductConsumer<P> {
+    fn consume(self, product: P);
+}
+
+impl<P> ProductConsumer<P> for () {
+    fn consume(self, _: P) {}
+}
+
+fn make_product_consumer<F: Factory>(_: F) -> impl ProductConsumer<F::Product> {
+    ()
+}
+
+fn main() {
+    let consumer = make_product_consumer(());
+    consumer.consume(());
+}