Mystery plant

In July this plant suddenly appeared in the edge of my Japanese spurge garden. The extra shows the leaves. I thought maybe it could be a pumpkin, squash, gourd, or cucumber plant since the yellow blooms look a-like to me. Since this was probably planted by a squirrel or an undetected eye after Halloween last year I let it be. Now it looks more like a watermelon, but since our summer has basically left us, I do not think much more growth will happen. Since it it crawling across our front walk it will need to be pulled after the first frost. I am waiting to cut into it see see if I then can detect its origin.....Any ideas???