Not a Rose, but what IS it?

The area I'm turning into my "rose garden", was a dichondra lawn nearly forty years ago. Since that time, it's been permitted to provide a home to whatever weeds wished to grow and which would endure regular weed whacking with no irrigation.

The roses are settling in, as are all manner of odd "volunteers". The Mesquite tree down the hill, way down the hill, is producing seedlings there. The Dodonea hedge I planted is also producing baby Hopseed Bushes.

Now, there is this...

Nothing I've planted anywhere on the hill produced this, nor anything I have dug or pulled. It kind of looks as if it should be some kind of melon or something, but those prickles! They are sharp and they are everywhere.

I kind of like odd plants. Solanum pyracanthum is quite happy here, and I find it rather interesting. At least, nothing seems to find it delicious! But, this thing...

It's pretty and those bright yellow flowers just do not fade. It seems to be setting a lot of "fruit". Might anyone have an idea just what the heck the thing is, please?