Let me clarify my earlier post by saying that I'm in Florida and further research has revealed that the 'panless' shower is or at least was a common feature here in the Sunshine State.

Still, it doesn't sound quite right to me...

Opinion? Your contractor(?) is full of it. You MUST have a pan, on top of a preslope with a mud bed over it. In the alternative, yes, you can build a proper shower without a pan. It's called Kerdi.

How does this genius contractor propose you keep the water from leaking under the walls of the shower without using one of the mainstream shower pan methods???

I can only guess that he expects the 5" concrete curb that's integral to the shower recess, which is integral to the slab foundation, to keep it from leaking. The finished wall material starts 5" above the finished shower floor.

I would be interested in hearing feedback from anybody who might be familiar with this method of construction, which apparently was/is common in Florida and which I believe is still permitted by Florida Building code (not sure on this/still researching). See the pertinent code section and particularly EXCEPTION 2 below:

P2709.2 Lining required. The adjoining walls and floor framing, enclosing on-site built-up shower receptors shall be lined with sheet lead, copper or a plastic liner material that complies with ASTM D 4068. The lining material shall extend not less than 3 inches (76 mm) beyond or around the rough jambs and not less than 3 inches (76 mm) above finished thresholds. Hot mopping shall be permitted in accordance with Section
P2709.2.1.

Exception:
1. Floor surfaces under showerheads provided for rising laid directly on the ground.
2. Shower compartments where the finished shower drain is depressed a minimum of 2inches below the surrounding finished floor on the first floor level and the shower recess is poured integrally with the adjoining
floor.

Here is a link that might be useful: Florida building code

Except in California.

There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy.