[Bug]: Aqueduct construction on slopes inconsistent with other bridges and tunnels

[Kuhnovic]

Version of OpenTTD

Latest master, also in 13.1

Expected result

After comparing aqueducts to tunnels, which show similar highlighting behavior, I noticed that a corner on the opposite tunnel end will automatically get terraformed if it's the wrong slope. Aqueducts don't do this, even though the highlighted area suggest you can actually build the aqueduct.

Actual result

If I select this side of the water the aqueduct is correctly highlighted, and it builds fine when you click.

However, hovering over the tile next to it highlights the aqueduct. But it can't be built due to the wrong slope at the other end. Clicking results in the "land sloped in wrong direction" message. Here I'd expect some automatic terraforming similar to bridges and tunnels.

Steps to reproduce

1. Generate a map with enough water
2. Do what I did in the screenshots ;)