It seems that the problem was caused by my having left one UART RXINT enable/disable call set to UART0. That was enough to upset the compiler!

I have a ESP32-DEVKITC-32UE board and it uses UART0 (Chan1 in Flowcode) to program via the built in USB bridge.
This works fine, but I also have a UART component in my program.
If I set that component's UART channel to anything other than chan1 (U0) - (including trying to use the software UART ...

When selecting INT1 as an interrupt source on the MX795F512L, it is shown as INT1/RD8 - whereas the data sheet shows that this is RE8 (pin 18 on the 100-pin TQFP).