That said, in Q- state it should ignore the overflow, looks like a bug that the compiler should have caught - with a warning - at compile time.
That's reasonable but, $Q is supposed to control runtime overflow checking code generation. At compile time, the compiler already knows it cannot represent the constant, consequently there is no way it can generate correct code.
The compiler could calculate the sum, let it overflow and use that result as the constant. I could see a C compiler doing such an atrocity but not Pascal.
It will be interesting to see what PascalDragon or another FPC developer says about it.
ETA:Well, it is "Delphi compatible" except for the macro's of course, but with direct literal values.
I didn't know that nor did I think it would ever do that.