That offsetof-like trick allows classes where they must be disallowed and disallows where they could be allowed.

Previously on “Offsetof-like Trick”: #39665 (closed), #39666 (closed).

This code erroneously compiles, as if Indirect was a record (with extra VMT pointer).

type
	Indirect = class
		hasNoOffsetInMyObj: int32;
	end;

	PMyObj = ^MyObj;
	MyObj = record
		c: Indirect;
	end;

const
	WrongOffset = PtrUint(@PMyObj(nil)^.c.hasNoOffsetInMyObj); // 8 on x64

begin
end.

BUT, while we at it, I want to make a feature request, too.

Allow classes (and arrays if #39666 (closed) will be fixed) as a top-level entity. Presently, this does not compile:

type
	SomeClass = class
		field: int32;
	end;

const
	FieldOffset = PtrUint(@SomeClass(nil).field);

begin
end.

but looking at how PtrUint(@PMyObj(nil)^.c.hasNoOffsetInMyObj) essentially gave the offset of hasNoOffsetInMyObj in Indirect instance, there are no obstacles to do it in valid scenarios.