On Thu, 17 Aug 2023, Mattias Gaertner via fpc-devel wrote:
Hi,
FPC and Delphi handle type alias differently and I wonder if this was
deliberate.
type
TMyDouble = type double;
TBird<T> = class
a: T;
end;
TDoubleBird = TBird<double>;
TMyDoubleBird = TBird<TMyDouble>;
Both FPC and Delphi create different classes for TDoubleBird and
TMyDoubleBird.
In Delphi they are not assign compatible, in FPC they are. For example:
var
a: TDoubleBird;
b: TMyDoubleBird;
begin
b:=TMyDoubleBird.Create;
a:=b; // forbidden in Delphi
writeln(a is TDoubleBird); // writes false
Bytewise the assignment is ok, but logic wise "a" is no longer a TDoubleBird.
Is this a bug or a feature?
Depends on how you look at it. Double and TMyDouble are assignment compatible.
There is no reason why the same should not hold true for TDoubleBird and
TMyDoubleBird ?
In Delphi
Type
TArrayInteger1 = Array of Integer;
TArrayInteger2 = Array of Integer;
var
A1 : TArrayInteger1;
A2 : TArrayInteger2;
begin
A1:=A2;
end.
Will not work, but in FPC it does. This is IMO similar.
Of course, the method addresses are different. Other than that and the
different RTTI (which has 100% the same structure) the classes are
identical. So based on that we could argue they are not assignment
compatible.
I am inclined to go for feature, but maybe there are arguments to tip the
balance in the direction of bug.
Michael.
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