[fpc-devel] Implicit function specialization precedence

[Ryan Joseph]

Finally some movement is happening on implicit function specialization and I'm almost finished now except some questions about precedence have been raised again. Initially I thought we decided on non-generic functions taking precedence in the case of *any* name collisions (the original thread https://lists.freepascal.org/pipermail/fpc-pascal/2018-December/055225.html) but Sven is saying now that this isn't the case (see the remarks on the bug report https://bugs.freepascal.org/view.php?id=35261). I'm asking this here not to go over Svens head but in hopes to get some answers quicker (it can take us weeks sometimes to round trip even simple questions).

Currently what I implemented is that in the case below non-generic Test() will take precedence even though Test<T> could be specialized and indeed even comes after.  My questions:

1) What is required for Delphi compatibility? I never used Delphi and I thought we decided this initially for Delphi compatibility. Of course we can make a Delphi mode only option if we need to.

2) Svens final remarks on the bug tracker are "Right now your code will pick the existing String overload even if specializing the generic might be the better choice. If the user really wants the String one (or if the specializing the generic does not work) the user can still force the String overload by casting the parameter to a String.". I'm confused about this because Test(String) and Test<String> are both identical and thus I don't see what is the "better choice".

Personally I feel like we should fallback to the non-generic function as a way to resolve ambiguity but I can also see why Test<T> should take precedence simply because it comes after Test().

================

{$mode objfpc}{$H+}
{$modeswitch IMPLICITFUNCTIONSPECIALIZATION}

function Test(const aStr: String): LongInt;
begin
  Result := 1;
end;

generic function Test<T>(aT: T): LongInt;
begin
  Result := 2;
end;

begin
  Writeln(Test('Hello World'));
end.

================

And just to be clear even if the name is the same but parameters do not match the generic function will be selected. For example below Test<T> will be specialized because Test() doesn't have a compatible signature.

================

function Test(const aStr: String): LongInt;
begin
  Result := 1;
end;

generic function Test<T>(aT: T; aInt: Integer): LongInt;
begin
  Result := 2;
end;

begin
  Writeln(Test('Hello World', 1));    // prints "2"
end.

================

Regards,
	Ryan Joseph