[fpc-pascal] generic proc inference

[Mattias Gaertner]

On Sun, 6 Oct 2019 12:23:57 -0400
Ryan Joseph <genericptr at gmail.com> wrote:

> > On Oct 6, 2019, at 12:18 PM, Mattias Gaertner via fpc-pascal
> > <fpc-pascal at lists.freepascal.org> wrote:
> > 
> > Ryan, is patch_3_25.diff the latest patch?
> > 
> > I get a lot of merge conflicts.
> > What fpc revision is needed for this?
> > 
> > And what is the git repo
> > https://github.com/genericptr/freepascal/tree/generic_implicit
> > ?
> > It does not contain the modeswitch.
> > 
> > Mattias  
> 
> It’s the latest patch but that’s many months old already so nothing
> is up to date. I failed multiple times to update my branch from the
> “upstream" so I can't fix errors easily (there was a long thread some
> months ago but I gave up).
> 
> Look at the changes page on Github and search for
> m_implicit_function_specialization.
> 
> https://github.com/graemeg/freepascal/compare/master...genericptr:generic_implicit


My mistake was to use git clone
https://github.com/genericptr/freepascal.git.
The zip contains a valid version.

You implemented it pretty Delphi compatible.
As you know, the Delphi help states wrongly, that type inference uses
32bit "integer" when possible, which would mean that often only one
specialization is created. In reality it uses the smallest integer
type, creating easily many different specializations, bloating up the
exe.
The Delphi compiler is really picky (dumb?) on type inference.
That being said, I noticed several differences to Delphi (10.3):

1.
FPC allows default params, Delphi does not. Nice.
I see no tests for this though, so maybe this was not on purpose?

2.
DoThis(1,200) gives range check warning instead of error. A warning
means there are some rare cases, where this code is correct. Is this a
todo or do you see a useful case?

3.
timpfuncspez2.pp
DoThis<T>
DoThis<T,U>
Delphi gives an error "Ambiguous call to DoThis". FPC silently selects
the one with only one param. IMO this is dangerous, it should give an
error.

4.
Why does timpfuncspez6 fail?
It works in Delphi.
The comment has an explanation, which looks wrong to me:

generic procedure DoThis<T,U>(msg: T; param1: U; param2: TObject);
begin
end;

begin
        DoThis('aa', 'aa', TObject.Create);
        // wil be specialized as DoThis(msg: integer; param1: TObject;
param2: TObject)
        // so we expect an incompatible type error
        DoThis(1, 1, TObject.Create);
end.


Mattias