[fpc-devel] Implicit function specialization precedence

Sven Barth pascaldragon at googlemail.com
Wed Apr 7 21:42:19 CEST 2021

Am 07.04.2021 um 18:16 schrieb Ryan Joseph via fpc-devel:
>> On Apr 6, 2021, at 11:34 PM, Sven Barth <pascaldragon at googlemail.com> wrote:
>> In the second case the compiler will have the non-generic Test(String) due to the implicit operator as well as Test<LongInt>(LongInt) due to the implicit specialization. Here it will pick the generic one, because a call without a type conversion is considered better.
> I finally get what you're saying and get the correct results now after moving my check inside the if/then block. I didn't realize that the operator overload was causing a type conversion that the overloading system knew about. So the non-generic still wins unless the type conversion happened due to the operator overload and then the generic wins.
> 	Writeln(Test('Hello World'));			// Test(String)
> 	Writeln(Test(42));					// Test<String>
> 	Writeln(Test(String(42)));			// Test(String)

Good. :)

> As for $H+ do we really want string literals likes 'ABC' to change? I wouldn't think so. Here's my string literal type symbol conversion in create_unamed_typesym I'm using now:
>                case tstringdef(def).stringtype of
>                  st_shortstring:
>                    newtype:=search_system_type('SHORTSTRING');
>                  st_longstring,
>                  st_ansistring:
>                    newtype:=search_system_type('ANSISTRING');
>                  st_widestring:
>                    newtype:=search_system_type('WIDESTRING');
>                  st_unicodestring:
>                    newtype:=search_system_type('UNICODESTRING');
>                end;
> 'Hello World' is parsed as st_shortstring so we use System.ShortString for specialization. Given that I don't think I need to do anything with $H+.

Yes, we want to change that for two reasons:
- the constant string might be larger than 255 characters
- ShortString is worse for passing as a by-value parameter (which will 
be the default after all) than AnsiString or UnicodeString as 
ShortString is completely copied while Ansi-/UnicodeString are only 
references with reference count adjustments

Also your check is wrong as the def that returns True for is_stringlike 
might not be a stringdef. In the case of a constant string it's an 
arraydef with ado_ConstantString in arrayoptions (and that's how you can 
detect constant strings which *need* this treatment).


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