Re: Template argument deduction fails with alias

[Paul Backus via Digitalmars-d-learn]

On Tuesday, 1 September 2020 at 02:48:08 UTC, Ben Jones wrote:


Thanks all.

I tried using alias this at first and then I get errors trying 
to construct AliasType objects:



auto pi = Payload!int(5);
auto pe = ParseError("error");
alias PRType = ParseResult!(Payload!int, ParseError);
auto pr = PRType(pi);

gives:

cannot implicitly convert expression `pi` of type `Payload!int` 
to `SumType!(Payload!int, ParseError)`


Unfortunately, `alias this` does not apply to constructors. You 
can either add your own constructor to the wrapper struct:


struct ParseResult(Ts...)
{
SumType!Ts data;
alias data this;

this(T)(T t)
if (staticIndexOf!(T, data.Types) >= 0)
{
data = t;
}
}

...or use a factory function like the following:

PRType parseResult(T)(T t)
{
return PRType(SumType!(Payload!int, ParseError)(t));
}

Re: Template argument deduction fails with alias

[Ben Jones via Digitalmars-d-learn]

On Tuesday, 1 September 2020 at 01:26:30 UTC, Paul Backus wrote:



Aside from using SumType directly in the function signature, 
another workaround is to use a wrapper struct instead of an 
alias:


struct AliasType(Args...) {
SumType!Args data;
alias data this;
}

bool func(T : AliasType!Args, Args...)(T t) { ... }


Thanks all.

I tried using alias this at first and then I get errors trying to 
construct AliasType objects:



auto pi = Payload!int(5);
auto pe = ParseError("error");
alias PRType = ParseResult!(Payload!int, ParseError);
auto pr = PRType(pi);

gives:

cannot implicitly convert expression `pi` of type `Payload!int` 
to `SumType!(Payload!int, ParseError)`

Re: Template argument deduction fails with alias

[Paul Backus via Digitalmars-d-learn]

On Tuesday, 1 September 2020 at 01:11:35 UTC, Ben Jones wrote:

I have an alias that looks like

static if(...){
  alias AliasType = SumType!(...);
}

which I use in a template constraint for a function template:

bool func(T: AliasType!Args, Args...)(T t){ ... }


When I try to call func with an AliasType object, the argument 
deduction fails with a message saying that the argument type (a 
SumType) doesn't match the template constraint (an AliasType)


Things do work if I change the template constraint to be a 
SumType rather an an AliasType


Is there a workaround to this?  Here's a complete example: 
https://run.dlang.io/is/buRGTs


This is a very old known bug:

https://issues.dlang.org/show_bug.cgi?id=1807

Attempts have been made to fix it, but the problem is a lot more 
complicated than it looks at first glance, so I wouldn't expect a 
solution any time soon.


Aside from using SumType directly in the function signature, 
another workaround is to use a wrapper struct instead of an alias:


struct AliasType(Args...) {
SumType!Args data;
alias data this;
}

bool func(T : AliasType!Args, Args...)(T t) { ... }

Re: Template argument deduction fails with alias

[Steven Schveighoffer via Digitalmars-d-learn]

On 8/31/20 9:11 PM, Ben Jones wrote:

I have an alias that looks like

static if(...){
   alias AliasType = SumType!(...);
}

which I use in a template constraint for a function template:

bool func(T: AliasType!Args, Args...)(T t){ ... }


When I try to call func with an AliasType object, the argument deduction 
fails with a message saying that the argument type (a SumType) doesn't 
match the template constraint (an AliasType)


Things do work if I change the template constraint to be a SumType 
rather an an AliasType


Is there a workaround to this?  Here's a complete example: 
https://run.dlang.io/is/buRGTs




Very old enhancement request (I doubt this will ever happen): 
https://issues.dlang.org/show_bug.cgi?id=1807


-Steve