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

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

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) { ... }

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)`

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));
}