• Ben Jones (13/13) Aug 31 2020 I have an alias that looks like
• Steven Schveighoffer (4/25) Aug 31 2020 Very old enhancement request (I doubt this will ever happen):
• Paul Backus (13/26) Aug 31 2020 This is a very old known bug:
• Ben Jones (11/19) Aug 31 2020 Thanks all.
• Paul Backus (18/28) Sep 01 2020 Unfortunately, `alias this` does not apply to constructors. You

Ben Jones <fake fake.fake> writes:

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

Steven Schveighoffer <schveiguy gmail.com> writes:

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

Paul Backus <snarwin gmail.com> writes:

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) { ... }

Ben Jones <fake fake.fake> writes:

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)`

Paul Backus <snarwin gmail.com> writes:

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));
}