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) { ... }