Type inference for constructors

[data pulverizer]

On Friday, 18 September 2020 at 06:43:12 UTC, Simen Kjærås wrote:
> On Friday, 18 September 2020 at 05:43:56 UTC, data pulverizer
>
> That's issue 1997: https://issues.dlang.org/show_bug.cgi?id=1997
>
> D's templates are turing complete, and there may be arbitrary 
> amounts of logic obscuring the mapping between type template 
> parameters and the specific argument types of the type's 
> constructor, so the problem is intractable in the general case. 
> E.g:
>
> template Foo(T) {
>     static if (is(T == int)) {
>         struct Foo {
>             this(int i) {}
>             // int-specific code
>         }
>     } else {
>         struct Foo {
>             this(T t) {}
>             // generic code
>         }
>     }
> }
>
> The above example is a simple one, yet mapping from a 
> constructor call to the correct template parameters is 
> difficult.
>

Thank you for the explanation and the reference.

>
> That's a template this parameter 
> (https://dlang.org/spec/template.html#template_this_parameter). 
> It takes on the type of 'this' at the point of instantiation. 
> That is:
>
> struct S {
>     void fun(this T)() { pragma(msg, T); }
> }
>
> unittest {
>     S s;
>     const S sc;
>     immutable S si;
>     shared S ss;
>     s.fun();  // prints S
>     sc.fun(); // prints const(S)
>     si.fun(); // prints immutable(S)
>     ss.fun(); // prints shared(S)
> }
>
> This allows the return type to have the same constness as 
> 'this', or for specific code to be executed when operating on 
> e.g. a non-mutable instance, where lazy initialization couldn't 
> be performed. In many cases, this is better served by using 
> inout:
>
> https://dlang.org/spec/function.html#inout-functions

After reading your explanation and looking at the part of the 
manual you referred to, it looks as if this pattern could have a 
lot of utility, the `Addable` example in the manual looks like 
polymorphism with dynamic dispatch but retains static type a bit 
like Rust's (runtime) traits.

Many thanks.