This DIP is a partial resurrection of DIP 40 (https://wiki.dlang.org/DIP40) by me and Dennis Korpel. Dennis is working on the implementation (https://github.com/dlang/dmd/pull/16910) while I am writing the DIP.

The purpose of this DIP is to propose a new avenue for Implicit Template Instantiation (ITI) via constructors. Currently (that I'm aware of) D only does Implicit Function Template Instantiation (IFTI); the proposal is to extend this implicit instantiation to types as well, via their constructors.

Before this DIP:

struct Pair(T, U)
{
    T t;
    U u;

    this(T t, U u)
    {
        this.t = t;
        this.u = u;
    }
}

void main()
{
    auto p1 = Pair(1, "asdf"); // Error: struct `Pair` is not callable using argument types `!()(int, string)`
                               // Candidate is: `Pair(T, U)`
}

After this DIP:

struct Pair(T, U)
{
    T t;
    U u;

    this(T t, U u)
    {
        this.t = t;
        this.u = u;
    }
}

void main()
{
    auto p1 = Pair(1, "asdf"); // Okay, T is deduced as int and U as string
}

The DIP:
https://github.com/MetaLang/DIPs/blob/dip1050/DIPs/DIP1050.md

On Wednesday, 12 March 2025 at 00:20:55 UTC, Meta wrote:

Glad to see someone working on this problem.

How would this apply to template aliases (e.g. DIP1023)?

On Wednesday, 12 March 2025 at 02:37:06 UTC, jmh530 wrote:

From my understanding, it should work the same way as IFTI, i.e., if eventually this code compiles:

struct TemplateType(T) { }
alias TemplateAlias(T) = TemplateType!T;
void templateFunction(T)(TemplateAlias!T arg) { }

void main()
{
    TemplateAlias!int ta;
    templateFunction(ta);
}

Then this code should as well:

struct TemplateType(T) { }
alias TemplateAlias(T) = TemplateType!T;
struct AnotherTemplateType(T)
{
    this(TemplateAlias!T arg) {}
}

void main()
{
    TemplateAlias!int ta;
    auto att = AnotherTemplateType(ta); // Currently does not compile, even if you add AnotherTemplateType!(TemplateAlias!int)(ta)
}