Poll: what should this program do?

[Ogion]

On Wednesday, 19 March 2025 at 03:40:28 UTC, Paul Backus wrote:

> **What SHOULD this program do?**
>
> **What do you think it ACTUALLY does?**

It should print “outer”, and it actually does.

Obviously, at first I expected it to print: “inner”. But when I 
saw the result and checked the spec, I think that it works 
correctly.

There are three non-obvious things ging in here.

Number one: `fun(T).fun(string)` is not a regular function, it’s 
a template. So it doesn’t collide with globally-defined 
`fun(string)`. This is why this program compiles.

Number two: inside eponymous template `fun`, `fun` refers to the 
template itself, not to its members. This gives a different 
result:
```D
template fun(T) {
     alias fun = f;
     string f(int) => f("hi");
     string f(string) => "inner";
}
string f(string) => "outer";

void main() {
     import std.stdio;
     writeln(fun!int(123)); // sorry, IFTI no longer works
}
```

Number three: using a type instead of template parameter (of the 
same type) is enough to break [implicit 
instantiation](https://dlang.org/spec/template.html#ifti-restrictions):

```D
template fun(T) {
     string fun(string) => "inner";
}

void main() {
     import std.stdio;
     writeln(fun("hi")); // error
     writeln(fun!string("hi")); // works
}
```
There are many other ways to break IFTI:
```D
template fun(T) {
     alias fun = fun;
     string f(T) => "inner";
}
```

```D
template fun(T) {
     template fun(T) {
         string fun(T) => "inner";
     }
}
```
I guess it can’t be helped. Usually, though, IFTI failure results 
in error. In this program, it leads to unforeseen behavior. But 
such is life on templates.