Mixin template parameter overloading bug?

[monkyyy]

On Saturday, 14 June 2025 at 02:16:58 UTC, Andrey Zherikov wrote:
> On Saturday, 14 June 2025 at 02:10:03 UTC, monkyyy wrote:
>> On Saturday, 14 June 2025 at 01:46:31 UTC, Andrey Zherikov 
>> wrote:
>>> On Saturday, 14 June 2025 at 00:02:32 UTC, Andrey Zherikov 
>>> wrote:
>>> Simplified test case a bit more.
>>> This works:
>>> ```d
>>> template f(void function(int) F) {}
>>> template f(int function(int) F) {}
>>>
>>> mixin f!((int _) {});
>>> mixin f!((int _) => 0);
>>>
>>> mixin f!((int) {});
>>> mixin f!((int) => 0);
>>>
>>> mixin f!((_) {});
>>> mixin f!((_) => 0);  // Error: cannot return non-void from 
>>> `void` function
>>> ```
>>>
>>> Can anyone explain why adding type of the parameter in lambda 
>>> (`int`) makes this working?
>>
>> https://dlang.org/spec/template-mixin.html
>>
>> Your still mixing syntax; mixin templates are suppose to be a 
>> separate system according to the spec.
>>
>> ```d
>> import std;
>> void f(int function(int) F)(){"int".writeln;}
>> void f(void function(int) F)(){"void".writeln;}
>> unittest{
>> 	f!((_) {});
>> 	f!((_) => 0);
>> }
>>
>> mixin template g(int function(int) F){string s1="int";}
>> mixin template g(void function(int) F){string s2="void";}
>> unittest{
>> 	mixin g!((_) {});
>> 	mixin g!((_) => 0);
>> 	s1.writeln;
>> 	s2.writeln;
>> }
>> ```
>
> Modified your example a bit.
> This works:
> ```d
> mixin template f(void function(int) F) { void s1() 
> {"void".writeln; } }
> mixin template f(int function(int) F) { void s2() 
> {"int".writeln; } }
>
> void main()
> {
> mixin f!((_) {});
> mixin f!((_) => 0);
>
>     s1();
>     s2();
> }
> ```
>
> This does not:
> ```d
> mixin template f(void function(int) F) { void s1() 
> {"void".writeln; } }
> mixin template f(int function(int) F) { void s2() 
> {"int".writeln; } }
>
> mixin f!((_) {});
> mixin f!((_) => 0); // Error: cannot return non-void from 
> `void` function
>
> void main()
> {
>     s1();
>     s2();
> }
> ```

```d
import std;
mixin template f(int function(int) F){}
mixin template f(void function(int) F){unittest{"void".writeln;}}

//mixin f!((_){}); //FAILS


mixin template g(void function(int) F){unittest{"void".writeln;}}
mixin template g(int function(int) F){}

mixin g!((_){});  //works
```
this example makes it purely a compiler bug

I cant escape any useful information(I tried `typeof(return)`), 
seems to be limited to just void return functions