Make shadowing mixin template names an error (5 years old problem)

[monkyyy]

On Sunday, 21 June 2026 at 18:59:24 UTC, mw wrote:
> On Saturday, 9 May 2026 at 12:04:45 UTC, Steven Schveighoffer 
> wrote:
>> On Tuesday, 5 May 2026 at 21:47:09 UTC, mw wrote:
>>> Make shadowing mixin template names an error
>>>
>>> https://github.com/dlang/dmd/issues/23060#issuecomment-4374242813
>>
>> Dennis is correct. This is how mixin templates are supposed to 
>> work, and if we changed that it would be massively disruptive.
>>
>> I would recommend using string mixins for the behavior you 
>> want.
>
> Using string mixin worked.
>
> But string mixin and template mixin have different behavior in 
> this case shows the language need to improve.
>
> We can do deprecation slowly.
>
> And at least for now, the compiler should generate a warning 
> message.
new attempt, now with classes:
```d
import std;

template innate(T,alias value,discrim...){
	T innate=value;
}
template innateclassfriendly(T,discrim...){
	T innateclassfriendly;
	static this(){
		innateclassfriendly= new T(allowedtoconstruct());
	}
}

mixin template Singleton(T) {

	@disable this();
	unittest{
		static assert( ! __traits(compiles, new T()),"please let me 
break, oo needs you to let less code work ;__;");
	}
	
	alias getSingleton=innate!(T,T.init,"singleton");
}

struct A {
	mixin Singleton!A;
	//this() {}  // uncommenting this breaks
	int i=5;
}

unittest{
	assert(A.getSingleton.i==5);
	A.getSingleton.i=3;
}
unittest{
	assert(A.getSingleton.i==3);
}

mixin template Singleton2() {
	alias THIS=typeof(this);
	@disable this();
	static assert(is(THIS==struct),"classes dont work with innate 
for some reason");
	alias getSingleton=innate!(THIS,THIS.init,"singleton");
}

struct B{
	mixin Singleton2!();
	//this() {}  // uncommenting this breaks
	float f=13.37;
}
unittest{
	assert(B.getSingleton.f.isClose(13.37));
}
struct allowedtoconstruct{}
mixin template Singleton3(){
	alias THIS=typeof(this);
	alias getSingleton=innateclassfriendly!(THIS,"singleton");
	this()(){
		static assert(0,"please use .getSingleton, or ()'s");
	}
	this(allowedtoconstruct){}
	static THIS opCall()=>getSingleton;
}
class C{
	mixin Singleton3!();
	string idk;
	bool handhold=true;
}
unittest{
	//C ohno=new C();//errors out
	C().handhold.writeln;
}
```