Why does templated interface function return something different than final function?

[Timoses]

On Wednesday, 18 July 2018 at 11:09:12 UTC, Timoses wrote:
> Why is the interface templated function not also returning the 
> class C toString return value "in C"??
>
> 	interface iface
> 	{
> 		void toString(scope void delegate(const(char)[]) sink) const;
>
> 		final string convert() inout
> 		{
> 			import std.conv;
> 			// assert(std.conv.to!string(this) == "in C"); // fails!
> 			return std.conv.to!string(this);
> 		}
>
> 		final string convert2() const
> 		{
> 			import std.conv;
> 			assert(std.conv.to!string(this) == "in C");
> 			return std.conv.to!string(this);
> 		}
> 	}
>
> 	class C : iface
> 	{
> 		void toString(scope void delegate(const(char)[]) sink) const
> 		{
> 			sink("in C");
> 		}
> 	}
>
> 	void main ()
> 	{
> 		iface i = new C();
> 		import std.stdio;
> 		writeln(i.convert); // "app.C"
> 		writeln(i.convert2()); // "in C"
> 	}
>
> It seems like `inout` triggers some odd behaviour??

Sorry, I experimented a bit and forgot to change the topic to 
something more fitting, e.g.
"Why does inout struct function return different results?"