So I just started learning D this weekend, and I'm trying to understand why the following example will not work... Essentially what I am trying to do is cast the templated child stored in the parent container as the correct form, return some T value, and use auto to return the correct value type.. It seems from the errors I get regarding mismatched return type inference of string and double, that the compiler, for whatever reason, is assuming auto is a double?

int main(string[] argv)
{
	Parent[] Array;
	Array ~= new Child!(double)(3.0);
	Array ~= new Child!(string)("text");
	foreach(item;Array)
	{
		auto val = value(item,item.type);
		//writeln(std.conv.to!string(value(item,item.type)));
	}
return 0;
}
auto value(Parent item, int type)
{
	if(item.type == 1)
	{
		return Fun!double(cast(Child!double)item);
	}
	else if(item.type==2)
		return Fun!string(cast(Child!string)item);
        else
                return 0;
}
auto Fun(T)(Child!T c)
{
	return c.Value;
}

public class Parent
{
	int type=-1;
}
public class Child(T) : Parent
{
	T value;
	public this(T value)
	{
		this.value=value;
		SetType();
	}
	private void SetType()
	{
		if(typeid(T) == typeid(int))
			type=0;
		if(typeid(T) == typeid(double))
			type=1;
		if(typeid(T) == typeid(string))
			type=2;
	}
	@property public T  Value(){return value;}
}

On Sunday, 3 August 2014 at 22:01:23 UTC, matt wrote:
> auto value(Parent item, int type)
> {
> 	if(item.type == 1)
> 	{
> 		return Fun!double(cast(Child!double)item);

Here you're returning a double.

> 	}
> 	else if(item.type==2)
> 		return Fun!string(cast(Child!string)item);

Here you're returning a string.

You're trying to return different, incompatible types depending
on runtime data (`type`). That's not possible. The return type
must be known at compile time.