template interface and delegates

[Steven Schveighoffer]

On Mon, 31 Mar 2014 14:58:30 -0400, anonymous <non at trash-mail.com> wrote:

> Hi,
> I'm new to D and played a bit with templates and delegates.
> Now i discovered some behaviore that i don't understand.
> Can somebody explain me why i get two different outputs?
>
>
> import std.stdio;
>
>
> interface A(T){
> 	bool GetBool();
> 	T getT();
> }
>
> class C:A!(double){
> 	override bool GetBool(){
> 		return false;
> 	}
> 	override double getT(){
> 		return 1;
> 	}
> }
>
> void mwriteln(T)( A!T delegate() dg){
> 	writeln(dg().getT());
> }
>
> void main()
> {
> 	auto c = new C();
> 	writeln(c.getT());
> 	mwriteln!double({return new C();});
> }

This is definitely a bug.

Reduced case:

import std.stdio;

interface A{
     void foo();
}

class C:A{
     override void foo(){
         writeln("here");
     }
}

void x( A delegate() dg){
     dg().foo();
}

void main()
{
     A c = new C;
     c.foo(); // prints "here"
     x({A a = new C; return a;}); // prints "here"
     x({return new C;}); // does not print
}

This is really an issue with delegate return type inferrence not working  
properly.

-Steve