import std.algorithm.iteration : map;
import std.algorithm : castSwitch;
import std.format : format;
	
class A { int value; this(int value) { this.value = value; }}
interface I { }
class B : I { }
	
Object[] arr = [new A(5), new B(), null];
auto results = arr.map!(castSwitch!(
	(A a) => "class A with a value of %d".format(a.value),
	(I i) => "derived from I",
	(B b) => "class B",
	() => "null reference",
))();

The codes get nonsense error like:
/home/bthach/dlang/dmd-2.071.0/linux/bin64/../../src/phobos/std/algorithm/comparison.d(277): Error: no property 'format' for type 'string'

But when I changed the order:
[...]
(B b) => "class B",
(I i) => "derived from I",

Suprisingly, it works.

On Tuesday, 7 June 2016 at 03:55:03 UTC, none wrote:
> import std.algorithm.iteration : map;
> import std.algorithm : castSwitch;
> import std.format : format;
> 	
> class A { int value; this(int value) { this.value = value; }}
> interface I { }
> class B : I { }
> 	
> Object[] arr = [new A(5), new B(), null];
> auto results = arr.map!(castSwitch!(
> 	(A a) => "class A with a value of %d".format(a.value),
> 	(I i) => "derived from I",
> 	(B b) => "class B",
> 	() => "null reference",
> ))();
>
> The codes get nonsense error like:
> /home/bthach/dlang/dmd-2.071.0/linux/bin64/../../src/phobos/std/algorithm/comparison.d(277): Error: no property 'format' for type 'string'
>

Note the location of the error: it is in Phobos, not in your code. It's a bug, it should actually print a valid error message. Filed here:
https://issues.dlang.org/show_bug.cgi?id=16135

It has nothing to do with the fact that you're using `format`, that's just coincidence.

> But when I changed the order:
> [...]
> (B b) => "class B",
> (I i) => "derived from I",
>
> Suprisingly, it works.

`castSwitch` tries the types you pass it in the order you specify. If you pass `I` first, the `B` case will never be matched because it is "shadowed" by the former (every `B` is also an `I`). `castSwitch` detects this situation and wants to print a message to warn you. However, someone forgot to import `std.format` (which is used for formatting the error message), resulting in the error you observed.