https://dlang.org/phobos/std_variant.html#.visit it said:
> If a function with an untyped parameter is specified, this function is called when the variant contains a type that does not match any other function. This can be used to apply the same function across multiple possible types. Exactly one generic function is allowed.

but this doesn't work:

---
> dmd --version
DMD32 D Compiler v2.091.0-beta.2-master-ec39fe5

> dmd code.d -vcolumns
code.d(7,15): Error: no property n for type code.B
...\variant.d(2293,55):        instantiated from here: visitImpl!(true, VariantN!(4u, A, B), function (B _) pure nothrow @nogc @safe => 42, (a) => a.n)
code.d(5,21):        instantiated from here: visit!(VariantN!(4u, A, B))
...\variant.d(2562,21): Error: static assert:  "__lambda2 is not a function or delegate"
...\variant.d(2293,55):        instantiated from here: visitImpl!(true, VariantN!(4u, A, B), function (B _) pure nothrow @nogc @safe => 42, (a) => a.n)
code.d(5,21):        instantiated from here: visit!(VariantN!(4u, A, B))


> type code.d
import std;

void main()
{
  Algebraic!(A, B)().visit!(
    (B _) => 42,
    (a  ) => a.n
  );
}

struct A{int n;}
struct B{      }
---