[Issue 21253] New: Can't compile Variant.visit!(...) with generic function

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21253

          Issue ID: 21253
           Summary: Can't compile Variant.visit!(...) with generic
                    function
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Windows
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: fitare2749 at araniera.net

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{      }
---

--