Assignment error when using recursive definition in sum types `type SumType = int | []SumType`

[impopular-guy]

V version: V 0.2.4 501b293
OS: windows, Microsoft Windows 10 Home Single Language v19044 64-bit

What did you do?
v test .

code5_test.v

type SumType = int | []SumType

struct Foo1 {
	a int
	b []int
}

fn test_func() {
	data := Foo1{120, [130, 140]}
	mut res1 := map[string]SumType{}
	$for field in Foo1.fields {
		$if field.typ is int {
			res1[field.name] = data.$(field.name)  // this fails during compilation
		} $else $if field.typ is []int{
			res1[field.name] = data.$(field.name)
		}
	}

	mut res2 := map[string]SumType{}
	$for field in Foo1.fields {
		$if field.typ is int {
			e := data.$(field.name)
			res2[field.name] = e    // this compiles successfully
		} $else $if field.typ is []int{
			e := data.$(field.name)
			res2[field.name] = e
		}
	}
	assert res1 == res2
}

code4_test.v

type SumType = int | []int // difference

struct Foo1 {
	a int
	b []int
}

fn test_func() {
	data := Foo1{120, [130, 140]}
	mut res1 := map[string]SumType{}
	$for field in Foo1.fields {
		$if field.typ is int {
			res1[field.name] = data.$(field.name)
		} $else $if field.typ is []int{
			res1[field.name] = data.$(field.name)
		}
	}

	mut res2 := map[string]SumType{}
	$for field in Foo1.fields {
		$if field.typ is int {
			e := data.$(field.name)
			res2[field.name] = e
		} $else $if field.typ is []int{
			e := data.$(field.name)
			res2[field.name] = e
		}
	}
	assert res1 == res2
}

What did you expect to see?
Both test files successful

What did you see instead?

C:\Users\dhrit\Desktop\ToReead\temp\vtest>v test .
---- Testing... --------------------------------------------------------------------------------------------------------
 FAIL  [1/2]   499.829 ms C:/Users/dhrit/Desktop/ToReead/temp/vtest/code5_test.v
code5_test.v:13:28: error: cannot assign to `res1[field.name]`: expected `SumType`, not `[]int`
   11 |     $for field in Foo1.fields {
   12 |         $if field.typ is int {
   13 |             res1[field.name] = data.$(field.name)
      |                                     ~~~~~~~~~~~~~
   14 |         } $else $if field.typ is []int{
   15 |             res1[field.name] = data.$(field.name)
code5_test.v:15:28: error: cannot assign to `res1[field.name]`: expected `SumType`, not `[]int`
   13 |             res1[field.name] = data.$(field.name)
   14 |         } $else $if field.typ is []int{
   15 |             res1[field.name] = data.$(field.name)
      |                                     ~~~~~~~~~~~~~
   16 |         }
   17 |     }

 FAIL  [2/2]   504.533 ms C:/Users/dhrit/Desktop/ToReead/temp/vtest/code4_test.v
C:\\Users\\dhrit\\Desktop\\ToReead\\temp\\vtest\\code4_test.v:29: ✗ fn test_func
  > assert res1 == res2
    Left value:
      {'a': SumType(120), 'b': SumType([130, 140])}
    Right value:
      {'a': SumType(120), 'b': SumType(4)}

------------------------------------------------------------------------------------------------------------------------
Failed command 1:    "C:\BIN\v\v.exe"   -o "C:\Users\dhrit\AppData\Local\Temp\v\test_session_4713100\code5_test.exe" "C:\Users\dhrit\Desktop\ToReead\temp\vtest\code5_test.v"
Failed command 2:    "C:\BIN\v\v.exe"   -o "C:\Users\dhrit\AppData\Local\Temp\v\test_session_4713100\code4_test.exe" "C:\Users\dhrit\Desktop\ToReead\temp\vtest\code4_test.v"
Summary for all V _test.v files: 2 failed, 2 total. Runtime: 505 ms, on 2 parallel jobs.

[impopular-guy]

In code4_test.v file, the generated C code has problems. Array_int is being casted to wrong type.

#elif 37 == 37
{
    Array_int e = data.b;
    map_set(&res2, &(string[]){field.name}, &(main__SumType[]) { int_to_sumtype_main__SumType(&e) });
    // should have used Array_int_to_sumtype_main__SumType
}
#endif

[impopular-guy]

Smaller example:

type SumType = int | []SumType

fn test_func() {
	mut res := map[string]SumType{}
	b := [120, 130]
	println(typeof(b).name) // []int
	res['a'] = b
}

[yuyi98]

Whether it should be banned?

[impopular-guy]

@yuyi98 Not sure if it should be banned.

I would expect type SumType = int | []SumType to work same as type SumType = int | []int. Although there seems to be some existing bug as mentioned in vlib/v/ast/table.v:1278 regarding casting of sumtypes.
And this syntax can be a bit cleaner when considering the following example.

type SumType = int | f32 | f64 | string | bool | []SumType
type SumType = int | f32 | f64 | string | bool | []int | []f32 | []f64 | []string | []bool | [][]int ....

[yuyi98]

ok, I see.

[enghitalo]

@enghitalo See it

[JalonSolov]

I'd lean towards banning it. Recursion like this can quickly get you lost when trying to debug something.

How would you even figure out that your current SumType was actually [][][][][][]SumType (by the time it got through a few levels...)?