checker: disallow array sum types without default field (#6892)

vlib/v/ast/ast.v

@@ -815,15 +815,15 @@ pub:
 
 pub struct ArrayInit {
 pub:
-	pos             token.Position
+	pos             token.Position // `[]` in []Type{} position
-	elem_type_pos   token.Position
+	elem_type_pos   token.Position // `Type` in []Type{} position
 	exprs           []Expr // `[expr, expr]` or `[expr]Type{}` for fixed array
 	ecmnts          [][]Comment // optional iembed comments after each expr
 	is_fixed        bool
 	has_val         bool // fixed size literal `[expr, expr]!!`
 	mod             string
-	len_expr        Expr
+	len_expr        Expr // len: expr
-	cap_expr        Expr
+	cap_expr        Expr // cap: expr
 	default_expr    Expr // init: expr
 	has_len         bool
 	has_cap         bool
@@ -832,8 +832,8 @@ pub mut:
 	is_interface    bool // array of interfaces e.g. `[]Animal` `[Dog{}, Cat{}]`
 	interface_types []table.Type // [Dog, Cat]
 	interface_type  table.Type // Animal
-	elem_type       table.Type
+	elem_type       table.Type // element type
-	typ             table.Type
+	typ             table.Type // array type
 }
 
 pub struct ChanInit {

vlib/v/checker/checker.v

@@ -2246,6 +2246,11 @@ pub fn (mut c Checker) array_init(mut array_init ast.ArrayInit) table.Type {
 		if array_init.has_default {
 			c.expr(array_init.default_expr)
 		}
+		if sym.kind in [.sum_type, .union_sum_type] {
+			if array_init.has_len && !array_init.has_default {
+				c.error('cannot initalize sum type array without default value', array_init.elem_type_pos)
+			}
+		}
 		if sym.kind == .placeholder {
 			c.error('unknown type `$sym.source_name`', array_init.elem_type_pos)
 		}

vlib/v/checker/tests/array_init_sum_type_without_init_value_and_len_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/array_init_sum_type_without_init_value_and_len_err.vv:4:9: error: cannot initalize sum type array without default value
+    2 | 
+    3 | fn main() {
+    4 |     a := []Foo{len:10}
+      |            ~~~
+    5 |     println(a)
+    6 |

vlib/v/checker/tests/array_init_sum_type_without_init_value_and_len_err.vv

@@ -0,0 +1,7 @@
+type Foo = string | int
+
+fn main() {
+	a := []Foo{len:10}
+	println(a)
+
+}