checker: check for `x := []Interface{len: 9}` without `init:`

vlib/v/checker/checker.v

@@ -3840,6 +3840,13 @@ pub fn (mut c Checker) array_init(mut array_init ast.ArrayInit) ast.Type {
 			}
 		}
 		if array_init.has_len {
+			if array_init.has_len && !array_init.has_default {
+				elem_type_sym := c.table.get_type_symbol(array_init.elem_type)
+				if elem_type_sym.kind == .interface_ {
+					c.error('cannot instantiate an array of interfaces without also giving a default `init:` value',
+						array_init.len_expr.position())
+				}
+			}
 			c.ensure_sumtype_array_has_default_value(array_init)
 		}
 		c.ensure_type_exists(array_init.elem_type, array_init.elem_type_pos) or {}

vlib/v/checker/tests/array_of_interfaces_with_len_without_init.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/array_of_interfaces_with_len_without_init.vv:14:37: error: cannot instantiate an array of interfaces without also giving a default `init:` value
+   12 | 
+   13 | fn main() {
+   14 |     mut parsed_lines := []MObject{len: 9}
+      |                                        ^
+   15 |     println(parsed_lines)
+   16 | }

vlib/v/checker/tests/array_of_interfaces_with_len_without_init.vv

@@ -0,0 +1,16 @@
+interface MObject {
+	give_string() string
+}
+
+struct LeStruct {
+	le_string string
+}
+
+fn (a LeStruct) give_string() string {
+	return 'V'
+}
+
+fn main() {
+	mut parsed_lines := []MObject{len: 9}
+	println(parsed_lines)
+}