checker: disallow interfaces inside sum types (#6232)

vlib/v/checker/checker.v

@@ -293,6 +293,8 @@ pub fn (mut c Checker) type_decl(node ast.TypeDecl) {
 				typ_sym := c.table.get_type_symbol(typ)
 				if typ_sym.kind == .placeholder {
 					c.error("type `$typ_sym.source_name` doesn't exist", node.pos)
+				} else if typ_sym.kind == .interface_ {
+					c.error("sum type cannot hold an interface", node.pos)
 				}
 			}
 		}