checker: disallow result callbacks functions like map/filter/all/any

vlib/v/checker/fn.v

@@ -2727,6 +2727,14 @@ fn (mut c Checker) check_map_and_filter(is_map bool, elem_typ ast.Type, node ast
 				}
 				c.error('type mismatch, `${arg_expr.name}` must return a bool', arg_expr.pos)
 			}
+			if arg_expr.return_type.has_flag(.result) && arg_expr.or_block.kind != .block {
+				if arg_expr.return_type.clear_option_and_result() in [ast.void_type, 0] {
+					c.error('cannot use Result type in `${node.name}`', arg_expr.pos)
+				} else {
+					c.error('cannot use Result type in `${node.name}` calls without unwrapping or using an or block',
+						arg_expr.pos)
+				}
+			}
 		}
 		ast.StringLiteral, ast.StringInterLiteral {
 			if !is_map {

vlib/v/checker/tests/map_result_callback_fn_err.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/map_result_callback_fn_err.vv:14:53: error: cannot use Result type in `map` calls without unwrapping or using an or block
+   12 | 
+   13 | fn execsync(cmd cli.Command) ! {
+   14 |     dens := os.read_lines('/etc/fox/dens')!.map(urllib.parse(it)!)
+      |                                                        ~~~~~~~~~
+   15 |     mut threads := []thread !{}
+   16 |     for den in dens {
+vlib/v/checker/tests/map_result_callback_fn_err.vv:19:17: error: cannot use Result type in `map`
+   17 |         threads << spawn update(den)
+   18 |     }
+   19 |     threads.map(it.wait()!)
+      |                    ~~~~~~~
+   20 | }
+   21 |

vlib/v/checker/tests/map_result_callback_fn_err.vv

@@ -0,0 +1,25 @@
+import os
+import cli
+import net.urllib
+
+pub fn sync() cli.Command {
+	return cli.Command{
+		name: 'sync'
+		description: 'sync local dens from remote'
+		execute: execsync
+	}
+}
+
+fn execsync(cmd cli.Command) ! {
+	dens := os.read_lines('/etc/fox/dens')!.map(urllib.parse(it)!)
+	mut threads := []thread !{}
+	for den in dens {
+		threads << spawn update(den)
+	}
+	threads.map(it.wait()!)
+}
+
+fn update(den urllib.URL) ! {
+	println(den.str())
+	return
+}