checker: fix missing check for diff type on map value declaration (#18522)

vlib/v/checker/containers.v

@@ -440,8 +440,10 @@ fn (mut c Checker) map_init(mut node ast.MapInit) ast.Type {
 					c.error('invalid map value: ${msg}', val.pos())
 				}
 			}
-			if !c.check_types(val_type, val0_type) || (i == 0 && val_type.is_number()
+			if !c.check_types(val_type, val0_type)
-				&& val0_type.is_number() && val0_type != ast.mktyp(val_type)) {
+				|| val0_type.has_flag(.option) != val_type.has_flag(.option)
+				|| (i == 0 && val_type.is_number() && val0_type.is_number()
+				&& val0_type != ast.mktyp(val_type)) {
 				msg := c.expected_msg(val_type, val0_type)
 				c.error('invalid map value: ${msg}', val.pos())
 			}

vlib/v/checker/tests/diff_type_map_value_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/diff_type_map_value_err.vv:10:44: error: invalid map value: expected `?MyStruct`, not `MyStruct`
+    8 | 
+    9 | fn empty() map[string]?MyStruct {
+   10 |   return {'key1': ?MyStruct(none), 'key2': MyStruct{field: 10}}
+      |                                            ~~~~~~~~~~~~~~~~~~~
+   11 | }
+   12 |

vlib/v/checker/tests/diff_type_map_value_err.vv

@@ -0,0 +1,16 @@
+struct MyStruct {
+    field int
+}
+
+struct MyStruct2 {
+    field int
+}
+
+fn empty() map[string]?MyStruct {
+  return {'key1': ?MyStruct(none), 'key2': MyStruct{field: 10}}
+}
+
+fn test_empty() {
+    expected := {'key': ?MyStruct(none)}
+    assert empty() == expected
+}