checker: check for `x := Abc { f: fn () ? {} }` mismatch, when `f` is `fn ()`

vlib/v/checker/check_types.v

@@ -144,6 +144,9 @@ pub fn (mut c Checker) check_matching_function_symbols(got_type_sym &ast.TypeSym
 	if got_fn.params.len != exp_fn.params.len {
 		return false
 	}
+	if got_fn.return_type.has_flag(.optional) != exp_fn.return_type.has_flag(.optional) {
+		return false
+	}
 	if !c.check_basic(got_fn.return_type, exp_fn.return_type) {
 		return false
 	}

vlib/v/checker/tests/fn_check_for_matching_optional_result_in_fields.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/fn_check_for_matching_optional_result_in_fields.vv:7:3: error: cannot assign to field `f`: expected `fn (voidptr)`, not `fn (voidptr) ?`
+    5 | fn main() {
+    6 |     a := Abc{
+    7 |         f: fn (data voidptr) ? {}
+      |         ~~~~~~~~~~~~~~~~~~~~~~~~~
+    8 |     }
+    9 |     println(a)

vlib/v/checker/tests/fn_check_for_matching_optional_result_in_fields.vv

@@ -0,0 +1,10 @@
+struct Abc {
+	f fn (voidptr)
+}
+
+fn main() {
+	a := Abc{
+		f: fn (data voidptr) ? {}
+	}
+	println(a)
+}