checker: check fn type mismatch of return result type fn (#16269)

vlib/v/checker/check_types.v

@@ -381,6 +381,9 @@ pub fn (mut c Checker) check_matching_function_symbols(got_type_sym &ast.TypeSym
 	if got_fn.return_type.has_flag(.optional) != exp_fn.return_type.has_flag(.optional) {
 		return false
 	}
+	if got_fn.return_type.has_flag(.result) != exp_fn.return_type.has_flag(.result) {
+		return false
+	}
 	if !c.check_basic(got_fn.return_type, exp_fn.return_type) {
 		return false
 	}

vlib/v/checker/tests/return_result_fn_mismatch.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/return_result_fn_mismatch.vv:15:10: error: cannot use `fn ()` as `fn () !` in argument 1 to `do_test`
+   13 |         foo() or { return }
+   14 |     }
+   15 |     do_test(test)
+      |             ~~~~
+   16 | }

vlib/v/checker/tests/return_result_fn_mismatch.vv

@@ -0,0 +1,16 @@
+type AnonOptionalFun = fn () !
+
+// do_test is used to test functionality
+pub fn do_test(cb AnonOptionalFun) {
+	cb() or { panic(err) }
+}
+
+fn foo () ! {
+}
+
+fn main() {
+	test := fn () {
+		foo() or { return }
+	}
+	do_test(test)
+}