checker: fix `instance.cb_field(mut arg)`

vlib/v/checker/checker.v

@@ -1534,10 +1534,13 @@ pub fn (mut c Checker) call_method(mut call_expr ast.CallExpr) table.Type {
 			call_expr.is_field = true
 			info := field_type_sym.info as table.FnType
 			call_expr.return_type = info.func.return_type
-			// TODO: check args (do it once for all of the above)
-			for arg in call_expr.args {
-				c.expr(arg.expr)
+			mut earg_types := []table.Type{}
+			for mut arg in call_expr.args {
+				targ := c.expr(arg.expr)
+				arg.typ = targ
+				earg_types << targ
 			}
+			call_expr.expected_arg_types = earg_types
 			return info.func.return_type
 		}
 	}

vlib/v/tests/struct_fields_storing_functions_test.v

@@ -0,0 +1,29 @@
+type Async_cb = fn (x []byte, mut y []byte) int
+
+fn async_cb(b []byte, mut res []byte) int {
+	if b.len > 0 {
+		res << b
+	}
+	return 0
+}
+
+struct Ep_arg {
+mut:
+	sfd int
+	cb  Async_cb
+}
+
+fn test_struct_fn_field_can_be_used_directly() {
+	buf := [byte(1), 2, 3]
+	mut res := []byte{}
+	res << 0x88
+	async_cb(buf[0..2], mut res)
+	data := Ep_arg{
+		sfd: 1234
+		cb: async_cb
+	}
+	data.cb(buf[1..2], mut res)
+	res << 0x99
+	eprintln(res)
+	assert res == [byte(0x88), 0x01, 0x02, 0x02, 0x99]
+}