make: empty() does not seem to work for loop variables

From: Yuri <yuri_at_aetern.org>
Date: Mon, 24 Jul 2023 22:41:33 UTC
Given the Makefile below, I get the following output:
---
$ make
make: "/home/yuri/tmp/Makefile" line 3: warning: check0
make: "/home/yuri/tmp/Makefile" line 11: warning: check2
---

Makefile:
---
FOO=    bar
.if !empty(FOO:Mbar)
.warning check0
.endif

.for i in foo bar baz
.if !empty(i:Mbar)
.warning check1
.endif
.if ${i:Mbar} != ""
.warning check2
.endif
.endfor

all:
---

So it looks like empty() does not work when applied to loop variable?
Or am I missing something here?