Not returning the value when using tag()

[naoliv]

Have this small validation rule:

*[foo = bar] {
        throwWarning: tr("something {0}={1}", "{0.key}", tag("{0.key}"));
}

and validate an object with foo=bar
We can see a message saying something foo=null

Of course the common usage is {0.value} instead tag("{0.key}"), but since tag(key_name) description says "get the value of the key key_name from the object in question", shouldn't we see the same result regardless which one we choose here?

JOSM:

Relative:URL: ^/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2018-01-02 00:31:23 +0100 (Tue, 02 Jan 2018)
Revision:13271
Build-Date:2018-01-02 02:33:50
URL:http://josm.openstreetmap.de/svn/trunk

Identification: JOSM/1.5 (13271 pt_BR) Linux Debian GNU/Linux unstable (sid)
Memory Usage: 576 MB / 5120 MB (173 MB allocated, but free)
Java version: 9.0.1+11-Debian-1, Oracle Corporation, OpenJDK 64-Bit Server VM
Screen: :0.0 1920x1080
Maximum Screen Size: 1920x1080
Java package: openjdk-9-jre:amd64-9.0.1+11-1
Java ATK Wrapper package: libatk-wrapper-java:all-0.33.3-13
VM arguments: [--illegal-access=warn, --add-modules=java.se.ee, -Dawt.useSystemAAFontSettings=on]
Dataset consistency test: No problems found

[naoliv]

As a real example (and for context), I was trying to test some rules for #15719:

*[distance =~ /^[0-9]+,[0-9]+$/],
*[ele =~ /^[0-9]+,[0-9]+$/],
*[height =~ /^[0-9]+,[0-9]+$/],
*[width =~ /^[0-9]+,[0-9]+$/] {
        throwWarning: tr("some message for the comma");
        fixAdd: concat("{0.key}=", eval(replace(tag("{0.key}"), ",", ".")));
}

But it deletes the key (since the value is null)

[Klumbumbus]

cc

[Klumbumbus]

more like an enhancement than a defect, see ticket:15719#comment:4

for now it works with examples similar to ticket:15719#comment:2 or r12215 or r13345