encoding/xml: empty namespace prefix definitions should be illegal

What does 'go version' print?
go version go1.2 linux/amd64


What steps reproduce the problem?
http://play.golang.org/p/f8UWfdF7Hb

What happened?
No-error.

What should have happened instead?
Should generate an error.

Per http://www.w3.org/TR/REC-xml-names/#dt-prefix a prefix must refer to a namespace
name, not the empty string. 

Please provide any additional information below.

Ran into this in implementing webdav, the litmus tests test this error condition [and
due to xml.Token hiding prefixing it is impossible to detect otherwise]

[griesemer]

Comment 1:

Labels changed: added repo-main, release-none.

[rogpeppe]

Comment 2:

> Per http://www.w3.org/TR/REC-xml-names/#dt-prefix a prefix must
> refer to a namespace name, not the empty string. 
I couldn't find this stated clearly in the spec. It says "The attribute's normalized
value MUST be either a URI reference — the namespace name identifying the namespace
— or an empty string.". It also explicitly allows the empty string for the xmlns
attribute itself (from section 6.2: "The attribute value in a default namespace
declaration MAY be empty. This has the same effect, within the scope of the declaration,
of there being no default namespace.")
That said, it's not clear what the semantics should be for an empty value.

Comment 3:

I must admit I was echo'ing the complaint from the WebDAV litmus test suite; which
claimed an empty prefix specification was invalid. 
I agree with your reading that it doesn't appear clear. 6.3. doesn't apply here, i'm
concerned specifically with namespace prefixes [the test in WebDAV is with xmlns:dav=""]

[nigeltao]

It could be that, at the time of writing of the WebDAV litmus test suite, an empty namespace prefix was illegal, but that has since become legal. The grammar differs between
http://www.w3.org/TR/2006/REC-xml-names-20060816/#ns-decl and
http://www.w3.org/TR/REC-xml-names/#dt-prefix

[gopherbot]

Change https://golang.org/cl/105636 mentions this issue: encoding/xml: fix invalid empty namespace without prefix

[iwdgo]

Namespace handling is not checking this case before adding the namespace to the namespace array of the decoder. A related fix is submitted which also rejects syntax xmlns:="..." which is invalid.