html/template: confusing or incorrect error

[rsc]

I have a template that says

<a href="{{if $.Draft}}/draft/{{.Name}}{{else}}/{{.Name}}{{end}}">

which works but is redundant.

If I change it to factor out the final /{{.Name}}, like in

<a href="{{if $.Draft}}/draft/{{end}}{{.Name}}">

then I get this error:

html/template:toc:65: (action: [(command: [F=[Name]])]) appears in an ambiguous URL
context

The error could perhaps say more about what is wrong, or say it
more clearly, but also I don't see why these two should be
different at all.

[gopherbot]

Comment 1 by mikesamuel:

I agree that the error message needs to be improved.
In the case where $.Draft is truthy, we have
    
where we need to HTML escape .Name but do nothing else.
In the other case, we have
    
where we need to ensure that .Name does not start with "javascript:" or another
disallowed protocol.
The "ambiguous URL context" error message indicates that it is impossible to tell
statically, whether to white-list protocols or not.
In your case, we could probably %-encode any colon, but that is not possible in general
because colons are overloaded and would be bending the rules of RFC 3986.
    
It is ambiguous whether the colon follows a non-hierarchical protocol or precedes a port.
We could factor the interpolation left into the conditional and that may match
programmer intent, but that is not possible across template call boundaries:
     

[adg]

Comment 2:

Labels changed: added priority-go1.

[gopherbot]

Comment 3 by krolaw:

Shouldn't the factored version be:

??

[gopherbot]

Comment 4 by mikesamuel:

krolaw, I think you're right that the factored form is missing a slash.  If it were
factored to

there would be no error.

[rsc]

Comment 5:

wacky.  thanks for the explanation.

[gopherbot]

Comment 6 by mikesamuel:

Now that you understand the problem, what would html.template ideally have done
differently?

[rsc]

Comment 7:

The original error was
(action: [(command: [F=[Name]])]) appears in an ambiguous URL context
The trivial thing is that it would be nice to use the template syntax
in the error, so that it would say
{.Name} appears in an ambiguous URL context
Slightly less trivially, it would be nice to list the contexts in some
friendly way.  For example, in this case,
{.Name} appears in an ambiguous context: either beginning or middle of URL

[robpike]

Comment 8:

Owner changed to builder@golang.org.

[rsc]

Comment 9:

Labels changed: added go1-must.

[remyoudompheng]

Comment 10:

I can't see the error with tip. It seems to have been fixed (there are even test cases).

[rsc]

Comment 11:

package main
import "html/template"
import "os"
func main() {
    var x struct { Draft bool; Name string }
    t := template.Must(template.New("x").Parse(``))
    if err := t.Execute(os.Stdout, &x); err != nil {
        panic(err.Error())
    }
}
This says
panic: html/template:x:1: {{.Name}} appears in an ambiguous URL context
Close enough for now.

Status changed to Fixed.