Comment 1 by pavadeli:

By the way, it works correctly for text/template.

Comment 2 by pavadeli:

It turns out that all errors that occur during template execution suffer from this bug.
For example, if you add an argument to the myFunc function call in the playground
example you also get the same problem:
const tmplString = `{{myFunc "arg1"}}`
I am currently considering, as a partial workaround, to create wrappers for all custom
functions that start panicking on any returned errors. For example, if you rewrite
myFunc in the go playground example as follows, then the error is returned:
func myFunc() (string, error) {
    panic(fmt.Errorf("My error!"))
}
Anyone a better idea? This works for custom functions, but of course not for errors that
occur in the template itself.

Comment 3:

Labels changed: added priority-later, go1.2, removed priority-triage.

Status changed to Accepted.

Comment 4:

The problem is that text/template/parse's Tree.text wasn't getting copied during the
clone. Unfortunately, I don't see any way to fix this without adding to the public API.
Proposed fix in https://golang.org/cl/12420044

Comment 5:

This issue was closed by revision eeb7585.

Status changed to Fixed.