fmt: surprising undocumented behavior for "String() string" panics

[bcmills]

The fmt package and other packages that transitively use it (html/template and
text/template in particular) have surprising behavior when the "String()
string" method panics.

I would expect the panic to continue to propagate back to the caller.  But it appears
that fmt.Sprint silently converts panics into strings of the form
"%v(PANIC=%s)".  This behavior is not documented in the fmt, html/template, or
text/template packages.

This leads to cases where non-trivial programs deadlock when one would otherwise expect
a crash with useful stack traces:
http://play.golang.org/p/G9ytQr6EPs

Ideally, the fmt package should not recover any panics at all.

If that's not feasible (e.g. for reasons of backward-compatibility), then at the very
least the behavior of packages that (transitively) use it should be documented.

[ianlancetaylor]

Comment 1:

This is working as intended.
But documenting it sounds like a good idea.

[ianlancetaylor]

Comment 2:

This issue was closed by revision 806dea8.

Status changed to Fixed.