text/template: panic on range over send-only channel

[jba]

What version of Go are you using (go version)?

go1.15.5

What did you do?

https://play.golang.org/p/Z9wXwjO_NVd

What did you expect to see?

An error returned.

What did you see instead?

A panic.

[mengzhuo]

I will take a look

[gopherbot]

Change https://golang.org/cl/276532 mentions this issue: text/template: error on range over send channel