text/template: Clone() does not clone options like it does for funcs

[bokwoon95]

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

$ go version
go version go1.15.4 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/bokwoon/Library/Caches/go-build"
GOENV="/Users/bokwoon/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/bokwoon/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/bokwoon/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/bokwoon/Documents/nusskylabx/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/dz/0qq60fyd6z3dbr2qqy23mfpm0000gn/T/go-build334239392=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

html/template .Clone() method will copy the funcmap set with .FuncMap() over to the cloned template (as mentioned here). However .Clone() does not seem to copy the options set with .Option() over to the cloned template. Instead the cloned template reverts back to the default option missingkey=invalid. Example code demonstrating how the funcmap is copied over, but the options are not:
https://play.golang.org/p/1M00gykKOkp

package main

import (
	"html/template"
	"fmt"
	"os"
)

func main() {
	t1 := template.Must(template.
		New("t").
		Option("missingkey=error").
		Funcs(map[string]interface{}{
			"SomeFunc": func() string { return "template" },
		}).
		Parse(`{{SomeFunc}} {{.tmpl}}: {{.jdlsadjklja}}`),
	)
	t2 := template.Must(t1.Clone())
	
	if err := t1.Execute(os.Stdout, map[string]interface{}{"tmpl": "t1"}); err != nil {
		fmt.Printf("t1 error: %s\n", err)
	}
	if err := t2.Execute(os.Stdout, map[string]interface{}{"tmpl": "t2"}); err != nil {
		fmt.Printf("t2 error: %s\n", err)
	}
}

What did you expect to see?

template t1: t1 error: template: t1:1:26: executing "t" at <.jdlsadjklja>: map has no entry for key "jdlsadjklja"
template t2: t2 error: template: t1:1:26: executing "t" at <.jdlsadjklja>: map has no entry for key "jdlsadjklja"

What did you see instead?

template t1: t1 error: template: t1:1:26: executing "t" at <.jdlsadjklja>: map has no entry for key "jdlsadjklja"
template t2:

This behaviour is also observed for text/template, only that text/template outputs <no value> instead of an empty string.

Currently I am cloning a bunch of templates, and in order to preserve the options I have to store the option string somewhere in a variable and re-configure using .Option() on every newly cloned template. Copying options on .Clone() is not mentioned in the documentation, but it would be nice if it did that. Given that .Clone() already copies the funcmap, it seems reasonable for it to copy the options as well.

[toothrot]

/cc @robpike