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

$ go version
go version go1.17.1 linux/amd64

This was also an issue on go1.16. I haven't tested any older releases.

Does this issue reproduce with the latest release?

Yes

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

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/bkeyes/.cache/go-build"
GOENV="/home/bkeyes/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/bkeyes/code/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/bkeyes/code/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17.1"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build447354891=/tmp/go-build -gno-record-gcc-switches"
GOROOT/bin/go version: go version go1.17.1 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.17.1
uname -sr: Linux 5.10.16.3-microsoft-standard-WSL2
Distributor ID: Ubuntu
Description:    Ubuntu 20.04.2 LTS
Release:        20.04
Codename:       focal
/lib/x86_64-linux-gnu/libc.so.6: GNU C Library (Ubuntu GLIBC 2.31-0ubuntu9.2) stable release version 2.31.

Also reproduces on the Go Playground.

What did you do?

Used AddParseTree to set the content of the root/default template then tried to execute it.

https://play.golang.org/p/4UPROPWwFFA

package main

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

func main() {
	base := text.Must(text.New("base").Parse("template content\n"))
	
	tcopy := text.New("base")
	text.Must(tcopy.AddParseTree("base", base.Tree))
	
	hcopy := html.New("base")
	html.Must(hcopy.AddParseTree("base", base.Tree))

	fmt.Println("== text ==")
	if err := tcopy.ExecuteTemplate(os.Stdout, "base", nil); err != nil {
		panic(err)
	}
	if err := tcopy.Execute(os.Stdout, nil); err != nil {
		panic(err)
	}
	
	fmt.Println("== html ==")
	if err := hcopy.ExecuteTemplate(os.Stdout, "base", nil); err != nil {
		panic(err)
	}
	if err := hcopy.Execute(os.Stdout, nil); err != nil {
		panic(err)
	}
}

What did you expect to see?

Executing the template prints template content\n to standard out, like it does for a text/template.Template created using the same methods.

What did you see instead?

Executing the HTML template returns an error: template: "base" is an incomplete or empty template.

Additional Information

Note that executing the HTML template by name works as expected. This seems to be because the HTML implementation of AddParseTree does not set the Tree field of the Template struct. If I manually set this field, it appears to fix the initial error, but this might miss fixing other internal state.

I encountered this while building a utility for template inheritance. I used AddParseTree because Clone does not support changing the template name and I was trying to avoid re-parsing the base template(s) for each leaf in the tree.