text/template: improve error reporting for executing an empty template

Fixes #4522.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7502044

1 files changed, 17 insertions, 1 deletions

diff --git a/src/pkg/text/template/exec.go b/src/pkg/text/template/exec.go
index b9c03d8f0b..12c40b70f1 100644
--- a/src/pkg/text/template/exec.go
+++ b/src/pkg/text/template/exec.go
@@ -5,6 +5,7 @@
 package template
 
 import (
+	"bytes"
 	"fmt"
 	"io"
 	"reflect"
@@ -125,8 +126,23 @@ func (t *Template) Execute(wr io.Writer, data interface{}) (err error) {
 		wr:   wr,
 		vars: []variable{{"$", value}},
 	}
+	t.init()
 	if t.Tree == nil || t.Root == nil {
-		state.errorf("%q is an incomplete or empty template", t.name)
+		var b bytes.Buffer
+		for name, tmpl := range t.tmpl {
+			if tmpl.Tree == nil || tmpl.Root == nil {
+				continue
+			}
+			if b.Len() > 0 {
+				b.WriteString(", ")
+			}
+			fmt.Fprintf(&b, "%q", name)
+		}
+		var s string
+		if b.Len() > 0 {
+			s = "; defined templates are: " + b.String()
+		}
+		state.errorf("%q is an incomplete or empty template%s", t.Name(), s)
 	}
 	state.walk(value, t.Root)
 	return