text/template: fix bug in pipelined variadics

Simple bug in argument processing: The final arg may
be the pipeline value, in which case it gets bound to the
fixed argument section. The code got that wrong. Easy
to fix.

Fixes issue 8950.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/161750043

2 files changed, 13 insertions, 1 deletions

diff --git a/src/text/template/exec.go b/src/text/template/exec.go
index f6eed662b..b00e10c7e 100644
--- a/src/text/template/exec.go
+++ b/src/text/template/exec.go
@@ -546,7 +546,7 @@ func (s *state) evalCall(dot, fun reflect.Value, node parse.Node, name string, a
 	argv := make([]reflect.Value, numIn)
 	// Args must be evaluated. Fixed args first.
 	i := 0
-	for ; i < numFixed; i++ {
+	for ; i < numFixed && i < len(args); i++ {
 		argv[i] = s.evalArg(dot, typ.In(i), args[i])
 	}
 	// Now the ... args.
diff --git a/src/text/template/exec_test.go b/src/text/template/exec_test.go
index e2cf2d370..69c213ed2 100644
--- a/src/text/template/exec_test.go
+++ b/src/text/template/exec_test.go
@@ -893,6 +893,18 @@ func TestMessageForExecuteEmpty(t *testing.T) {
 	}
 }
 
+func TestFinalForPrintf(t *testing.T) {
+	tmpl, err := New("").Parse(`{{"x" | printf}}`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var b bytes.Buffer
+	err = tmpl.Execute(&b, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
 type cmpTest struct {
 	expr  string
 	truth string