exp/template: don't panic on range of nil interface
This avoids a non-obvious panic when range is used on a
nil interface, and fixes it by behaving as if the range
was empty.
The new behavior is equivalent to the outcome of iterating
on a nil map or slice, and is useful because it allows
generic structures such as used in json (map[string]interface{})
to behave correctly if a key generally set to a list or map
isn't present.
R=golang-dev, r, gustavo
CC=golang-dev
https://golang.org/cl/4876046
diff --git a/src/pkg/exp/template/exec_test.go b/src/pkg/exp/template/exec_test.go
index a8ef64d..8a610da 100644
--- a/src/pkg/exp/template/exec_test.go
+++ b/src/pkg/exp/template/exec_test.go
@@ -372,6 +372,7 @@
{"range map else", "{{range .MSI | .MSort}}-{{.}}-{{else}}EMPTY{{end}}", "-one--three--two-", tVal, true},
{"range empty map else", "{{range .MSIEmpty}}-{{.}}-{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
{"range empty interface", "{{range .Empty3}}-{{.}}-{{else}}EMPTY{{end}}", "-7--8-", tVal, true},
+ {"range empty nil", "{{range .Empty0}}-{{.}}-{{end}}", "", tVal, true},
{"range $x SI", "{{range $x := .SI}}<{{$x}}>{{end}}", "<3><4><5>", tVal, true},
{"range $x $y SI", "{{range $x, $y := .SI}}<{{$x}}={{$y}}>{{end}}", "<0=3><1=4><2=5>", tVal, true},
{"range $x MSIone", "{{range $x := .MSIone}}<{{$x}}>{{end}}", "<1>", tVal, true},
