fmt: add test of NaN map keys

R=r
CC=golang-dev
https://golang.org/cl/5564063

src/pkg/fmt/fmt_test.go

@@ -443,6 +443,14 @@ var fmttests = []struct {
 	{"%s", nil, "%!s(<nil>)"},
 	{"%T", nil, "<nil>"},
 	{"%-1", 100, "%!(NOVERB)%!(EXTRA int=100)"},
+
+	// The "<nil>" show up because maps are printed by
+	// first obtaining a list of keys and then looking up
+	// each key.  Since NaNs can be map keys but cannot
+	// be fetched directly, the lookup fails and returns a
+	// zero reflect.Value, which formats as <nil>.
+	// This test is just to check that it shows the two NaNs at all.
+	{"%v", map[float64]int{math.NaN(): 1, math.NaN(): 2}, "map[NaN:<nil> NaN:<nil>]"},
 }
 
 func TestSprintf(t *testing.T) {