math: make portable Tan(Pi/2) return NaN

The panic NaN was a translation error.
The earliest version said panic "return sys.NaN()",
and when sys.NaN came along, it changed
to "panic sys.NaN()" instead of "return sys.NaN()".

R=r
CC=golang-dev
https://golang.org/cl/2106049

src/pkg/math/all_test.go

@@ -7,6 +7,7 @@ package math_test
 import (
 	"fmt"
 	. "math"
+	"runtime"
 	"testing"
 )
 
@@ -2100,6 +2101,16 @@ func TestTan(t *testing.T) {
 			t.Errorf("Tan(%g) = %g, want %g\n", vfsinSC[i], f, sinSC[i])
 		}
 	}
+
+	// Make sure portable Tan(Pi/2) doesn't panic (it used to).
+	// The portable implementation returns NaN.
+	// Assembly implementations might not,
+	// because Pi/2 is not exactly representable.
+	if runtime.GOARCH != "386" {
+		if f := Tan(Pi / 2); !alike(f, NaN()) {
+			t.Errorf("Tan(%g) = %g, want %g\n", Pi/2, f, NaN())
+		}
+	}
 }
 
 func TestTanh(t *testing.T) {

src/pkg/math/tan.go

@@ -54,7 +54,7 @@ func Tan(x float64) float64 {
 
 	if flag {
 		if temp == 0 {
-			panic(NaN())
+			return NaN()
 		}
 		temp = 1 / temp
 	}