3 files changed, 45 insertions, 5 deletions
diff --git a/libgo/go/math/cmplx/cmath_test.go b/libgo/go/math/cmplx/cmath_test.go
index d904be8..7a5c485 100644
--- a/libgo/go/math/cmplx/cmath_test.go
+++ b/libgo/go/math/cmplx/cmath_test.go
@@ -759,6 +759,14 @@ func TestTanh(t *testing.T) {
 	}
 }
 
+// See issue 17577
+func TestInfiniteLoopIntanSeries(t *testing.T) {
+	want := Inf()
+	if got := Cot(0); got != want {
+		t.Errorf("Cot(0): got %g, want %g", got, want)
+	}
+}
+
 func BenchmarkAbs(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		Abs(complex(2.5, 3.5))
diff --git a/libgo/go/math/cmplx/example_test.go b/libgo/go/math/cmplx/example_test.go
new file mode 100644
index 0000000..be87cff
--- /dev/null
+++ b/libgo/go/math/cmplx/example_test.go
@@ -0,0 +1,30 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build ignore
+
+package cmplx_test
+
+import (
+	"fmt"
+	"math"
+	"math/cmplx"
+)
+
+func ExampleAbs() {
+	fmt.Printf("%.1f", cmplx.Abs(3+4i))
+	// Output: 5.0
+}
+
+// ExampleExp computes Euler's identity.
+func ExampleExp() {
+	fmt.Printf("%.1f", cmplx.Exp(1i*math.Pi)+1)
+	// Output: (0.0+0.0i)
+}
+
+func ExamplePolar() {
+	r, theta := cmplx.Polar(2i)
+	fmt.Printf("r: %.1f, θ: %.1f*π", r, theta/math.Pi)
+	// Output: r: 2.0, θ: 0.5*π
+}
diff --git a/libgo/go/math/cmplx/tan.go b/libgo/go/math/cmplx/tan.go
index 9485315..2990552 100644
--- a/libgo/go/math/cmplx/tan.go
+++ b/libgo/go/math/cmplx/tan.go
@@ -120,9 +120,9 @@ func tanSeries(z complex128) float64 {
 	rn := 0.0
 	d := 0.0
 	for {
-		rn += 1
+		rn++
 		f *= rn
-		rn += 1
+		rn++
 		f *= rn
 		x2 *= x
 		y2 *= y
@@ -130,16 +130,18 @@ func tanSeries(z complex128) float64 {
 		t /= f
 		d += t
 
-		rn += 1
+		rn++
 		f *= rn
-		rn += 1
+		rn++
 		f *= rn
 		x2 *= x
 		y2 *= y
 		t = y2 - x2
 		t /= f
 		d += t
-		if math.Abs(t/d) <= MACHEP {
+		if !(math.Abs(t/d) > MACHEP) {
+			// Caution: Use ! and > instead of <= for correct behavior if t/d is NaN.
+			// See issue 17577.
 			break
 		}
 	}