math/cmplx: avoid panic in Pow(x, NaN())

Fixes #30088

Change-Id: I08cec17feddc86bd08532e6b135807e3c8f4c1b2
Reviewed-on: https://go-review.googlesource.com/c/161197
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/cmplx/cmath_test.go

@@ -400,9 +400,11 @@ var polarSC = []ff{
 }
 var vcPowSC = [][2]complex128{
 	{NaN(), NaN()},
+	{0, NaN()},
 }
 var powSC = []complex128{
 	NaN(),
+	NaN(),
 }
 var vcSinSC = []complex128{
 	NaN(),
@@ -734,8 +736,8 @@ func TestPow(t *testing.T) {
 		}
 	}
 	for i := 0; i < len(vcPowSC); i++ {
-		if f := Pow(vcPowSC[i][0], vcPowSC[i][0]); !cAlike(powSC[i], f) {
-			t.Errorf("Pow(%g, %g) = %g, want %g", vcPowSC[i][0], vcPowSC[i][0], f, powSC[i])
+		if f := Pow(vcPowSC[i][0], vcPowSC[i][1]); !cAlike(powSC[i], f) {
+			t.Errorf("Pow(%g, %g) = %g, want %g", vcPowSC[i][0], vcPowSC[i][1], f, powSC[i])
 		}
 	}
 	for _, pt := range branchPoints {

src/math/cmplx/pow.go

@@ -48,6 +48,9 @@ import "math"
 //	Pow(0, c) for real(c)<0 returns Inf+0i if imag(c) is zero, otherwise Inf+Inf i.
 func Pow(x, y complex128) complex128 {
 	if x == 0 { // Guaranteed also true for x == -0.
+		if IsNaN(y) {
+			return NaN()
+		}
 		r, i := real(y), imag(y)
 		switch {
 		case r == 0: