math/big: panic if n <= 0 for ProbablyPrime

Fixes #9509

Change-Id: I3b86745d38e09093fe2f4b918d774bd6608727d7
Reviewed-on: https://go-review.googlesource.com/2313Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/int.go

@@ -736,8 +736,11 @@ func (z *Int) binaryGCD(a, b *Int) *Int {
 
 // ProbablyPrime performs n Miller-Rabin tests to check whether x is prime.
 // If it returns true, x is prime with probability 1 - 1/4^n.
-// If it returns false, x is not prime.
+// If it returns false, x is not prime. n must be >0.
 func (x *Int) ProbablyPrime(n int) bool {
+	if n <= 0 {
+		panic("non-positive n for ProbablyPrime")
+	}
 	return !x.neg && x.abs.probablyPrime(n)
 }
 

src/math/big/int_test.go

@@ -989,6 +989,21 @@ func TestProbablyPrime(t *testing.T) {
 			break
 		}
 	}
+
+	// check that ProbablyPrime panics if n <= 0
+	c := NewInt(11) // a prime
+	for _, n := range []int{-1, 0, 1} {
+		func() {
+			defer func() {
+				if n <= 0 && recover() == nil {
+					t.Fatalf("expected panic from ProbablyPrime(%d)", n)
+				}
+			}()
+			if !c.ProbablyPrime(n) {
+				t.Fatalf("%v should be a prime", c)
+			}
+		}()
+	}
 }
 
 type intShiftTest struct {