math/big: don't clobber shared underlying array in pow5 computation

Rearranged code slightly to make lifetime of underlying array of
pow5 more explicit in code.

Fixes #31184.

Change-Id: I063081f0e54097c499988d268a23813746592654
Reviewed-on: https://go-review.googlesource.com/c/go/+/170641Reviewed-by: Filippo Valsorda <filippo@golang.org>

src/math/big/ratconv.go

@@ -162,36 +162,31 @@ func (z *Rat) SetString(s string) (*Rat, bool) {
 	}
 	// exp consumed - not needed anymore
 
-	// compute pow5 if needed
-	pow5 := z.b.abs
+	// apply exp5 contributions
+	// (start with exp5 so the numbers to multiply are smaller)
 	if exp5 != 0 {
 		n := exp5
 		if n < 0 {
 			n = -n
 		}
-		pow5 = pow5.expNN(natFive, nat(nil).setWord(Word(n)), nil)
-	}
-
-	// apply dividend contributions of exponents
-	// (start with exp5 so the numbers to multiply are smaller)
+		pow5 := z.b.abs.expNN(natFive, nat(nil).setWord(Word(n)), nil) // use underlying array of z.b.abs
 		if exp5 > 0 {
 			z.a.abs = z.a.abs.mul(z.a.abs, pow5)
-		exp5 = 0
+			z.b.abs = z.b.abs.setWord(1)
+		} else {
+			z.b.abs = pow5
+		}
+	} else {
+		z.b.abs = z.b.abs.setWord(1)
 	}
+
+	// apply exp2 contributions
 	if exp2 > 0 {
 		if int64(uint(exp2)) != exp2 {
 			panic("exponent too large")
 		}
 		z.a.abs = z.a.abs.shl(z.a.abs, uint(exp2))
-		exp2 = 0
-	}
-
-	// apply divisor contributions of exponents
-	z.b.abs = z.b.abs.setWord(1)
-	if exp5 < 0 {
-		z.b.abs = pow5
-	}
-	if exp2 < 0 {
+	} else if exp2 < 0 {
 		if int64(uint(-exp2)) != -exp2 {
 			panic("exponent too large")
 		}

src/math/big/ratconv_test.go

@@ -574,3 +574,18 @@ func TestFloat64SpecialCases(t *testing.T) {
 		}
 	}
 }
+
+func TestIssue31184(t *testing.T) {
+	var x Rat
+	for _, want := range []string{
+		"-213.090",
+		"8.192",
+		"16.000",
+	} {
+		x.SetString(want)
+		got := x.FloatString(3)
+		if got != want {
+			t.Errorf("got %s, want %s", got, want)
+		}
+	}
+}