strconv: fix %.2g, 40

Fixes #845.

R=rsc
CC=golang-dev
https://golang.org/cl/1673049

src/pkg/strconv/ftoa.go

@@ -154,21 +154,27 @@ func genericFtoa(bits uint64, fmt byte, prec int, flt *floatInfo) string {
 	case 'f':
 		return fmtF(neg, d, prec)
 	case 'g', 'G':
-		// trailing zeros are removed.
-		if prec > d.nd {
-			prec = d.nd
+		// trailing fractional zeros in 'e' form will be trimmed.
+		eprec := prec
+		if eprec > d.nd && d.nd >= d.dp {
+			eprec = d.nd
 		}
 		// %e is used if the exponent from the conversion
 		// is less than -4 or greater than or equal to the precision.
 		// if precision was the shortest possible, use precision 6 for this decision.
-		eprec := prec
 		if shortest {
 			eprec = 6
 		}
 		exp := d.dp - 1
 		if exp < -4 || exp >= eprec {
+			if prec > d.nd {
+				prec = d.nd
+			}
 			return fmtE(neg, d, prec-1, fmt+'e'-'g')
 		}
+		if prec > d.dp {
+			prec = d.nd
+		}
 		return fmtF(neg, d, max(prec-d.dp, 0))
 	}
 

src/pkg/strconv/ftoa_test.go

@@ -34,6 +34,17 @@ var ftoatests = []ftoaTest{
 	ftoaTest{200000, 'g', -1, "200000"},
 	ftoaTest{2000000, 'g', -1, "2e+06"},
 
+	// g conversion and zero suppression
+	ftoaTest{400, 'g', 2, "4e+02"},
+	ftoaTest{40, 'g', 2, "40"},
+	ftoaTest{4, 'g', 2, "4"},
+	ftoaTest{.4, 'g', 2, "0.4"},
+	ftoaTest{.04, 'g', 2, "0.04"},
+	ftoaTest{.004, 'g', 2, "0.004"},
+	ftoaTest{.0004, 'g', 2, "0.0004"},
+	ftoaTest{.00004, 'g', 2, "4e-05"},
+	ftoaTest{.000004, 'g', 2, "4e-06"},
+
 	ftoaTest{0, 'e', 5, "0.00000e+00"},
 	ftoaTest{0, 'f', 5, "0.00000"},
 	ftoaTest{0, 'g', 5, "0"},