strconv: document parsing of leading +/-

Explicitly document the handling of a sign prefix, and the interaction
between the sign and base prefixes.

Fixes #46641.

Change-Id: I3cd6773e3f074fe671a944a05a79d2408137fcd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/325875
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

2 files changed, 18 insertions, 3 deletions

diff --git a/src/strconv/atoi.go b/src/strconv/atoi.go
index c9ba0383b3..631b487d97 100644
--- a/src/strconv/atoi.go
+++ b/src/strconv/atoi.go
@@ -57,6 +57,8 @@ const IntSize = intSize
 const maxUint64 = 1<<64 - 1
 
 // ParseUint is like ParseInt but for unsigned numbers.
+//
+// A sign prefix is not permitted.
 func ParseUint(s string, base int, bitSize int) (uint64, error) {
 	const fnParseUint = "ParseUint"
 
@@ -159,10 +161,13 @@ func ParseUint(s string, base int, bitSize int) (uint64, error) {
 // ParseInt interprets a string s in the given base (0, 2 to 36) and
 // bit size (0 to 64) and returns the corresponding value i.
 //
+// The string may begin with a leading sign: "+" or "-".
+//
 // If the base argument is 0, the true base is implied by the string's
-// prefix: 2 for "0b", 8 for "0" or "0o", 16 for "0x", and 10 otherwise.
-// Also, for argument base 0 only, underscore characters are permitted
-// as defined by the Go syntax for integer literals.
+// prefix following the sign (if present): 2 for "0b", 8 for "0" or "0o",
+// 16 for "0x", and 10 otherwise. Also, for argument base 0 only,
+// underscore characters are permitted as defined by the Go syntax for
+// integer literals.
 //
 // The bitSize argument specifies the integer type
 // that the result must fit into. Bit sizes 0, 8, 16, 32, and 64
diff --git a/src/strconv/atoi_test.go b/src/strconv/atoi_test.go
index 178fb01ea7..867fa66a14 100644
--- a/src/strconv/atoi_test.go
+++ b/src/strconv/atoi_test.go
@@ -33,6 +33,9 @@ var parseUint64Tests = []parseUint64Test{
 	{"_12345", 0, ErrSyntax},
 	{"1__2345", 0, ErrSyntax},
 	{"12345_", 0, ErrSyntax},
+	{"-0", 0, ErrSyntax},
+	{"-1", 0, ErrSyntax},
+	{"+1", 0, ErrSyntax},
 }
 
 type parseUint64BaseTest struct {
@@ -140,8 +143,10 @@ var parseInt64Tests = []parseInt64Test{
 	{"", 0, ErrSyntax},
 	{"0", 0, nil},
 	{"-0", 0, nil},
+	{"+0", 0, nil},
 	{"1", 1, nil},
 	{"-1", -1, nil},
+	{"+1", 1, nil},
 	{"12345", 12345, nil},
 	{"-12345", -12345, nil},
 	{"012345", 12345, nil},
@@ -236,6 +241,11 @@ var parseInt64BaseTests = []parseInt64BaseTest{
 	{"0__12345", 0, 0, ErrSyntax},
 	{"01234__5", 0, 0, ErrSyntax},
 	{"012345_", 0, 0, ErrSyntax},
+
+	{"+0xf", 0, 0xf, nil},
+	{"-0xf", 0, -0xf, nil},
+	{"0x+f", 0, 0, ErrSyntax},
+	{"0x-f", 0, 0, ErrSyntax},
 }
 
 type parseUint32Test struct {