strconv: add Unwrap to custom error types

Updates #30322

This change adds the Unwrap method to NumError. NumError is the only custom error type of the strconv that has a nested exported error.

Change-Id: I8774886348880365a83f72a1d106276def27dffe
GitHub-Last-Rev: 712f3df8842f48f988cebfc527476781a7cf7140
GitHub-Pull-Request: golang/go#34213
Reviewed-on: https://go-review.googlesource.com/c/go/+/194563
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/strconv/atoi.go

@@ -31,6 +31,8 @@ func (e *NumError) Error() string {
 	return "strconv." + e.Func + ": " + "parsing " + Quote(e.Num) + ": " + e.Err.Error()
 }
 
+func (e *NumError) Unwrap() error { return e.Err }
+
 func syntaxError(fn, str string) *NumError {
 	return &NumError{fn, str, ErrSyntax}
 }

src/strconv/atoi_test.go

@@ -592,6 +592,13 @@ func TestNumError(t *testing.T) {
 	}
 }
 
+func TestNumErrorUnwrap(t *testing.T) {
+	err := &NumError{Err: ErrSyntax}
+	if !errors.Is(err, ErrSyntax) {
+		t.Error("errors.Is failed, wanted success")
+	}
+}
+
 func BenchmarkParseInt(b *testing.B) {
 	b.Run("Pos", func(b *testing.B) {
 		benchmarkParseInt(b, 1)