encoding/asn1: unmarshal doesn't support the same encoding field tags as marshal

[rolandshoemaker]

When using implicit tags, Marshal/Unmarshal provide a number of struct field tags that allow forcing specific encodings to be used for confusable types.

Marshal supports a larger set of these tags than Unmarshal, causing some data to be impossible to roundtrip.

package main

import (
	"encoding/asn1"
	"fmt"
	"time"
)

func main() {
	out, err := asn1.MarshalWithParams(time.Now(), "tag:2,generalized")
	if err != nil {
		fmt.Printf("failed to marshal: %v\n", err)
	}
	fmt.Printf("out: %x\n", out)
	var t time.Time
	_, err = asn1.UnmarshalWithParams(out, &t, "tag:2,generalized")
	if err != nil {
		fmt.Printf("failed to unmarshal: %v\n", err)
	}
	fmt.Printf("time: %v\n", t)
}

Unmarshal should support the same set of field tags.

[gopherbot]

Change https://go.dev/cl/654275 mentions this issue: encoding/asn1: make sure implicit fields roundtrip