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proposal: encoding/json: add support of wrapped error in addErrorContext function #45449

DhZero opened this issue Apr 8, 2021 · 2 comments


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@DhZero DhZero commented Apr 8, 2021

What version of Go are you using (go version)?

$ go version
go version go1.15.5 darwin/amd64

Does this issue reproduce with the latest release?


What operating system and processor architecture are you using (go env)?

go env Output
$ go env

What did you do?

What did you expect to see?

Expected to see proper values set to err.Field and err.Struct even if error was wrapped by fmt.Errorf() in UnmarshalJSON of custom structure.

What did you see instead?

In decode.go in addErrorContext function, switch won't work as expected because type mismatch.
So err.Field and err.Struct will be empty even if d.errorContext.Struct != nil || len(d.errorContext.FieldStack) > 0 condition was passed.

What did you propose?

Maybe it is better to use errors.As() instead of switch?
Like this:

// addErrorContext returns a new error enhanced with information from d.errorContext
func (d *decodeState) addErrorContext(err error) error {
	if d.errorContext.Struct != nil || len(d.errorContext.FieldStack) > 0 {
		var e *UnmarshalTypeError
		if errors.As(err, &e) {
			e.Struct = d.errorContext.Struct.Name()
			e.Field = strings.Join(d.errorContext.FieldStack, ".")
	return err
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@rsc rsc commented May 5, 2021

/cc @dsnet

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@rsc rsc commented May 5, 2021

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

@rsc rsc moved this from Incoming to Active in Proposals May 5, 2021
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3 participants