implement ".Unwrap() error" on Error type

[LeGEC]

Description

The 'Error' type defined in errors.go does not implement ".Unwrap() error", which prevents the usage of "errors.Is()" and "errors.As()"

How to reproduce

package main

import (
	"github.com/gin-gonic/gin"
)

type TestStrErr string

func (e TestStrErr) Error() string { return string(e) }

func main() {
	testErr := TestStrErr("some error")

	err := &gin.Error{Err: testErr}

	if errors.Is(err, testErr) {
	    fmt.Println("match with testErr ok")
	}
	
	var unwrap TestStrErr
	if errors.As(err, &unwrap) {
	    fmt.Println("succesfully unwrapped as a TestStrErr")
	}
}

Expectations

output :

match with testErr ok
succesfully unwrapped as a TestStrErr

Actual result

no output

Environment

• go version: go version go1.14.2 linux/amd64
• gin version (or commit ref): 540b1ef
• operating system: linux

[LeGEC]

A more useful use case was asked by this user on StackOverflow :
How to assert error type json.UnmarshalTypeError when caught by gin c.BindJSON ?

He wants to tell apart "this is a JSON parsing error" from "the JSON is ok, this is a validation error"

With the correct implementation of the .Unwrap() error method, this boils down to :

// gErr is a gin.Error
var jsErr *json.UnmarshalTypeError
if errors.As(gErr, &jsErr) {
    fmt.Println("the json is invalid")
} else {
    fmt.Println("this is something else")
}

[thejdavid]

It would also help for other issues for other data types such as trying to un parse an invalid time.Time. in such case it wouldn't return a json parsing error but a time parsing error.
It might be useful to differentiate / group parsing errors from binding errors.

[thinkerou]

#2526 merged

[jhannah-mm]

@thejdavid wrote:

It would also help for other issues for other data types such as trying to un parse an invalid time.Time. in such case it wouldn't return a json parsing error but a time parsing error.
It might be useful to differentiate / group parsing errors from binding errors.

Ya, it seems that even under 1.7.1 there's still no useful information regarding where in a JSON POST the UnmarshalType casting failure occurred...

POST {
   "start_date": "invalid_date"    // time.Time
}
parsing time ""invalid_date"" as ""2006-01-02T15:04:05Z07:00"": cannot parse "invalid_date"" as "2006"

Hoping it would say start_date... somewhere / somehow