runtime: PanicNilError error string should have "runtime error: " prefix

[raghvenders]

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

$ go version
1.21.3

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
Windows, amd64, 1.21.3

What did you do?

package main

import (
	"fmt"
	"runtime"
)

type I interface {
	M()
}

func main() {

	defer func() {
		if r := recover(); r != nil {
			if e, ok := r.(runtime.Error); ok {
				fmt.Println(e.Error())
			}
		}
	}()

	defer func() {
		if r := recover(); r != nil {
			if e, ok := r.(runtime.Error); ok {
				fmt.Println(e.Error())
				panic(nil)
			}
		}
	}()

	defer func() {
		if r := recover(); r != nil {
			if e, ok := r.(runtime.Error); ok {
				fmt.Println(e.Error())
				a := 1
				b := 0

				fmt.Println(a / b)
			}
		}
	}()

	var i I
	i.M()

	//panic(nil)

	//fmt.Println("hello")

}

What did you expect to see?

Since Panic error is a different runtime error as mentioned in the documentation - https://pkg.go.dev/builtin#panic and it has implementation of method - RuntimeError
// RuntimeError is a no-op function but // serves to distinguish types that are run time // errors from ordinary errors: a type is a // run time error if it has a RuntimeError method. RuntimeError()

I would like to see the error message like other runtime errors - "runtime error:" as mentioned in the above example

What did you see instead?

I rather the see the error message as - panic called with nil argument

[mauri870]

This is also confirmed by the Go specification:

The return value of recover is nil when the goroutine is not panicking or recover was not called directly by a deferred function. Conversely, if a goroutine is panicking and recover was called directly by a deferred function, the return value of recover is guaranteed not to be nil. To ensure this, calling panic with a nil interface value (or an untyped nil) causes a run-time panic.

This behavior was introduced in CL 461956

[mauri870]

@gopherbot please open a backport issue for Go 1.21.

Errors that implement runtime.Error should have a "runtime error: " prefix, with the solo exception of runtime.plainError on purpose. Calling panic(nil) results in a PanicNilError that violates this constraint.

[gopherbot]

Backport issue(s) opened: #63815 (for 1.21).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

[mauri870]

Looks like runtime.TypeAssertionError is also a runtime.Error that does not follow this convention.

[gopherbot]

Change https://go.dev/cl/538496 mentions this issue: runtime: add missing runtime error prefix to PanicNilError

[randall77]

I think it is past time to update runtime.TypeAssertionError. That's error has been around a long time and is not worth fixing.
(We grandfathered a bunch of errors without the runtime error: prefix using plainError back as part of #14965. This one wasn't so marked, and maybe we can do that, but it would not be a user-visible change I think).

@rsc Do we want to fix PanicNilError to add a runtime error:?