Custom String() method of maps got invalid receiver value

[chenzhuoyu]

Repro:

package main

import (
	"fmt"

	"github.com/davecgh/go-spew/spew"
)

type X map[int]int

func (x X) String() string {
	return fmt.Sprintf("{M1:%d}", len(x))
}

func main() {
	spew.Dump(X{1: 2})
}

Output:

(main.X) (len=1) (PANIC=runtime error: invalid memory address or nil pointer dereference){
 (int) 1: (int) 2
}

Panic occures when taking the len() of the receiver x, which is an invalid pointer.

This happens on all versions of go I installed (1.16 to 1.18).

[zavla]

A workaround is:
func main() {
spew.Dump(&X{1: 2})
}

[chenzhuoyu]

A workaround is: func main() { spew.Dump(&X{1: 2}) }

Not really. Sometimes the map is not directly accessable, and maybe nested in a deep structure.

[dwlnetnl]

I've fixed this issue in my fork of the package: spewerspew/spew@62a6506

[dwlnetnl]

Dupe of #115