json.Unmarshaler interface not considered for nested types

[peterstace]

I believe I may have found a false positive. The following minimum reproduction shows the false positive:

package main

import (
    "encoding/json"
    "fmt"
)

type Outer struct {
    Inner Inner `json:"inner"`
}

type Inner struct {
    String string
}

func (i *Inner) UnmarshalJSON(p []byte) error {
    var s string
    if err := json.Unmarshal(p, &s); err != nil {
        return err
    }
    i.String = s
    return nil
}

func main() {
    buf := []byte(`
        {
            "inner": "my string"
        }
    `)
    var outer Outer
    if err := json.Unmarshal(buf, &outer); err != nil {
        panic(err)
    }
    fmt.Println(outer)
}

The output of the program is:

{{my string}}

See https://go.dev/play/p/HEcyBYmfO_Y

However, musttag with the latest tag (v0.12.1) shows a false positive:

main.go:32:32: the given struct  should be annotated with the `json` tag

It doesn't seem to ignore the absence of JSON tags on the Inner type, even though the Inner type implements the jsonUnmarshaler interface.

[tmzane]

Hi, thank you for the detailed report!

Looks like we only check if a type implements a (un)marshaler interface, but should also do it for a pointer-type.

I'll push the fix soon.