reflect: Value.Elem() inconsistency betwen go1.4.3 and go.15

[odeke-em]

In some code whose struct

type pushCmd struct {
    Hidden         *bool   `json:"hidden"`
    Source         *string `json:"source"`
    Convert        *bool   `json:"convert"`
    Ocr            *bool   `json:"ocr"`
    IgnoreChecksum *bool   `json:"ignore-checksum"`
}

I perform some value extraction by json tag and fill in a fresh pushCmd value with a mix of already defined values from a resource configuration and those already entered in the from value. However, running the exact same program http://play.golang.org/p/4NRzI-ejuc on go1.4.3 and then on go.1.5 gives different results

The code example and results are at https://github.com/odeke-em/bugs/tree/master/golang/reflect-inconsistency

[nodirt]

Distilled, fmt.Println(reflect.ValueOf(true)) changed. In Go 1.4 this prints <bool Value>. In Go 1.5, this prints true.

This doesn't violate Go 1 compatibility guidelines, which says The Go specification tries to be explicit about most properties of the language, but there are some aspects that are undefined. Programs that depend on such unspecified behavior may break in future releases.

In Go 1.4.3 fmt didn't specify how it formats reflect.Value. It have been specified only in Go 1.5

Note that reflect.ValueOf(true).String() value didn't change.

[nodirt]

Actually, I admit it was a breaking change because fmt specification essentially changed: the rule

If an operand implements method String() string, that method
will be invoked to convert the object to a string, which will then
be formatted as required by the verb (if any).

applied to reflect.Value in the past. I still don't think anything will be done about this.

[ianlancetaylor]

The change is documented in https://golang.org/go1.5.html. I don't consider it to be a Go 1 compatibility breaking change: if we changed reflect.Value.String that wouldn't be a breaking change either. Breaking user types would be a breaking change. The handling of library defined types is up to the library.

[odeke-em]

Alright, thank you folks for taking a look at this. My way around it was to invoke ".Interface()"

-   fmt.Sprintf("%v", elem)
+   fmt.Sprintf("%v", elem.Interface())

and that produces the desired result between go 1.5 and 1.4.3.