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

$ go version
latest (playground)

Does this issue reproduce with the latest release?

yes

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

$ go env
playground

What did you do?

Unmarshal json (https://play.golang.org/p/Ka8kdddUHKu):

s := struct{
	Val int `json:",string,omitempty"`
}{}

println(json.Unmarshal([]byte(`{"val":""}`), &s))

What did you expect to see?

Since val is "", the error as such is expected, however I would expect the omitempty to make a difference and hence return nil error. According to https://golang.org/pkg/encoding/json/:

The "omitempty" option specifies that the field should be omitted from the encoding if the field has an empty value, defined as false, 0, a nil pointer, a nil interface value, and any empty array, slice, map, or string.

What did you see instead?

Error: invalid use of ,string struct tag, trying to unmarshal "" into int

Reasoning

To give flexibility in handling external json input it is preferable to handle the omitempty tag before handling further validations like string. In this case the order seems to be different.

If effect, ,string,omitempty should allow "" to be unmarshaled into numeric zero values.

UPDATE I realize this might indeed be more a proposal as omitempty seems to be documented and take effect only for marshaling but not unmarshaling.