func Tag(tag string, expectedValue interface{}) TestDeep

Tag is a smuggler operator. It only allows to name expectedValue, which can be an operator or a value. The data is then compared against expectedValue as if Tag was never called. It is only useful as JSON operator parameter, to name placeholders. See JSON operator for more details.

td.Cmp(t, gotValue,
  td.JSON(`{"fullname": $name, "age": $age, "gender": $gender}`,
    td.Tag("name", td.HasPrefix("Foo")), // matches $name
    td.Tag("age", td.Between(41, 43)),   // matches $age
    td.Tag("gender", "male")))           // matches $gender

TypeBehind method is delegated to expectedValue one if expectedValue is a TestDeep operator, otherwise it returns the type of expectedValue (or nil if it is originally untyped nil).

See also Tag godoc.