What does dual-context mean?

[wjoe via Digitalmars-d-learn]

Hello,

what's a dual context as in the deprecation message?
```d
struct MockFile {
  [...]

  void writef(alias fmt, A...)(A args) { // Deprecation: function 
'writef': function requires a dual-context, which is deprecated
    import std.format: format;
    write(format!fmt(args));
  }

  [...]
}

unittest {
  auto f = MockFile();
  immutable fmt = "%d";
  f.writef!fmt(0); // instatiated from here
}
```