```
import std.json;

struct MyCustomType
{
    public string toString () const @system { return null; }
    alias toString this;
}

void main () @system
{
    JSONValue json;
    MyCustomType ilovedlang;
    json = ilovedlang;
}
```

Results in:
```
/usr/local/opt/dmd/include/dlang/dmd/std/json.d(459): Error: @safe function std.json.JSONValue.assign!(MyCustomType).assign cannot call @system function foo.MyCustomType.toString
foo.d(5):        foo.MyCustomType.toString is declared here
/usr/local/opt/dmd/include/dlang/dmd/std/json.d(593): Error: template instance std.json.JSONValue.assign!(MyCustomType) error instantiating
foo.d(13):        instantiated from here: opAssign!(MyCustomType)
```

Since `assign` is a template, why is it annotated ?
From the discussion in: https://forum.dlang.org/post/tgpqkdghinutpumkouvo@forum.dlang.org