[Issue 20874] New: std.json.assign requires '@safe'

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20874

          Issue ID: 20874
           Summary: std.json.assign requires '@safe'
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: safe
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: pro.mathias.lang at gmail.com

```
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

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