Strange error when compiling with dmd, not with ldc

[user1234]

On Wednesday, 29 November 2017 at 06:18:09 UTC, Fra Mecca wrote:
> I have this struct:
>
> immutable struct Configuration {
>     string title;
>     string baseurl;
>     string url;
>     string email;
>     string author;
>     string parser;
>     string target;
>     string urlFormat;
>     string urlFormatCmd;
>
>     short port;
>
>     string[] ignore;
>     string[] extensions;
>
>     @property string toString()
>     {
>         auto urlF = (urlFormatCmd ? "url_format_cmd: " ~ 
> urlFormatCmd : "") ~ "\n";
>         return
>         "title: "        ~ title ~ "\n" ~
>         "baseurl: "      ~ baseurl ~ "\n" ~
>         "url: "          ~ url ~ "\n" ~
>         "email: "        ~ email ~ "\n" ~
>         "author: "       ~ author ~ "\n" ~
>         "parser: "       ~ parser ~ "\n" ~
>         "target: "       ~ target ~ "\n" ~
>         "url_format: "   ~ urlFormat ~ "\n" ~
>         "ignore: "       ~ to!string(ignore)[1 .. $ - 1] ~ "\n" 
> ~
>         "extensions: "   ~ to!string(extensions)[1 .. $ - 1] ~ 
> "\n" ~
>         urlF;
>     }
> }
>
> and this function:
>
> void show_config()
> {
>     writef("%s", parse_config(
>                 exists("config.sdl") ? "config.sdl" : 
> "").toString);
> }
>
>
> Whenever I compile with ldc2 I get no errors, while with dmd I 
> get:
>
> source/configuration.d(105,27): Error: immutable method 
> configuration.Configuration.toString is not callable using a 
> mutable object
>
>
> What is the problem?

You must also use a type constructor later, when a Configuration 
is declared:

```
immutable(Configuration) config;
config.toString.writeln; // okay this time
```

What happens is that all the member functions have the 
`immutable` attribute, but the instance you declared was not 
itself `immutable`.

actually this:

```
immutable struct Configuration {
     @property string toString(){return "";}
}
```

is like:

```
struct Configuration {
     @property string toString() immutable {return "";}
}
```

I would personally prefer the second form. Why ? Because the 
variable members will be set immutable anyway when an instance is 
declared.