Thread overview
[DIP1000] Something I don't quite understand regarding 'scope'
Jun 29
ag0aep6g
June 29
void local(Args...)(Args args)
{
}

void main() @safe
{
    import std.stdio;
    scope int* p;
    local(p);   // Ok
    writeln(p); // Error: scope variable p assigned to non-scope parameter _param_0 calling std.stdio.writeln!(int*).writeln
}

The signatures of `std.stdio.writeln` and `local` are the same (see `writeln` [1]). Yet, with '$ dmd -preview=dip1000' the call to `local` compiles, while the call to `writeln` doesn't.

Is that an instance of [2]?

[1] https://github.com/dlang/phobos/blob/v2.092.1/std/stdio.d#L3865
[2] https://issues.dlang.org/show_bug.cgi?id=20023
June 29
On 29.06.20 02:28, Stanislav Blinov wrote:
> void local(Args...)(Args args)
> {
> }
> 
> void main() @safe
> {
>      import std.stdio;
>      scope int* p;
>      local(p);   // Ok
>      writeln(p); // Error: scope variable p assigned to non-scope parameter _param_0 calling std.stdio.writeln!(int*).writeln
> }
> 
> The signatures of `std.stdio.writeln` and `local` are the same (see `writeln` [1]). Yet, with '$ dmd -preview=dip1000' the call to `local` compiles, while the call to `writeln` doesn't.

Since `local` and `writeln` are templates, the attributes for their parameters are inferred from their bodies. `local!(int*)` doesn't do anything with the parameter, so it's inferred as `scope`. `writeln!(int*)` apparently does something that prevents `scope` from being inferred.
June 29
On Monday, 29 June 2020 at 06:21:43 UTC, ag0aep6g wrote:

> Since `local` and `writeln` are templates, the attributes for their parameters are inferred from their bodies. `local!(int*)` doesn't do anything with the parameter, so it's inferred as `scope`. `writeln!(int*)` apparently does something that prevents `scope` from being inferred.

Thanks. It would appear indeed that inference fails for some reason. Explicitly marking args as 'scope' for `writeln` and `File.write` lets them compile. I wonder if there's a way to find exactly what it is in either of those that prevents the compiler from inferring 'scope'.