http://d.puremagic.com/issues/show_bug.cgi?id=10138

           Summary: Regression (2.063 git-head): Using ReturnType with
                    scoped no longer works
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: regression
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: andrej.mitrovich@gmail.com


--- Comment #0 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-05-22 19:38:35 PDT ---
-----
import std.traits;
import std.typecons;

class A
{
}

class B
{
    ReturnType!(scoped!A) a;  // L12

    this()
    {
        a = scoped!A();
    }
}

void main()
{
}
-----

2.062: ok
2.063 git-head:

test.d(12): Error: template instance ReturnType!(scoped) does not match
template declaration ReturnType(func...) if (func.length == 1 &&
isCallable!(func))

This is a breaking change. The workaround is to use 'typeof(scoped!A())'. This
workaround was not documented either.

Note that we should consider moving the inner Voldemort type to be a
module-scoped templated struct (e.g. 'Scoped'), so the API is easier to use if
you use scoped!() for member variables, e.g.:

-----
class B
{
    Scoped!A a;  // better API

    this()
    {
        a = scoped!A();
    }
}
-----

This is all based on the assumption that using scoped for member variables is something that is supported. There are currently no unittests that try to declare scoped variables, hence why this should either become an official feature or it should be properly documented as disallowed.

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--- Comment #1 from Kenji Hara <k.hara.pg@gmail.com> 2013-05-22 20:41:14 PDT ---
That is changed by: https://github.com/D-Programming-Language/phobos/pull/1215

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Kenji Hara <k.hara.pg@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |INVALID


--- Comment #2 from Kenji Hara <k.hara.pg@gmail.com> 2013-05-27 06:13:54 PDT ---
The code had a hidden bug with 2.062 dmd and earlier.

If class A has two ctors - one is default, another has arguments, and initialize scoped value in B's ctor by calling argument version ctor, it had caused errors.

import std.traits;
import std.typecons;

class A
{
    this() {}
    this(int) {}
}

class B
{
    ReturnType!(scoped!A) a;  // L12

    this()
    {
        a = scoped!A(1);    // L16
    }
}

void main()
{
}

Errors with 2.062:
test.d(16): Error: function
std.typecons.scoped!(A).scoped.Scoped!(A).Scoped.opAssign (Scoped!(A) p) is not
callable using argument types (Scoped!(A))
test.d(16): Error: cannot implicitly convert expression (scoped(1)) of type
Scoped!(A) to Scoped!(A)

In fact, the type you had extract by using ReturnType!(scoped!A) had not been correct scoped value type. The bug has fixed by phobos/pull/1215, therefore the OP code is invalid right now.

In conclusion, this is not a regression.

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--- Comment #3 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-05-27 06:16:05 PDT ---
(In reply to comment #2)
> therefore the OP code is invalid right now.
> In conclusion, this is not a regression.

Ok, but we should at least add a comment somewhere. I think I'll add it to the changelog perhaps..

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--- Comment #4 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-05-27 14:51:41 PDT ---
(In reply to comment #2)
> In fact, the type you had extract by using ReturnType!(scoped!A) had not been correct scoped value type. The bug has fixed by phobos/pull/1215, therefore the OP code is invalid right now.
> 
> In conclusion, this is not a regression.

Here's the thing, though. In that phobos pull, monarchdodra commented with:

"Finally, I gave Scoped an @disabled this() to prevent our smarter users from trying to build a Scoped using typeof."

It seems like he is actively trying to *disallow* using either ReturnType or typeof in order to create a scoped field variable.

But, typeof still works. In 2.063 your example code will become:

-----
class A
{
    this() {}
    this(int) {}
}

class B
{
    typeof(scoped!A()) a;

    this()
    {
        a = scoped!A();   // ok
        a = scoped!A(1);  // ok
    }
}
-----

So now I can't tell whether we *support* using scoped field variables or is the above just a hack? I need to know what to put in the changelog.

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--- Comment #5 from Kenji Hara <k.hara.pg@gmail.com> 2013-05-27 15:39:22 PDT ---
(In reply to comment #4)
> Here's the thing, though. In that phobos pull, monarchdodra commented with:
> 
> "Finally, I gave Scoped an @disabled this() to prevent our smarter users from trying to build a Scoped using typeof."
> 
> It seems like he is actively trying to *disallow* using either ReturnType or typeof in order to create a scoped field variable.
> 
> But, typeof still works. In 2.063 your example code will become:
[snip]

You are confusing about @disable for `Scoped` struct default construction and calling `scoped` function with zero argument.

If new A() is valid, scoped!A() is also valid. The latter is corresponding to
the former. It's the design of scoped utility.

> So now I can't tell whether we *support* using scoped field variables or is the above just a hack? I need to know what to put in the changelog.

The rejected case from 2.063 is this:

class A
{
    this() {}
    this(int) {}
}

class B
{
    typeof(scoped!A()) a;

    this()  // L12
    {
        //a = scoped!A();
        //a = scoped!A(1);
    }
}

Output:
test.d(12): Error: constructor test.B.this field a must be initialized in
constructor

The field B.a is must be initialized in constructor, comes from the @disable
this() defined in typeof(scoped!A()) struct.

However, getting the type of Scoped struct by using typeof is still allowed. So

typeof(scoped!A()) a;

is valid from 2.063.

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