[scope] Fwd reference error with nested struct

[dlangBugzillaToGithub]

Radu Racariu (@rracariu) reported this on 2017-02-17T07:21:16Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=17194

CC List

• Walter Bright (@WalterBright)
• Elie Morisse

Description

The following:

struct V
{
    W w;
    
    struct W
    {
        this(scope ref V v)
        {
            this.v = &v;
        }
        V* v;
    }
}

void main()
{
	V v;
}

Produces this:

Error: struct app.V no size because of forward reference

Expecting: to compile.

Error is also not pointing the line number the issue was suppose to be.

Work around:

1. Removing the "scope" keyword makes the program compile.

2. Changing W ctor signature to: 

this(scope V* v)
{
 this.v = v;
}

also makes it work.

Compiled with dmd 2.073.1 on win32

[dlangBugzillaToGithub]

syniurge commented on 2017-07-14T21:30:12Z

It's not specific to nested classes:

struct S {
    S2 a;
}

struct S2 {
    void foo(scope S s) { }
}

=> Error: struct S no size because of forward reference

Kinda related: issue 17548 was another bogus forward ref error originating from the same TypeStruct.hasPointers call in TypeFuntion.semantic (but as I understand it that call isn't the actual problem).

[dlangBugzillaToGithub]

dlang-bugzilla (@CyberShadow) commented on 2017-07-15T06:12:04Z

Elle's example stopped working after https://github.com/dlang/dmd/pull/5897 but I don't know enough about scope whether know if this can be marked as a regression.

[dlangBugzillaToGithub]

bugzilla (@WalterBright) commented on 2017-08-31T06:33:35Z

This is a bit of a hopeless circular tangle. 'scope' is ignored if the type has no pointers. So S is checked for pointers. S.a is of type S2, which then must be checked for pointers. Checking S2 for pointers means evaluating each member to see if it is a field, which gets us back to looking at the 'scope'.

Not sure if this is reasonably fixable.

[dlangBugzillaToGithub]

radu.racariu (@rracariu) commented on 2017-08-31T08:17:21Z

I think the original description is different than Elie's example.

For one, the argument is sent by ref - the issue I was describing is related to the interaction of 'scope' and 'ref'.

I expected that 'scope ref V' be the same as 'scope V*'

[dlangBugzillaToGithub]

radu.racariu (@rracariu) commented on 2017-11-06T15:49:53Z

Just to point that "scope" is to blame here, removing "scope" from the inner stuct ctor like:

+++++++++++++++++++
struct V
{
    W w;
    
    struct W
    {
        this(/*scope*/ ref V v)
        {
            this.v = &v;
        }
        V* v;
    }
}

void main()
{
	V v;
}
+++++++++++++++++++

compiles without errors.