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digitalmars.D.bugs - [Issue 6969] New: Forward reference on template class triangle

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=6969

           Summary: Forward reference on template class triangle
           Product: D
           Version: D2
          Platform: Other
        OS/Version: Windows
            Status: NEW
          Keywords: rejects-valid
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: cbkbbejeap mailinator.com
            Blocks: 340


--- Comment #0 from Nick Sabalausky <cbkbbejeap mailinator.com> 2011-11-17
18:20:08 PST ---
class A()
{
    alias C!() C1;
}
class B
{
    alias A!() A1;
}
class C() : B
{
}


dmd -c text.d


test.d(9): Error: class test.C!().C has forward references
test.d(7): Error: template instance test.A!() error instantiating

Problem cannot be worked around by rearranging order of declaration. All 6
possible orders exhibit the "has forward references" error.

Might be related to #3834

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Nov 17 2011
next sibling parent d-bugmail puremagic.com writes:
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Nick Sabalausky <cbkbbejeap mailinator.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |critical


--- Comment #1 from Nick Sabalausky <cbkbbejeap mailinator.com> 2011-11-21
13:50:16 PST ---
Severity -> critical

This is blocking a major refactoring in my project.

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--- Comment #2 from Nick Sabalausky <cbkbbejeap mailinator.com> 2011-12-06
16:57:34 PST ---
I'm not very familiar with DMD's internals wrt semantics, but from what I can
tell so far, the problem may have to do with a certain section near the end of
TemplateInstance::semantic(Scope *sc, Expressions *fargs):

...
    /* The problem is when to parse the initializer for a variable.
     * Perhaps VarDeclaration::semantic() should do it like it does
     * for initializers inside a function.
     */
//    if (sc->parent->isFuncDeclaration())

        /* BUG 782: this has problems if the classes this depends on
         * are forward referenced. Find a way to defer semantic()
         * on this template.
         */
        semantic2(sc2);
...

That call to semantic2 is the call stack when the "has forward references"
error is thrown. Note that this occurs before tryMain() reaches semantic2.

Do to my inexperience with DMD's internals, I have no idea if there's some
reason that call to semantic2 is supposed to be there. And I don't understand
what those comments are trying to say. But if I comment out that semantic2
call, the test case passes. However, I don't know whether that breaks anything
else.

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--- Comment #3 from Nick Sabalausky <cbkbbejeap mailinator.com> 2011-12-06
16:59:05 PST ---
*Ahem*:

"That call to semantic2 is *IN* the call stack..."

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Nick Sabalausky <cbkbbejeap mailinator.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|critical                    |blocker


--- Comment #4 from Nick Sabalausky <cbkbbejeap mailinator.com> 2012-04-23
17:58:53 PDT ---
Commenting out that call to semantic2 no longer fixes the problem in latest DMD
in Git (and Phobos fails to rebuild with the change). So I've got no clue. But
this is still blocking me from some major refactoring I need to do.

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Apr 23 2012
prev sibling next sibling parent d-bugmail puremagic.com writes:
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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bugzilla digitalmars.com


--- Comment #5 from Walter Bright <bugzilla digitalmars.com> 2012-04-28
02:04:29 PDT ---
It's not really a forward reference, it's a circular one. B is defined in terms
of itself. I'm not sure how this could ever work.

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--- Comment #6 from Nick Sabalausky <cbkbbejeap mailinator.com> 2012-04-29
21:23:36 PDT ---
This works:

class A
{
    alias C C1;
}
class B
{
    alias A A1;
}
class C : B
{
}

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Nick Sabalausky <cbkbbejeap mailinator.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|blocker                     |critical


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Rob T <alanb ucora.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |alanb ucora.com


--- Comment #7 from Rob T <alanb ucora.com> 2012-11-10 00:39:04 PST ---
(In reply to comment #6)

 This works:
 
 class A
 {
     alias C C1;
 }
 class B
 {
     alias A A1;
 }
 class C : B
 {
 }


It works because there are no actual circular references, and that's because
the class alias are in reality pointers, and pointers are known data types
irrespective of what they may represent (the definition expansion should stop
at the pointer). The compiler is certainly able to determine what the pointers
represent for dereferencing purposes as the class structures and contents are
100% knowable.

Note that if you convert the pointers to represent structs, ie change class to
struct, and fix up struct C to contain struct B due to a lack of inheritance,
it will no longer work, which is to be expected. To make the struct version
work again, you simply change the contained B to a pointer.

// this fails, and it should fail, so we're good.
struct A()
{
    C!() C1;
}
struct B
{
    A!() A1;
}
struct C() 
{
    B s;
}

// this works, and it should work, so we're good.
struct A()
{
    C!() C1;
}
struct B
{
    A!() A1;
}
struct C() 
{
    B* s; // changed to pointer
}

The problem at hand is definitely a bug from what I see.

--rt

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--- Comment #8 from Rob T <alanb ucora.com> 2012-11-10 01:34:57 PST ---
(In reply to comment #7)

 (In reply to comment #6)


I get weird behavior if I put in only the alias definitions.

This stuct version compiles, and it probably should because sA and sB are empty
with no circular referencing.

struct sA()
{
    alias sC!() sC1;
}
struct sB
{
    alias  sA!() sA1;
}
struct sC() 
{
    sB s;
}

But adding sA1 into sB will not compile, but I think it should compile because
sA is empty and there's no circular reference back to sC.

struct sA()
{
    alias sC!() sC1;
}
struct sB
{
    alias  sA!() sA1;
    sA1 a;
}
struct sC() 
{
    sB b;
}

In fact it does compile in the non-template version as expected. 

struct sA
{
    alias sC sC1;
}
struct sB
{
    alias  sA sA1;
    sA1 a;
}
struct sC() 
{
    sB b;
}

Finally, closing the loop will not compile as expected

struct sA
{
    alias sC sC1;
    sC1 c; // bang, we're dead.
}
struct sB
{
    alias  sA sA1;
    sA1 a;
}
struct sC() 
{
    sB b;
}

Templates and non-templates are being evaluated inconsistently, so there's
definitely a bug in there somewhere.

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |pull


--- Comment #9 from Kenji Hara <k.hara.pg gmail.com> 2012-11-13 04:19:05 PST ---
https://github.com/D-Programming-Language/dmd/pull/1280

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--- Comment #10 from github-bugzilla puremagic.com 2012-11-13 12:27:37 PST ---
Commits pushed to master at https://github.com/D-Programming-Language/dmd

https://github.com/D-Programming-Language/dmd/commit/75b0fafc74eda2fed816415e31dcbfde8283077d
fix Issue 6969 - Forward reference on template class triangle

https://github.com/D-Programming-Language/dmd/commit/c34c5fb0e67f7c48bb683ef409378fde6db33088
Merge pull request #1280 from 9rnsr/fix8990

Issue 6969 & 8990 - Forward reference error between three template
instantiations

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Walter Bright <bugzilla digitalmars.com> changed:

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


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