r199490 - Permit redeclaration of tags introduced by using decls

[Richard Smith]

On Fri Jan 17 2014 at 4:41:22 PM, Alp Toker <alp at nuanti.com> wrote:

> I think we're OK here. 3.3.1/4 describes the rule as applying to a set
> of declarations "each of which specifies the same unqualified name" and
> we're guaranteed that using declarations will be qualified while the
> redeclarations will be unqualified (and the shadows are an
> implementation detail). So there's no conflict even in a pedantic reading..
>

7.3.3/13, and its example, clarify that using-declarations are considered
here. I've mailed the core reflector to get the wording in 3.3.1/4
clarified.


> The "namespace surrounding the declaration" rule also doesn't appear to
> apply here given that one of the purposes of using declarations to work
> around strict lexical scope. Perhaps shades of PR17337 here but I'm
> tending to side with the mainstream again.
>
> Like you say the the standard is unclear but with this reading we get
> compatibility with g++ and MSVC and it does "feel right".
>

EDG, g++, and MSVC all have different behavior here (different from each
other and different from us).


> Awaiting that clarification you requested from the core reflector -- if
> anything it's an interesting discussion.
>

Discussion on core so far suggests that an unqualified name *always*
declares a name in the lexical context in which it appears, meaning our
former behavior on this example was more appropriate than our new behavior.

Alp.
>
>
> On 17/01/2014 20:59, Richard Smith wrote:
> > Hi Alp,
> >
> > This change doesn't look correct.
> >
> > On Fri Jan 17 2014 at 5:04:31 AM, Alp Toker <alp at nuanti.com
> > <mailto:alp at nuanti.com>> wrote:
> >
> >     Author: alp
> >     Date: Fri Jan 17 06:57:21 2014
> >     New Revision: 199490
> >
> >     URL: http://llvm.org/viewvc/llvm-project?rev=199490&view=rev
> >     Log:
> >     Permit redeclaration of tags introduced by using decls
> >
> >     This valid construct appears in MSVC headers where it's used to
> >     provide a
> >     definition for the '::type_info' compiler builtin type.
> >
> >     Modified:
> >         cfe/trunk/lib/Sema/SemaDecl.cpp
> >         cfe/trunk/test/SemaCXX/using-decl-1.cpp
> >
> >     Modified: cfe/trunk/lib/Sema/SemaDecl.cpp
> >     URL:
> >     http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/
> SemaDecl.cpp?rev=199490&r1=199489&r2=199490&view=diff
> >     ============================================================
> ==================
> >     --- cfe/trunk/lib/Sema/SemaDecl.cpp (original)
> >     +++ cfe/trunk/lib/Sema/SemaDecl.cpp Fri Jan 17 06:57:21 2014
> >     @@ -10681,7 +10681,8 @@ Decl *Sema::ActOnTag(Scope *S, unsigned
> >        }
> >
> >        if (!Previous.empty()) {
> >     -    NamedDecl *PrevDecl = (*Previous.begin())->getUnderlyingDecl();
> >     +    NamedDecl *DirectPrevDecl = *Previous.begin();
> >     +    NamedDecl *PrevDecl = DirectPrevDecl->getUnderlyingDecl();
> >
> >          // It's okay to have a tag decl in the same scope as a typedef
> >          // which hides a tag decl in the same scope.  Finding this
> >     @@ -10713,7 +10714,7 @@ Decl *Sema::ActOnTag(Scope *S, unsigned
> >            // in the same scope (so that the definition/declaration
> >     completes or
> >            // rementions the tag), reuse the decl.
> >            if (TUK == TUK_Reference || TUK == TUK_Friend ||
> >     -          isDeclInScope(PrevDecl, SearchDC, S,
> >     +          isDeclInScope(DirectPrevDecl, SearchDC, S,
> >                              SS.isNotEmpty() ||
> >     isExplicitSpecialization)) {
> >              // Make sure that this wasn't declared as an enum and now
> >     used as a
> >              // struct or something similar.
> >
> >     Modified: cfe/trunk/test/SemaCXX/using-decl-1.cpp
> >     URL:
> >     http://llvm.org/viewvc/llvm-project/cfe/trunk/test/
> SemaCXX/using-decl-1.cpp?rev=199490&r1=199489&r2=199490&view=diff
> >     ============================================================
> ==================
> >     --- cfe/trunk/test/SemaCXX/using-decl-1.cpp (original)
> >     +++ cfe/trunk/test/SemaCXX/using-decl-1.cpp Fri Jan 17 06:57:21 2014
> >     @@ -119,6 +119,27 @@ namespace foo
> >        };
> >      }
> >
> >     +namespace using_tag_redeclaration
> >     +{
> >     +  struct S;
> >     +  namespace N {
> >     +    using ::using_tag_redeclaration::S;
> >     +    struct S {}; // expected-note {{previous definition is here}}
> >
> >
> > This is ill-formed, by 3.3.1/4. It appears that your change makes the
> > second declaration into a redeclaration of
> > ::using_tag_redeclaration::S; that's incorrect. (Our previous handling
> > of this case was also incorrect, albeit in a different way, since we
> > didn't enforce the 3.3.1/4 rule here.)
> >
> > If we need to handle something of this form for MSVC compatibility,
> > we'll need to put it behind MSVCCompat.
> >
> >     +  }
> >     +  void f() {
> >     +    N::S s1;
> >
> >     +    S s2;
> >     +  }
> >     +  void g() {
> >     +    struct S; // expected-note {{forward declaration of 'S'}}
> >     +    S s3; // expected-error {{variable has incomplete type 'S'}}
> >     +  }
> >     +  void h() {
> >     +    using ::using_tag_redeclaration::S;
> >     +    struct S {}; // expected-error {{redefinition of 'S'}}
> >
> >     +  }
> >     +}
> >     +
> >      // Don't suggest non-typenames for positions requiring typenames.
> >      namespace using_suggestion_tyname_val {
> >      namespace N { void FFF() {} }
> >
> >
> >     _______________________________________________
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> >     cfe-commits at cs.uiuc.edu <mailto:cfe-commits at cs.uiuc.edu>
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> >
>
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