r199490 - Permit redeclaration of tags introduced by using decls
Alp Toker
alp at nuanti.com
Fri Jan 17 17:09:55 PST 2014
On 18/01/2014 00:56, Richard Smith wrote:
> On Fri Jan 17 2014 at 4:41:22 PM, Alp Toker <alp at nuanti.com
> <mailto: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.
Thanks for the indication, I've restricted the new behaviour to MSVCMode
in r199531 for now.
Will shortly also post the equivalent fix for PR13786.
Alp.
>
> 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>
> > <mailto: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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