r199490 - Permit redeclaration of tags introduced by using decls

[Richard Smith]

Hi Alp,

This change doesn't look correct.

On Fri Jan 17 2014 at 5:04:31 AM, Alp Toker <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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