Re: [PATCH] c++: wrong ambiguity in accessing static field [PR112744]

[Patrick Palka <ppalka@redhat.com>]

On Wed, 29 Nov 2023, Marek Polacek wrote:

> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> Now that I'm posting this patch, I think you'll probably want me to use
> ba_any unconditionally.  That works too; g++.dg/tc1/dr52.C just needs
> a trivial testsuite tweak:
>   'C' is not an accessible base of 'X'
> v.
>   'C' is an inaccessible base of 'X'
> We should probably unify those messages...
> 
> -- >8 --
> Given
> 
>   struct A { constexpr static int a = 0; };
>   struct B : A {};
>   struct C : A {};
>   struct D : B, C {};
> 
> we give the "'A' is an ambiguous base of 'D'" error for
> 
>   D{}.A::a;
> 
> which seems wrong: 'a' is a static data member so there is only one copy
> so it can be unambiguously referred to even if there are multiple A
> objects.  clang++/MSVC/icx agree.
> 
> 	PR c++/112744
> 
> gcc/cp/ChangeLog:
> 
> 	* typeck.cc (finish_class_member_access_expr): When accessing
> 	a static data member, use ba_any for lookup_base.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/lookup/scoped11.C: New test.
> 	* g++.dg/lookup/scoped12.C: New test.
> 	* g++.dg/lookup/scoped13.C: New test.
> ---
>  gcc/cp/typeck.cc                       | 21 ++++++++++++++++++---
>  gcc/testsuite/g++.dg/lookup/scoped11.C | 14 ++++++++++++++
>  gcc/testsuite/g++.dg/lookup/scoped12.C | 14 ++++++++++++++
>  gcc/testsuite/g++.dg/lookup/scoped13.C | 14 ++++++++++++++
>  4 files changed, 60 insertions(+), 3 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/lookup/scoped11.C
>  create mode 100644 gcc/testsuite/g++.dg/lookup/scoped12.C
>  create mode 100644 gcc/testsuite/g++.dg/lookup/scoped13.C
> 
> diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
> index e995fb6ddd7..c4de8bb2616 100644
> --- a/gcc/cp/typeck.cc
> +++ b/gcc/cp/typeck.cc
> @@ -3476,7 +3476,7 @@ finish_class_member_access_expr (cp_expr object, tree name, bool template_p,
>  			   name, scope);
>  		  return error_mark_node;
>  		}
> -	      
> +
>  	      if (TREE_SIDE_EFFECTS (object))
>  		val = build2 (COMPOUND_EXPR, TREE_TYPE (val), object, val);
>  	      return val;
> @@ -3493,9 +3493,24 @@ finish_class_member_access_expr (cp_expr object, tree name, bool template_p,
>  	      return error_mark_node;
>  	    }
>  
> +	  /* NAME may refer to a static data member, in which case there is
> +	     one copy of the data member that is shared by all the objects of
> +	     the class.  So NAME can be unambiguously referred to even if
> +	     there are multiple indirect base classes containing NAME.  */
> +	  const base_access ba = [scope, name] ()
> +	    {
> +	      if (identifier_p (name))
> +		{
> +		  tree m = lookup_member (scope, name, /*protect=*/0,
> +					  /*want_type=*/false, tf_none);
> +		  if (!m || VAR_P (m))
> +		    return ba_any;

I wonder if we want to return ba_check_bit instead of ba_any so that we
still check access of the selected base?

  struct A { constexpr static int a = 0; };
  struct D : private A {};

  void f() {
    D{}.A::a; // #1 GCC (and Clang) currently rejects
  }

  template<class T>
  void g() {
    D{}.T::a; // #2 GCC currently rejects, Clang accepts?!
  }

  template void g<A>();

> +		}
> +	      return ba_check;
> +	    } ();
> +
>  	  /* Find the base of OBJECT_TYPE corresponding to SCOPE.  */
> -	  access_path = lookup_base (object_type, scope, ba_check,
> -				     NULL, complain);
> +	  access_path = lookup_base (object_type, scope, ba, NULL, complain);
>  	  if (access_path == error_mark_node)
>  	    return error_mark_node;
>  	  if (!access_path)
> diff --git a/gcc/testsuite/g++.dg/lookup/scoped11.C b/gcc/testsuite/g++.dg/lookup/scoped11.C
> new file mode 100644
> index 00000000000..be743522fce
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/lookup/scoped11.C
> @@ -0,0 +1,14 @@
> +// PR c++/112744
> +// { dg-do compile }
> +
> +struct A { const static int a = 0; };
> +struct B : A {};
> +struct C : A {};
> +struct D : B, C {};
> +
> +int main()
> +{
> +  D d;
> +  (void) d.a;
> +  (void) d.A::a;
> +}
> diff --git a/gcc/testsuite/g++.dg/lookup/scoped12.C b/gcc/testsuite/g++.dg/lookup/scoped12.C
> new file mode 100644
> index 00000000000..ffa145598fd
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/lookup/scoped12.C
> @@ -0,0 +1,14 @@
> +// PR c++/112744
> +// { dg-do compile }
> +
> +class A { const static int a = 0; };
> +struct B : A {};
> +struct C : A {};
> +struct D : B, C {};
> +
> +int main()
> +{
> +  D d;
> +  (void) d.a;	  // { dg-error "private" }
> +  (void) d.A::a;  // { dg-error "private" }
> +}
> diff --git a/gcc/testsuite/g++.dg/lookup/scoped13.C b/gcc/testsuite/g++.dg/lookup/scoped13.C
> new file mode 100644
> index 00000000000..970e1aa833e
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/lookup/scoped13.C
> @@ -0,0 +1,14 @@
> +// PR c++/112744
> +// { dg-do compile }
> +
> +struct A { const static int a = 0; };
> +struct B : A {};
> +struct C : A {};
> +struct D : B, C {};
> +
> +int main()
> +{
> +  D d;
> +  (void) d.x;	  // { dg-error ".struct D. has no member named .x." }
> +  (void) d.A::x;  // { dg-error ".struct A. has no member named .x." }
> +}
> 
> base-commit: 3d104d93a7011146b0870ab160613147adb8d9b3
> -- 
> 2.42.0
> 
>