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

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

[Marek Polacek]

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;
+		}
+	      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

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

[Patrick Palka]

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
> 
> 

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

[Marek Polacek]

On Wed, Nov 29, 2023 at 12:23:46PM -0500, Patrick Palka wrote:
> 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?

That would certainly make sense to me.  I didn't do that because
I'd not seen ba_check_bit being used except as part of ba_check,
but that may not mean much.

So either I can tweak the lambda to return ba_check_bit rather
than ba_any or use ba_check_bit unconditionally.  Any opinions on that?

>   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>();

Thanks for looking at the patch and the testcase.  I'll add it.

> > +		}
> > +	      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
> > 
> > 
> 

Marek

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

[Jason Merrill]

On 11/29/23 12:43, Marek Polacek wrote:
> On Wed, Nov 29, 2023 at 12:23:46PM -0500, Patrick Palka wrote:
>> 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?
> 
> That would certainly make sense to me.  I didn't do that because
> I'd not seen ba_check_bit being used except as part of ba_check,
> but that may not mean much.
> 
> So either I can tweak the lambda to return ba_check_bit rather
> than ba_any or use ba_check_bit unconditionally.  Any opinions on that?

The relevant passage seems to be
https://eel.is/c++draft/class.access.base#6
after DR 52, which seems to have clarified that the pointer conversion 
only applies to non-static members.

>>    struct A { constexpr static int a = 0; };
>>    struct D : private A {};
>>
>>    void f() {
>>      D{}.A::a; // #1 GCC (and Clang) currently rejects
>>    }

I see that MSVC also rejects it, while EDG accepts.

https://eel.is/c++draft/class.access.base#5.1 seems to say that a is 
accessible when named in A.

https://eel.is/c++draft/expr.ref#7 also only constrains references to 
non-static members.

But first we need to look up A in D, and A's injected-class-name looked 
up as a member of D is not accessible; it's private, and f() is not a 
friend, and we correctly complain about that.

If we avoid the lookup of A in D with

D{}.::A::a;

clang accepts it, which is consistent with accepting the template 
version, and seems correct.

So, I think ba_any is what we want here.

Jason

[PATCH v2] c++: wrong ambiguity in accessing static field [PR112744]

[Marek Polacek]

On Wed, Nov 29, 2023 at 03:28:44PM -0500, Jason Merrill wrote:
> On 11/29/23 12:43, Marek Polacek wrote:
> > On Wed, Nov 29, 2023 at 12:23:46PM -0500, Patrick Palka wrote:
> > > 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?
> > 
> > That would certainly make sense to me.  I didn't do that because
> > I'd not seen ba_check_bit being used except as part of ba_check,
> > but that may not mean much.
> > 
> > So either I can tweak the lambda to return ba_check_bit rather
> > than ba_any or use ba_check_bit unconditionally.  Any opinions on that?
> 
> The relevant passage seems to be
> https://eel.is/c++draft/class.access.base#6
> after DR 52, which seems to have clarified that the pointer conversion only
> applies to non-static members.
> 
> > >    struct A { constexpr static int a = 0; };
> > >    struct D : private A {};
> > > 
> > >    void f() {
> > >      D{}.A::a; // #1 GCC (and Clang) currently rejects
> > >    }
> 
> I see that MSVC also rejects it, while EDG accepts.
> 
> https://eel.is/c++draft/class.access.base#5.1 seems to say that a is
> accessible when named in A.
> 
> https://eel.is/c++draft/expr.ref#7 also only constrains references to
> non-static members.
> 
> But first we need to look up A in D, and A's injected-class-name looked up
> as a member of D is not accessible; it's private, and f() is not a friend,
> and we correctly complain about that.
> 
> If we avoid the lookup of A in D with
> 
> D{}.::A::a;
> 
> clang accepts it, which is consistent with accepting the template version,
> and seems correct.
> 
> So, I think ba_any is what we want here.

Wow, that is not intuitive (to me at least).  So I had it right but
only by accident.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >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.

The rationale for using ba_any is explained at
<https://gcc.gnu.org/pipermail/gcc-patches/2023-November/638676.html>.

	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.
	* g++.dg/lookup/scoped14.C: New test.
	* g++.dg/lookup/scoped15.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 ++++++++++++++
 gcc/testsuite/g++.dg/lookup/scoped14.C | 14 ++++++++++++++
 gcc/testsuite/g++.dg/lookup/scoped15.C | 20 ++++++++++++++++++++
 6 files changed, 94 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
 create mode 100644 gcc/testsuite/g++.dg/lookup/scoped14.C
 create mode 100644 gcc/testsuite/g++.dg/lookup/scoped15.C

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 0839d0a4167..bf8ffaa7e75 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -3467,7 +3467,7 @@ 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;
@@ -3484,9 +3484,24 @@ 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 || shared_member_p (m))
+		    return ba_any;
+		}
+	      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." }
+}
diff --git a/gcc/testsuite/g++.dg/lookup/scoped14.C b/gcc/testsuite/g++.dg/lookup/scoped14.C
new file mode 100644
index 00000000000..141aa0d2b1a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/lookup/scoped14.C
@@ -0,0 +1,14 @@
+// PR c++/112744
+// { dg-do compile { target c++11 } }
+
+struct A { int a = 0; };
+struct B : A {};
+struct C : A {};
+struct D : B, C {};
+
+int main()
+{
+  D d;
+  (void) d.a;	  // { dg-error "request for member .a. is ambiguous" }
+  (void) d.A::a;  // { dg-error ".A. is an ambiguous base of .D." }
+}
diff --git a/gcc/testsuite/g++.dg/lookup/scoped15.C b/gcc/testsuite/g++.dg/lookup/scoped15.C
new file mode 100644
index 00000000000..d450a41a617
--- /dev/null
+++ b/gcc/testsuite/g++.dg/lookup/scoped15.C
@@ -0,0 +1,20 @@
+// PR c++/112744
+// { dg-do compile { target c++11 } }
+
+struct A { constexpr static int a = 0; };
+struct D : private A {};
+
+// See <https://gcc.gnu.org/pipermail/gcc-patches/2023-November/638676.html>
+// for rationale.
+
+void f() {
+  D{}.A::a; // { dg-error "inaccessible" }
+  D{}.::A::a;
+}
+
+template<class T>
+void g() {
+  D{}.T::a;
+}
+
+template void g<A>();

base-commit: 220fe41fd4085e91a49e62dd815628ec4883a4ea
-- 
2.42.0

Re: [PATCH v2] c++: wrong ambiguity in accessing static field [PR112744]

[Jason Merrill]

On 11/29/23 17:01, Marek Polacek wrote:
> On Wed, Nov 29, 2023 at 03:28:44PM -0500, Jason Merrill wrote:
>> On 11/29/23 12:43, Marek Polacek wrote:
>>> On Wed, Nov 29, 2023 at 12:23:46PM -0500, Patrick Palka wrote:
>>>> 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?
>>>
>>> That would certainly make sense to me.  I didn't do that because
>>> I'd not seen ba_check_bit being used except as part of ba_check,
>>> but that may not mean much.
>>>
>>> So either I can tweak the lambda to return ba_check_bit rather
>>> than ba_any or use ba_check_bit unconditionally.  Any opinions on that?
>>
>> The relevant passage seems to be
>> https://eel.is/c++draft/class.access.base#6
>> after DR 52, which seems to have clarified that the pointer conversion only
>> applies to non-static members.
>>
>>>>     struct A { constexpr static int a = 0; };
>>>>     struct D : private A {};
>>>>
>>>>     void f() {
>>>>       D{}.A::a; // #1 GCC (and Clang) currently rejects
>>>>     }
>>
>> I see that MSVC also rejects it, while EDG accepts.
>>
>> https://eel.is/c++draft/class.access.base#5.1 seems to say that a is
>> accessible when named in A.
>>
>> https://eel.is/c++draft/expr.ref#7 also only constrains references to
>> non-static members.
>>
>> But first we need to look up A in D, and A's injected-class-name looked up
>> as a member of D is not accessible; it's private, and f() is not a friend,
>> and we correctly complain about that.
>>
>> If we avoid the lookup of A in D with
>>
>> D{}.::A::a;
>>
>> clang accepts it, which is consistent with accepting the template version,
>> and seems correct.
>>
>> So, I think ba_any is what we want here.
> 
> Wow, that is not intuitive (to me at least).  So I had it right but
> only by accident.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> -- >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.
> 
> The rationale for using ba_any is explained at
> <https://gcc.gnu.org/pipermail/gcc-patches/2023-November/638676.html>.

I'd prefer not to cite the mailing list for rationales.

To summarize:
[class.access.base] requires conversion to a unique base subobject for 
non-static data members, but it does not require that the base be unique 
or accessible for static data members.

> 	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.
> 	* g++.dg/lookup/scoped14.C: New test.
> 	* g++.dg/lookup/scoped15.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 ++++++++++++++
>   gcc/testsuite/g++.dg/lookup/scoped14.C | 14 ++++++++++++++
>   gcc/testsuite/g++.dg/lookup/scoped15.C | 20 ++++++++++++++++++++
>   6 files changed, 94 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
>   create mode 100644 gcc/testsuite/g++.dg/lookup/scoped14.C
>   create mode 100644 gcc/testsuite/g++.dg/lookup/scoped15.C
> 
> diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
> index 0839d0a4167..bf8ffaa7e75 100644
> --- a/gcc/cp/typeck.cc
> +++ b/gcc/cp/typeck.cc
> @@ -3467,7 +3467,7 @@ 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;
> @@ -3484,9 +3484,24 @@ 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 || shared_member_p (m))
> +		    return ba_any;
> +		}
> +	      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." }
> +}
> diff --git a/gcc/testsuite/g++.dg/lookup/scoped14.C b/gcc/testsuite/g++.dg/lookup/scoped14.C
> new file mode 100644
> index 00000000000..141aa0d2b1a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/lookup/scoped14.C
> @@ -0,0 +1,14 @@
> +// PR c++/112744
> +// { dg-do compile { target c++11 } }
> +
> +struct A { int a = 0; };
> +struct B : A {};
> +struct C : A {};
> +struct D : B, C {};
> +
> +int main()
> +{
> +  D d;
> +  (void) d.a;	  // { dg-error "request for member .a. is ambiguous" }
> +  (void) d.A::a;  // { dg-error ".A. is an ambiguous base of .D." }
> +}
> diff --git a/gcc/testsuite/g++.dg/lookup/scoped15.C b/gcc/testsuite/g++.dg/lookup/scoped15.C
> new file mode 100644
> index 00000000000..d450a41a617
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/lookup/scoped15.C
> @@ -0,0 +1,20 @@
> +// PR c++/112744
> +// { dg-do compile { target c++11 } }
> +
> +struct A { constexpr static int a = 0; };
> +struct D : private A {};
> +
> +// See <https://gcc.gnu.org/pipermail/gcc-patches/2023-November/638676.html>
> +// for rationale.

The injected-class-name of A is private when named in D, but if A is 
named some other way, there is no requirement in [class.access.base] for 
static data members that it be an accessible base.

OK with those comment adjustments.

Jason

Re: [PATCH v2] c++: wrong ambiguity in accessing static field [PR112744]

[Marek Polacek]

On Thu, Nov 30, 2023 at 11:42:36AM -0500, Jason Merrill wrote:
> On 11/29/23 17:01, Marek Polacek wrote:
> > On Wed, Nov 29, 2023 at 03:28:44PM -0500, Jason Merrill wrote:
> > > On 11/29/23 12:43, Marek Polacek wrote:
> > > > On Wed, Nov 29, 2023 at 12:23:46PM -0500, Patrick Palka wrote:
> > > > > 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?
> > > > 
> > > > That would certainly make sense to me.  I didn't do that because
> > > > I'd not seen ba_check_bit being used except as part of ba_check,
> > > > but that may not mean much.
> > > > 
> > > > So either I can tweak the lambda to return ba_check_bit rather
> > > > than ba_any or use ba_check_bit unconditionally.  Any opinions on that?
> > > 
> > > The relevant passage seems to be
> > > https://eel.is/c++draft/class.access.base#6
> > > after DR 52, which seems to have clarified that the pointer conversion only
> > > applies to non-static members.
> > > 
> > > > >     struct A { constexpr static int a = 0; };
> > > > >     struct D : private A {};
> > > > > 
> > > > >     void f() {
> > > > >       D{}.A::a; // #1 GCC (and Clang) currently rejects
> > > > >     }
> > > 
> > > I see that MSVC also rejects it, while EDG accepts.
> > > 
> > > https://eel.is/c++draft/class.access.base#5.1 seems to say that a is
> > > accessible when named in A.
> > > 
> > > https://eel.is/c++draft/expr.ref#7 also only constrains references to
> > > non-static members.
> > > 
> > > But first we need to look up A in D, and A's injected-class-name looked up
> > > as a member of D is not accessible; it's private, and f() is not a friend,
> > > and we correctly complain about that.
> > > 
> > > If we avoid the lookup of A in D with
> > > 
> > > D{}.::A::a;
> > > 
> > > clang accepts it, which is consistent with accepting the template version,
> > > and seems correct.
> > > 
> > > So, I think ba_any is what we want here.
> > 
> > Wow, that is not intuitive (to me at least).  So I had it right but
> > only by accident.
> > 
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > 
> > -- >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.
> > 
> > The rationale for using ba_any is explained at
> > <https://gcc.gnu.org/pipermail/gcc-patches/2023-November/638676.html>.
> 
> I'd prefer not to cite the mailing list for rationales.
> 
> To summarize:
> [class.access.base] requires conversion to a unique base subobject for
> non-static data members, but it does not require that the base be unique or
> accessible for static data members.
> 
> > 	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.
> > 	* g++.dg/lookup/scoped14.C: New test.
> > 	* g++.dg/lookup/scoped15.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 ++++++++++++++
> >   gcc/testsuite/g++.dg/lookup/scoped14.C | 14 ++++++++++++++
> >   gcc/testsuite/g++.dg/lookup/scoped15.C | 20 ++++++++++++++++++++
> >   6 files changed, 94 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
> >   create mode 100644 gcc/testsuite/g++.dg/lookup/scoped14.C
> >   create mode 100644 gcc/testsuite/g++.dg/lookup/scoped15.C
> > 
> > diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
> > index 0839d0a4167..bf8ffaa7e75 100644
> > --- a/gcc/cp/typeck.cc
> > +++ b/gcc/cp/typeck.cc
> > @@ -3467,7 +3467,7 @@ 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;
> > @@ -3484,9 +3484,24 @@ 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 || shared_member_p (m))
> > +		    return ba_any;
> > +		}
> > +	      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." }
> > +}
> > diff --git a/gcc/testsuite/g++.dg/lookup/scoped14.C b/gcc/testsuite/g++.dg/lookup/scoped14.C
> > new file mode 100644
> > index 00000000000..141aa0d2b1a
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/lookup/scoped14.C
> > @@ -0,0 +1,14 @@
> > +// PR c++/112744
> > +// { dg-do compile { target c++11 } }
> > +
> > +struct A { int a = 0; };
> > +struct B : A {};
> > +struct C : A {};
> > +struct D : B, C {};
> > +
> > +int main()
> > +{
> > +  D d;
> > +  (void) d.a;	  // { dg-error "request for member .a. is ambiguous" }
> > +  (void) d.A::a;  // { dg-error ".A. is an ambiguous base of .D." }
> > +}
> > diff --git a/gcc/testsuite/g++.dg/lookup/scoped15.C b/gcc/testsuite/g++.dg/lookup/scoped15.C
> > new file mode 100644
> > index 00000000000..d450a41a617
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/lookup/scoped15.C
> > @@ -0,0 +1,20 @@
> > +// PR c++/112744
> > +// { dg-do compile { target c++11 } }
> > +
> > +struct A { constexpr static int a = 0; };
> > +struct D : private A {};
> > +
> > +// See <https://gcc.gnu.org/pipermail/gcc-patches/2023-November/638676.html>
> > +// for rationale.
> 
> The injected-class-name of A is private when named in D, but if A is named
> some other way, there is no requirement in [class.access.base] for static
> data members that it be an accessible base.
> 
> OK with those comment adjustments.

Thanks; pushed with that adjusted.

I'm not planning to backport the patch.

Marek

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

[Jason Merrill]

On 11/29/23 10:45, 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...

Hmm, won't using ba_any unconditionally break ambiguous base checking 
for non-static data members?

> @@ -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] ()

Why a lambda?

> +	    {
> +	      if (identifier_p (name))
> +		{
> +		  tree m = lookup_member (scope, name, /*protect=*/0,
> +					  /*want_type=*/false, tf_none);
> +		  if (!m || VAR_P (m))

Do you want shared_member_p here?

Jason

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

[Marek Polacek]

On Wed, Nov 29, 2023 at 01:58:31PM -0500, Jason Merrill wrote:
> On 11/29/23 10:45, 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...
> 
> Hmm, won't using ba_any unconditionally break ambiguous base checking for
> non-static data members?

Yes.  I thought not but that's only because we weren't properly testing
that case (added scoped14.C, patch to follow).  So that settles that.
 
> > @@ -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] ()
> 
> Why a lambda?

Only so that I can set 'ba' once and make it const.  I don't believe it
deserves a named function.  It seems more readable than using a ?:.

> > +	    {
> > +	      if (identifier_p (name))
> > +		{
> > +		  tree m = lookup_member (scope, name, /*protect=*/0,
> > +					  /*want_type=*/false, tf_none);
> > +		  if (!m || VAR_P (m))
> 
> Do you want shared_member_p here?

That looks like the right thing to use, thanks.

Marek