[C++ Patch/RFC] PR 43906

[C++ Patch/RFC] PR 43906

[Paolo Carlini]

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Hi,

I suppose we can quickly resolve, one way or another, this rather old 
issue. Considering:

extern void z();
void h() { if ( z != (void*)0 ); }

we -Waddress warn in C and we don't in C++, due to the rather subtle 
differences between null_pointer_constant_p and null_ptr_cst_p. I 
believe we could as well warn in C++ too, but then I'm afraid we have to 
handle the case specially, like in the below. What do you think?

Thanks!
Paolo.

//////////////////////


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Index: cp/typeck.c
===================================================================
--- cp/typeck.c	(revision 213573)
+++ cp/typeck.c	(working copy)
@@ -4353,12 +4353,11 @@ cp_build_binary_op (location_t location,
 	  && (code1 == INTEGER_TYPE || code1 == REAL_TYPE
 	      || code1 == COMPLEX_TYPE || code1 == ENUMERAL_TYPE))
 	short_compare = 1;
-      else if ((code0 == POINTER_TYPE && code1 == POINTER_TYPE)
-	       || (TYPE_PTRDATAMEM_P (type0) && TYPE_PTRDATAMEM_P (type1)))
-	result_type = composite_pointer_type (type0, type1, op0, op1,
-					      CPO_COMPARISON, complain);
       else if ((code0 == POINTER_TYPE || TYPE_PTRDATAMEM_P (type0))
-	       && null_ptr_cst_p (op1))
+	       && (null_ptr_cst_p (op1)
+		   /* Handle (void*)0 too.  */
+		   || (TYPE_PTR_P (type1) && VOID_TYPE_P (TREE_TYPE (type1))
+		       && integer_zerop (op1))))
 	{
 	  if (TREE_CODE (op0) == ADDR_EXPR
 	      && decl_with_nonnull_addr_p (TREE_OPERAND (op0, 0)))
@@ -4371,7 +4370,10 @@ cp_build_binary_op (location_t location,
 	  result_type = type0;
 	}
       else if ((code1 == POINTER_TYPE || TYPE_PTRDATAMEM_P (type1))
-	       && null_ptr_cst_p (op0))
+	       && (null_ptr_cst_p (op0)
+		   /* Handle (void*)0 too.  */
+		   || (TYPE_PTR_P (type0) && VOID_TYPE_P (TREE_TYPE (type0))
+		       && integer_zerop (op0))))
 	{
 	  if (TREE_CODE (op1) == ADDR_EXPR 
 	      && decl_with_nonnull_addr_p (TREE_OPERAND (op1, 0)))
@@ -4383,6 +4385,10 @@ cp_build_binary_op (location_t location,
 	    }
 	  result_type = type1;
 	}
+      else if ((code0 == POINTER_TYPE && code1 == POINTER_TYPE)
+	       || (TYPE_PTRDATAMEM_P (type0) && TYPE_PTRDATAMEM_P (type1)))
+	result_type = composite_pointer_type (type0, type1, op0, op1,
+					      CPO_COMPARISON, complain);
       else if (null_ptr_cst_p (op0) && null_ptr_cst_p (op1))
 	/* One of the operands must be of nullptr_t type.  */
         result_type = TREE_TYPE (nullptr_node);
Index: testsuite/g++.dg/warn/Waddress-1.C
===================================================================
--- testsuite/g++.dg/warn/Waddress-1.C	(revision 0)
+++ testsuite/g++.dg/warn/Waddress-1.C	(working copy)
@@ -0,0 +1,7 @@
+// PR c++/43906
+// { dg-options "-Waddress" }
+
+extern void z();
+void f() { if ( z ) z(); }              // { dg-warning "address" }
+void g() { if ( z != 0 ) z(); }         // { dg-warning "address" }
+void h() { if ( z != (void*)0 ) z(); }  // { dg-warning "address" }

Re: [C++ Patch/RFC] PR 43906

[Jason Merrill]

On 08/04/2014 12:24 PM, Paolo Carlini wrote:
> +		   || (TYPE_PTR_P (type1) && VOID_TYPE_P (TREE_TYPE (type1))

Why check for VOID_TYPE_P?  I'd think we would want to warn about 
comparing to other null pointer values as well.

Jason

Re: [C++ Patch/RFC] PR 43906

[Paolo Carlini]

Hi,

On 08/04/2014 10:45 PM, Jason Merrill wrote:
> On 08/04/2014 12:24 PM, Paolo Carlini wrote:
>> +           || (TYPE_PTR_P (type1) && VOID_TYPE_P (TREE_TYPE (type1))
>
> Why check for VOID_TYPE_P?  I'd think we would want to warn about 
> comparing to other null pointer values as well.
In fact I wondered about that a few minutes after sending my message... 
And this is what I figured out: normally we have hard errors from 
composite_pointer_type (eg, try scalar types, class types), even for 
null values. The only exception I have been able to find earlier today 
is that of pointer to the same function type, eg:

extern void z();
typedef void (*ptr)();
void i() { if ( z != (ptr)0 ); }

but in this case the C front-end too doesn't warn. In short, the case of 
(void*)0 seems very special.

However, something I did *not* notice earlier today, is that comparing a 
pointer to function to a generic void* leads to a pedwarn at the 
beginning of composite_pointer_type about the comparison itself. Thus 
it's debatable whether we also want the -Waddress warning... If you ask 
me, closing the bug with a testcase which checks that we warn for 
-pedantic about the comparison (if we don't have one already) would be 
Ok (vs EDG 4.9 too) What do you think?

Paolo.

Re: [C++ Patch/RFC] PR 43906

[Jason Merrill]

On 08/04/2014 07:01 PM, Paolo Carlini wrote:
> In fact I wondered about that a few minutes after sending my message...
> And this is what I figured out: normally we have hard errors from
> composite_pointer_type (eg, try scalar types, class types), even for
> null values. The only exception I have been able to find earlier today
> is that of pointer to the same function type, eg:
>
> extern void z();
> typedef void (*ptr)();
> void i() { if ( z != (ptr)0 ); }
>
> but in this case the C front-end too doesn't warn.

I don't see why we wouldn't want to warn in this case; it's still the 
case thet the comparison will always be false.

We can also see this situation for non-function pointers:

void f()
{
   int i;
   if (&i != (int*)0);
}

Jason

Re: [C++ Patch/RFC] PR 43906

[Paolo Carlini]

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Hi,

On 08/05/2014 03:58 AM, Jason Merrill wrote:
> On 08/04/2014 07:01 PM, Paolo Carlini wrote:
>> In fact I wondered about that a few minutes after sending my message...
>> And this is what I figured out: normally we have hard errors from
>> composite_pointer_type (eg, try scalar types, class types), even for
>> null values. The only exception I have been able to find earlier today
>> is that of pointer to the same function type, eg:
>>
>> extern void z();
>> typedef void (*ptr)();
>> void i() { if ( z != (ptr)0 ); }
>>
>> but in this case the C front-end too doesn't warn.
> I don't see why we wouldn't want to warn in this case; it's still the 
> case thet the comparison will always be false.
In general, I agree of course. I was trying to understand if keeping the 
issue as minimally one of consistency with the C front-end simplified it.
> We can also see this situation for non-function pointers:
>
> void f()
> {
>   int i;
>   if (&i != (int*)0);
> }
Sure. Then, however, we must be careful about the actual pointer types, 
otherwise we change hard errors to warnings. And void* is an exception 
to the general rule. I tried using ptr_reasonably_similar to avoid the 
explicit call of comptypes + separate VOID_TYPE_P, but unfortunately it 
appears too loose about at least pointers to function types.

Thanks!
Paolo.

//////////////////////////

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Index: cp/typeck.c
===================================================================
--- cp/typeck.c	(revision 213631)
+++ cp/typeck.c	(working copy)
@@ -4353,12 +4353,14 @@ cp_build_binary_op (location_t location,
 	  && (code1 == INTEGER_TYPE || code1 == REAL_TYPE
 	      || code1 == COMPLEX_TYPE || code1 == ENUMERAL_TYPE))
 	short_compare = 1;
-      else if ((code0 == POINTER_TYPE && code1 == POINTER_TYPE)
-	       || (TYPE_PTRDATAMEM_P (type0) && TYPE_PTRDATAMEM_P (type1)))
-	result_type = composite_pointer_type (type0, type1, op0, op1,
-					      CPO_COMPARISON, complain);
       else if ((code0 == POINTER_TYPE || TYPE_PTRDATAMEM_P (type0))
-	       && null_ptr_cst_p (op1))
+	       && (null_ptr_cst_p (op1)
+		   /* Handle, eg, (void*)0 (c++/43906), and more.  */
+		   || (TYPE_PTR_P (type1) && integer_zerop (op1)
+		       && (VOID_TYPE_P (TREE_TYPE (type1))
+			   || comptypes (TYPE_MAIN_VARIANT (TREE_TYPE (type0)),
+					 TYPE_MAIN_VARIANT (TREE_TYPE (type1)),
+					 COMPARE_BASE | COMPARE_DERIVED)))))
 	{
 	  if (TREE_CODE (op0) == ADDR_EXPR
 	      && decl_with_nonnull_addr_p (TREE_OPERAND (op0, 0)))
@@ -4371,7 +4373,13 @@ cp_build_binary_op (location_t location,
 	  result_type = type0;
 	}
       else if ((code1 == POINTER_TYPE || TYPE_PTRDATAMEM_P (type1))
-	       && null_ptr_cst_p (op0))
+	       && (null_ptr_cst_p (op0)
+		   /* Handle, eg, (void*)0 (c++/43906), and more.  */
+		   || (TYPE_PTR_P (type0) && integer_zerop (op0)
+		       && (VOID_TYPE_P (TREE_TYPE (type0))
+			   || comptypes (TYPE_MAIN_VARIANT (TREE_TYPE (type0)),
+					 TYPE_MAIN_VARIANT (TREE_TYPE (type1)),
+					 COMPARE_BASE | COMPARE_DERIVED)))))
 	{
 	  if (TREE_CODE (op1) == ADDR_EXPR 
 	      && decl_with_nonnull_addr_p (TREE_OPERAND (op1, 0)))
@@ -4383,6 +4391,10 @@ cp_build_binary_op (location_t location,
 	    }
 	  result_type = type1;
 	}
+      else if ((code0 == POINTER_TYPE && code1 == POINTER_TYPE)
+	       || (TYPE_PTRDATAMEM_P (type0) && TYPE_PTRDATAMEM_P (type1)))
+	result_type = composite_pointer_type (type0, type1, op0, op1,
+					      CPO_COMPARISON, complain);
       else if (null_ptr_cst_p (op0) && null_ptr_cst_p (op1))
 	/* One of the operands must be of nullptr_t type.  */
         result_type = TREE_TYPE (nullptr_node);
Index: testsuite/g++.dg/warn/Waddress-1.C
===================================================================
--- testsuite/g++.dg/warn/Waddress-1.C	(revision 0)
+++ testsuite/g++.dg/warn/Waddress-1.C	(working copy)
@@ -0,0 +1,42 @@
+// PR c++/43906
+// { dg-options "-Waddress" }
+
+extern void z();
+typedef void (*ptrf) ();
+typedef int (*ptrfn) (int);
+int n;
+const int m = 1;
+struct S { };
+struct T : S { };
+struct U;
+S s;
+T t;
+double d;
+
+void f()  { if (z) z(); }               // { dg-warning "address" }
+
+void gl() { if (z != 0) z(); }          // { dg-warning "address" }
+void hl() { if (z != (ptrf)0) z(); }    // { dg-warning "address" }
+void il() { if (z != (void*)0) z(); }   // { dg-warning "address" }
+void jl() { if (&n != (int*)0) z(); }   // { dg-warning "address" }
+void kl() { if (&m != (int*)0) z(); }   // { dg-warning "address" }
+void ll() { if (&s != (T*)0) z(); }     // { dg-warning "address" }
+void ml() { if (&t != (S*)0) z(); }     // { dg-warning "address" }
+
+void nl() { if (z != (S*)0) z(); }      // { dg-error "comparison" }
+void pl() { if (z != (ptrfn)0) z(); }   // { dg-error "comparison" }
+void ql() { if (&d != (int*)0) z(); }   // { dg-error "comparison" }
+void rl() { if (&s != (U*)0) z(); }     // { dg-error "comparison" }
+
+void gr() { if (0 != z) z(); }          // { dg-warning "address" }
+void hr() { if ((ptrf)0 != z) z(); }    // { dg-warning "address" }
+void ir() { if ((void*)0 != z) z(); }   // { dg-warning "address" }
+void jr() { if ((int*)0 != &n) z(); }   // { dg-warning "address" }
+void kr() { if ((int*)0 != &m) z(); }   // { dg-warning "address" }
+void lr() { if ((T*)0 != &s) z(); }     // { dg-warning "address" }
+void mr() { if ((S*)0 != &t) z(); }     // { dg-warning "address" }
+
+void nr() { if ((S*)0 != z) z(); }      // { dg-error "comparison" }
+void pr() { if ((ptrfn)0 != z) z(); }   // { dg-error "comparison" }
+void qr() { if ((int*)0 != &d) z(); }   // { dg-error "comparison" }
+void rr() { if ((U*)0 != &s) z(); }     // { dg-error "comparison" }

Re: [C++ Patch/RFC] PR 43906

[Paolo Carlini]

.. a clarification. As I tried to briefly explain yesterday, this kind 
of change means that:

extern void z();
void il() { if (z != (void*)0) z(); }

doesn't trigger anymore the pedwarn at beginning of 
composite_pointer_type about the comparison itself, for the simple 
reason that we don't call it anymore. I suppose that's Ok. Otherwise we 
have to change something, it's a bit tricky tough. We could even decide 
that we don't want to handle the above for -Waddress because 
conceptually the issue with the comparison itself comes before, as again 
I tried to explain a bit yesterday, but that triggers a warning only 
with -pedantic (EDG has it by default).

Paolo.

Re: [C++ Patch/RFC] PR 43906

[Jason Merrill]

On 08/05/2014 08:10 AM, Paolo Carlini wrote:
> .. a clarification. As I tried to briefly explain yesterday, this kind
> of change means that:
>
> extern void z();
> void il() { if (z != (void*)0) z(); }
>
> doesn't trigger anymore the pedwarn at beginning of
> composite_pointer_type about the comparison itself, for the simple
> reason that we don't call it anymore.

So let's keep calling it when the RHS is also a pointer?

Jason

Re: [C++ Patch/RFC] PR 43906

[Paolo Carlini]

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Hi,

On 08/05/2014 02:32 PM, Jason Merrill wrote:
> On 08/05/2014 08:10 AM, Paolo Carlini wrote:
>> .. a clarification. As I tried to briefly explain yesterday, this kind
>> of change means that:
>>
>> extern void z();
>> void il() { if (z != (void*)0) z(); }
>>
>> doesn't trigger anymore the pedwarn at beginning of
>> composite_pointer_type about the comparison itself, for the simple
>> reason that we don't call it anymore.
> So let's keep calling it when the RHS is also a pointer?
Indeed ;) Then I'm finishing testing the below.

Note: I also rearranged the conditionals, splitting out the 
TYPE_PTRDATAMEM_P case, which was causing confusion in some cases: we 
were feeding a TYPE_PTRDATAMEM_P and a TYPE_PTR_P to 
composite_pointer_type, thus obtaining immediately verbose diagnostic, 
instead of the expected clean one talking about invalid operands to 
operator!=.

Thanks,
Paolo.

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Index: cp/typeck.c
===================================================================
--- cp/typeck.c	(revision 213631)
+++ cp/typeck.c	(working copy)
@@ -4353,13 +4353,22 @@ cp_build_binary_op (location_t location,
 	  && (code1 == INTEGER_TYPE || code1 == REAL_TYPE
 	      || code1 == COMPLEX_TYPE || code1 == ENUMERAL_TYPE))
 	short_compare = 1;
-      else if ((code0 == POINTER_TYPE && code1 == POINTER_TYPE)
-	       || (TYPE_PTRDATAMEM_P (type0) && TYPE_PTRDATAMEM_P (type1)))
-	result_type = composite_pointer_type (type0, type1, op0, op1,
-					      CPO_COMPARISON, complain);
-      else if ((code0 == POINTER_TYPE || TYPE_PTRDATAMEM_P (type0))
-	       && null_ptr_cst_p (op1))
+      else if (((code0 == POINTER_TYPE || TYPE_PTRDATAMEM_P (type0))
+		&& null_ptr_cst_p (op1))
+	       /* Handle, eg, (void*)0 (c++/43906), and more.  */
+	       || (code0 == POINTER_TYPE
+		   && TYPE_PTR_P (type1) && integer_zerop (op1)
+		   && (VOID_TYPE_P (TREE_TYPE (type1))
+		       || comptypes (TYPE_MAIN_VARIANT (TREE_TYPE (type0)),
+				     TYPE_MAIN_VARIANT (TREE_TYPE (type1)),
+				     COMPARE_BASE | COMPARE_DERIVED))))
 	{
+	  if (TYPE_PTR_P (type1))
+	    result_type = composite_pointer_type (type0, type1, op0, op1,
+						  CPO_COMPARISON, complain);
+	  else
+	    result_type = type0;
+
 	  if (TREE_CODE (op0) == ADDR_EXPR
 	      && decl_with_nonnull_addr_p (TREE_OPERAND (op0, 0)))
 	    {
@@ -4368,11 +4377,23 @@ cp_build_binary_op (location_t location,
 		warning (OPT_Waddress, "the address of %qD will never be NULL",
 			 TREE_OPERAND (op0, 0));
 	    }
-	  result_type = type0;
 	}
-      else if ((code1 == POINTER_TYPE || TYPE_PTRDATAMEM_P (type1))
-	       && null_ptr_cst_p (op0))
+      else if (((code1 == POINTER_TYPE || TYPE_PTRDATAMEM_P (type1))
+		&& null_ptr_cst_p (op0))
+	       /* Handle, eg, (void*)0 (c++/43906), and more.  */
+	       || (code1 == POINTER_TYPE
+		   && TYPE_PTR_P (type0) && integer_zerop (op0)
+		   && (VOID_TYPE_P (TREE_TYPE (type0))
+		       || comptypes (TYPE_MAIN_VARIANT (TREE_TYPE (type0)),
+				     TYPE_MAIN_VARIANT (TREE_TYPE (type1)),
+				     COMPARE_BASE | COMPARE_DERIVED))))
 	{
+	  if (TYPE_PTR_P (type0))
+	    result_type = composite_pointer_type (type0, type1, op0, op1,
+						  CPO_COMPARISON, complain);
+	  else
+	    result_type = type1;
+
 	  if (TREE_CODE (op1) == ADDR_EXPR 
 	      && decl_with_nonnull_addr_p (TREE_OPERAND (op1, 0)))
 	    {
@@ -4381,8 +4402,11 @@ cp_build_binary_op (location_t location,
 		warning (OPT_Waddress, "the address of %qD will never be NULL",
 			 TREE_OPERAND (op1, 0));
 	    }
-	  result_type = type1;
 	}
+      else if ((code0 == POINTER_TYPE && code1 == POINTER_TYPE)
+	       || (TYPE_PTRDATAMEM_P (type0) && TYPE_PTRDATAMEM_P (type1)))
+	result_type = composite_pointer_type (type0, type1, op0, op1,
+					      CPO_COMPARISON, complain);
       else if (null_ptr_cst_p (op0) && null_ptr_cst_p (op1))
 	/* One of the operands must be of nullptr_t type.  */
         result_type = TREE_TYPE (nullptr_node);
Index: testsuite/g++.dg/warn/Waddress-1.C
===================================================================
--- testsuite/g++.dg/warn/Waddress-1.C	(revision 0)
+++ testsuite/g++.dg/warn/Waddress-1.C	(working copy)
@@ -0,0 +1,42 @@
+// PR c++/43906
+// { dg-options "-Waddress -pedantic" }
+
+extern void z();
+typedef void (*ptrf) ();
+typedef int (*ptrfn) (int);
+int n;
+const int m = 1;
+struct S { };
+struct T : S { };
+struct U;
+S s;
+T t;
+double d;
+
+void f()  { if (z) z(); }               // { dg-warning "address" }
+
+void gl() { if (z != 0) z(); }          // { dg-warning "address" }
+void hl() { if (z != (ptrf)0) z(); }    // { dg-warning "address" }
+void il() { if (z != (void*)0) z(); }   // { dg-warning "address|comparison" }
+void jl() { if (&n != (int*)0) z(); }   // { dg-warning "address" }
+void kl() { if (&m != (int*)0) z(); }   // { dg-warning "address" }
+void ll() { if (&s != (T*)0) z(); }     // { dg-warning "address" }
+void ml() { if (&t != (S*)0) z(); }     // { dg-warning "address" }
+
+void nl() { if (z != (S*)0) z(); }      // { dg-error "comparison" }
+void pl() { if (z != (ptrfn)0) z(); }   // { dg-error "comparison" }
+void ql() { if (&d != (int*)0) z(); }   // { dg-error "comparison" }
+void rl() { if (&s != (U*)0) z(); }     // { dg-error "comparison" }
+
+void gr() { if (0 != z) z(); }          // { dg-warning "address" }
+void hr() { if ((ptrf)0 != z) z(); }    // { dg-warning "address" }
+void ir() { if ((void*)0 != z) z(); }   // { dg-warning "address|comparison" }
+void jr() { if ((int*)0 != &n) z(); }   // { dg-warning "address" }
+void kr() { if ((int*)0 != &m) z(); }   // { dg-warning "address" }
+void lr() { if ((T*)0 != &s) z(); }     // { dg-warning "address" }
+void mr() { if ((S*)0 != &t) z(); }     // { dg-warning "address" }
+
+void nr() { if ((S*)0 != z) z(); }      // { dg-error "comparison" }
+void pr() { if ((ptrfn)0 != z) z(); }   // { dg-error "comparison" }
+void qr() { if ((int*)0 != &d) z(); }   // { dg-error "comparison" }
+void rr() { if ((U*)0 != &s) z(); }     // { dg-error "comparison" }

Re: [C++ Patch/RFC] PR 43906

[Jason Merrill]

On 08/05/2014 10:48 AM, Paolo Carlini wrote:
> +		   && (VOID_TYPE_P (TREE_TYPE (type1))
> +		       || comptypes (TYPE_MAIN_VARIANT (TREE_TYPE (type0)),
> +				     TYPE_MAIN_VARIANT (TREE_TYPE (type1)),
> +				     COMPARE_BASE | COMPARE_DERIVED))))

Can we drop this now that we're calling composite_pointer_type?

Jason

Re: [C++ Patch/RFC] PR 43906

[Paolo Carlini]

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Hi,

On 08/06/2014 05:19 PM, Jason Merrill wrote:
> On 08/05/2014 10:48 AM, Paolo Carlini wrote:
>> +           && (VOID_TYPE_P (TREE_TYPE (type1))
>> +               || comptypes (TYPE_MAIN_VARIANT (TREE_TYPE (type0)),
>> +                     TYPE_MAIN_VARIANT (TREE_TYPE (type1)),
>> +                     COMPARE_BASE | COMPARE_DERIVED))))
>
> Can we drop this now that we're calling composite_pointer_type?
Yes we can, sorry for not investigating that earlier. I only have to 
tweak a bit the testcase because then in the malformed cases we emit 
first the permerror and then the -Waddress warning too. I suppose it's 
Ok because after all those are in most of the cases permerrors and I 
don't think the additional verbosity should be that common, we are 
talking about comparing a "null" pointer of the wrong type, not a 
generic pointer. Otherwise we would have to tweak composite_pointer_type 
to precisely inform the caller when an actual error was emitted.

Thanks,
Paolo.

/////////////////////

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Index: cp/typeck.c
===================================================================
--- cp/typeck.c	(revision 213654)
+++ cp/typeck.c	(working copy)
@@ -4353,13 +4353,18 @@ cp_build_binary_op (location_t location,
 	  && (code1 == INTEGER_TYPE || code1 == REAL_TYPE
 	      || code1 == COMPLEX_TYPE || code1 == ENUMERAL_TYPE))
 	short_compare = 1;
-      else if ((code0 == POINTER_TYPE && code1 == POINTER_TYPE)
-	       || (TYPE_PTRDATAMEM_P (type0) && TYPE_PTRDATAMEM_P (type1)))
-	result_type = composite_pointer_type (type0, type1, op0, op1,
-					      CPO_COMPARISON, complain);
-      else if ((code0 == POINTER_TYPE || TYPE_PTRDATAMEM_P (type0))
-	       && null_ptr_cst_p (op1))
+      else if (((code0 == POINTER_TYPE || TYPE_PTRDATAMEM_P (type0))
+		&& null_ptr_cst_p (op1))
+	       /* Handle, eg, (void*)0 (c++/43906), and more.  */
+	       || (code0 == POINTER_TYPE
+		   && TYPE_PTR_P (type1) && integer_zerop (op1)))
 	{
+	  if (TYPE_PTR_P (type1))
+	    result_type = composite_pointer_type (type0, type1, op0, op1,
+						  CPO_COMPARISON, complain);
+	  else
+	    result_type = type0;
+
 	  if (TREE_CODE (op0) == ADDR_EXPR
 	      && decl_with_nonnull_addr_p (TREE_OPERAND (op0, 0)))
 	    {
@@ -4368,11 +4373,19 @@ cp_build_binary_op (location_t location,
 		warning (OPT_Waddress, "the address of %qD will never be NULL",
 			 TREE_OPERAND (op0, 0));
 	    }
-	  result_type = type0;
 	}
-      else if ((code1 == POINTER_TYPE || TYPE_PTRDATAMEM_P (type1))
-	       && null_ptr_cst_p (op0))
+      else if (((code1 == POINTER_TYPE || TYPE_PTRDATAMEM_P (type1))
+		&& null_ptr_cst_p (op0))
+	       /* Handle, eg, (void*)0 (c++/43906), and more.  */
+	       || (code1 == POINTER_TYPE
+		   && TYPE_PTR_P (type0) && integer_zerop (op0)))
 	{
+	  if (TYPE_PTR_P (type0))
+	    result_type = composite_pointer_type (type0, type1, op0, op1,
+						  CPO_COMPARISON, complain);
+	  else
+	    result_type = type1;
+
 	  if (TREE_CODE (op1) == ADDR_EXPR 
 	      && decl_with_nonnull_addr_p (TREE_OPERAND (op1, 0)))
 	    {
@@ -4381,8 +4394,11 @@ cp_build_binary_op (location_t location,
 		warning (OPT_Waddress, "the address of %qD will never be NULL",
 			 TREE_OPERAND (op1, 0));
 	    }
-	  result_type = type1;
 	}
+      else if ((code0 == POINTER_TYPE && code1 == POINTER_TYPE)
+	       || (TYPE_PTRDATAMEM_P (type0) && TYPE_PTRDATAMEM_P (type1)))
+	result_type = composite_pointer_type (type0, type1, op0, op1,
+					      CPO_COMPARISON, complain);
       else if (null_ptr_cst_p (op0) && null_ptr_cst_p (op1))
 	/* One of the operands must be of nullptr_t type.  */
         result_type = TREE_TYPE (nullptr_node);
Index: testsuite/g++.dg/warn/Waddress-1.C
===================================================================
--- testsuite/g++.dg/warn/Waddress-1.C	(revision 0)
+++ testsuite/g++.dg/warn/Waddress-1.C	(working copy)
@@ -0,0 +1,50 @@
+// PR c++/43906
+// { dg-options "-Waddress -pedantic" }
+
+extern void z();
+typedef void (*ptrf) ();
+typedef int (*ptrfn) (int);
+int n;
+const int m = 1;
+struct S { };
+struct T : S { };
+struct U;
+S s;
+T t;
+double d;
+
+void f()  { if (z) z(); }               // { dg-warning "address" }
+
+void gl() { if (z != 0) z(); }          // { dg-warning "address" }
+void hl() { if (z != (ptrf)0) z(); }    // { dg-warning "address" }
+void il() { if (z != (void*)0) z(); }   // { dg-warning "address|comparison" }
+void jl() { if (&n != (int*)0) z(); }   // { dg-warning "address" }
+void kl() { if (&m != (int*)0) z(); }   // { dg-warning "address" }
+void ll() { if (&s != (T*)0) z(); }     // { dg-warning "address" }
+void ml() { if (&t != (S*)0) z(); }     // { dg-warning "address" }
+
+void nl() { if (z != (S*)0) z(); }      // { dg-error "comparison" }
+// { dg-warning "address" "" { target *-*-* } 26 }
+void pl() { if (z != (ptrfn)0) z(); }   // { dg-error "comparison" }
+// { dg-warning "address" "" { target *-*-* } 28 }
+void ql() { if (&d != (int*)0) z(); }   // { dg-error "comparison" }
+// { dg-warning "address" "" { target *-*-* } 30 }
+void rl() { if (&s != (U*)0) z(); }     // { dg-error "comparison" }
+// { dg-warning "address" "" { target *-*-* } 32 }
+
+void gr() { if (0 != z) z(); }          // { dg-warning "address" }
+void hr() { if ((ptrf)0 != z) z(); }    // { dg-warning "address" }
+void ir() { if ((void*)0 != z) z(); }   // { dg-warning "address|comparison" }
+void jr() { if ((int*)0 != &n) z(); }   // { dg-warning "address" }
+void kr() { if ((int*)0 != &m) z(); }   // { dg-warning "address" }
+void lr() { if ((T*)0 != &s) z(); }     // { dg-warning "address" }
+void mr() { if ((S*)0 != &t) z(); }     // { dg-warning "address" }
+
+void nr() { if ((S*)0 != z) z(); }      // { dg-error "comparison" }
+// { dg-warning "address" "" { target *-*-* } 43 }
+void pr() { if ((ptrfn)0 != z) z(); }   // { dg-error "comparison" }
+// { dg-warning "address" "" { target *-*-* } 45 }
+void qr() { if ((int*)0 != &d) z(); }   // { dg-error "comparison" }
+// { dg-warning "address" "" { target *-*-* } 47 }
+void rr() { if ((U*)0 != &s) z(); }     // { dg-error "comparison" }
+// { dg-warning "address" "" { target *-*-* } 49 }

Re: [C++ Patch/RFC] PR 43906

[Jason Merrill]

OK, thanks.

Jason