[C++ Patch/RFC] PR 43906

[Paolo Carlini <paolo.carlini@oracle.com>]

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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" }