[PATCH] Emit -Waddress warnings for comparing address of reference against NULL

[Patrick Palka <patrick at parcs dot ath dot cx>]

Here is an updated version of the patch with, hopefully, all your
suggestions made.  I decided to add calls to STRIP_NOPS before emitting
the warning so that we properly warn for cases where there's a cast in
between the whole thing, e.g.

  if (!&(int &)a)

I also added guards to emit the warnings only when the stripped operand
is actually a decl so that we don't pass a non-decl argument to
warning_at() which can happen in a case like

  if (!&(int &)*(int *)0)

Does this look OK now after testing?

gcc/c-family/ChangeLog:

	PR c++/65168
	* c-common.c (c_common_truthvalue_conversion): Warn when
	converting an address of a reference to a truth value.

gcc/cp/ChangeLog:

	PR c++/65168
	* typeck.c (cp_build_binary_op): Warn when comparing an address
	of a reference against NULL.

gcc/testsuite/ChangeLog:

	PR c++/65168
	g++.dg/warn/Walways-true-3.C: New test.
---
 gcc/c-family/c-common.c                    | 14 ++++++++++
 gcc/cp/typeck.c                            | 34 ++++++++++++++++++++++
 gcc/testsuite/g++.dg/warn/Walways-true-3.C | 45 ++++++++++++++++++++++++++++++
 3 files changed, 93 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/warn/Walways-true-3.C

diff --git a/gcc/c-family/c-common.c b/gcc/c-family/c-common.c
index 9797e17..c353c72 100644
--- a/gcc/c-family/c-common.c
+++ b/gcc/c-family/c-common.c
@@ -4806,6 +4806,20 @@ c_common_truthvalue_conversion (location_t location, tree expr)
 	tree totype = TREE_TYPE (expr);
 	tree fromtype = TREE_TYPE (TREE_OPERAND (expr, 0));
 
+	if (POINTER_TYPE_P (totype)
+	    && TREE_CODE (fromtype) == REFERENCE_TYPE)
+	  {
+	    tree inner = expr;
+	    STRIP_NOPS (inner);
+
+	    if (DECL_P (inner))
+	      warning_at (location,
+			  OPT_Waddress,
+			  "the compiler can assume that the address of "
+			  "%qD will always evaluate to %<true%>",
+			  inner);
+	  }
+
 	/* Don't cancel the effect of a CONVERT_EXPR from a REFERENCE_TYPE,
 	   since that affects how `default_conversion' will behave.  */
 	if (TREE_CODE (totype) == REFERENCE_TYPE
diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
index 91db32a..13fb401 100644
--- a/gcc/cp/typeck.c
+++ b/gcc/cp/typeck.c
@@ -4423,6 +4423,23 @@ cp_build_binary_op (location_t location,
 		warning (OPT_Waddress, "the address of %qD will never be NULL",
 			 TREE_OPERAND (op0, 0));
 	    }
+
+	  if (CONVERT_EXPR_P (op0)
+	      && TREE_CODE (TREE_TYPE (TREE_OPERAND (op0, 0)))
+		 == REFERENCE_TYPE)
+	    {
+	      tree inner_op0 = op0;
+	      STRIP_NOPS (inner_op0);
+
+	      if ((complain & tf_warning)
+		  && c_inhibit_evaluation_warnings == 0
+		  && !TREE_NO_WARNING (op0)
+		  && DECL_P (inner_op0))
+		warning (OPT_Waddress,
+			 "the compiler can assume that the address of "
+			 "%qD will never be NULL",
+			 inner_op0);
+	    }
 	}
       else if (((code1 == POINTER_TYPE || TYPE_PTRDATAMEM_P (type1))
 		&& null_ptr_cst_p (op0))
@@ -4445,6 +4462,23 @@ cp_build_binary_op (location_t location,
 		warning (OPT_Waddress, "the address of %qD will never be NULL",
 			 TREE_OPERAND (op1, 0));
 	    }
+
+	  if (CONVERT_EXPR_P (op1)
+	      && TREE_CODE (TREE_TYPE (TREE_OPERAND (op1, 0)))
+		 == REFERENCE_TYPE)
+	    {
+	      tree inner_op1 = op1;
+	      STRIP_NOPS (inner_op1);
+
+	      if ((complain & tf_warning)
+		  && c_inhibit_evaluation_warnings == 0
+		  && !TREE_NO_WARNING (op1)
+		  && DECL_P (inner_op1))
+		warning (OPT_Waddress,
+			 "the compiler can assume that the address of "
+			 "%qD will never be NULL",
+			 inner_op1);
+	    }
 	}
       else if ((code0 == POINTER_TYPE && code1 == POINTER_TYPE)
 	       || (TYPE_PTRDATAMEM_P (type0) && TYPE_PTRDATAMEM_P (type1)))
diff --git a/gcc/testsuite/g++.dg/warn/Walways-true-3.C b/gcc/testsuite/g++.dg/warn/Walways-true-3.C
new file mode 100644
index 0000000..0d13d3f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Walways-true-3.C
@@ -0,0 +1,45 @@
+// { dg-options "-Waddress" }
+
+void foo (void);
+
+int d;
+int &c = d;
+
+void
+bar (int &a)
+{
+  int &b = a;
+
+  if ((int *)&a) // { dg-warning "address of" }
+    foo ();
+
+  if (&b) // { dg-warning "address of" }
+    foo ();
+
+  if (!&c) // { dg-warning "address of" }
+    foo ();
+
+  if (!&(int &)(int &)a) // { dg-warning "address of" }
+    foo ();
+
+  if (&a == 0) // { dg-warning "never be NULL" }
+    foo ();
+
+  if (&b != 0) // { dg-warning "never be NULL" }
+    foo ();
+
+  if (0 == &(int &)(int &)c) // { dg-warning "never be NULL" }
+    foo ();
+
+  if (&a != (int *)0) // { dg-warning "never be NULL" }
+    foo ();
+}
+
+bool
+bar_1 (int &a)
+{
+  if (d == 5)
+    return &a; // { dg-warning "address of" }
+  else
+    return !&(int &)(int &)a; // { dg-warning "address of" }
+}
-- 
2.4.0.rc2.31.g7c597ef