[PATCH] Another -Wnonnull-compare false positive fix (PR c++/69902)

[Jakub Jelinek]

Hi!

Here is a fix for another -Wnonnull-compare false positive - the problem
is that during folding the NE_EXPR of a nonnull_arg_p with NULL (on which
the C++ FE set TREE_NO_WARNING, because it is an artificial comparison
for dynamic_cast) is changed by fold-const.c into EQ_EXPR, and the
TREE_NO_WARNING flag is lost.  Unfortunately it is deep enough in an
expression passed to fold that cp_fold can't easily tweak that.

Fixed thusly, bootstrapped/regtested on x86_64-linux and i686-linux, ok for
trunk?

2016-02-22  Jakub Jelinek  <ja...@redhat.com>

        PR c++/69902
        * fold-const.c (fold_truth_not_expr): Propagate TREE_NO_WARNING
        when inverting comparison.

        * g++.dg/warn/Wnonnull-compare-5.C: New test.

--- gcc/fold-const.c.jj 2016-02-19 08:55:05.000000000 +0100
+++ gcc/fold-const.c    2016-02-22 17:46:26.870468937 +0100
@@ -3589,8 +3589,11 @@ fold_truth_not_expr (location_t loc, tre
       if (code == ERROR_MARK)
        return NULL_TREE;
 
-      return build2_loc (loc, code, type, TREE_OPERAND (arg, 0),
-                        TREE_OPERAND (arg, 1));
+      tree ret = build2_loc (loc, code, type, TREE_OPERAND (arg, 0),
+                            TREE_OPERAND (arg, 1));
+      if (TREE_NO_WARNING (arg))
+       TREE_NO_WARNING (ret) = 1;
+      return ret;
     }
 
   switch (code)
--- gcc/testsuite/g++.dg/warn/Wnonnull-compare-5.C.jj   2016-02-22 
17:48:16.996963704 +0100
+++ gcc/testsuite/g++.dg/warn/Wnonnull-compare-5.C      2016-02-22 
17:56:58.235839294 +0100
@@ -0,0 +1,18 @@
+// PR c++/69902
+// { dg-do compile }
+// { dg-options "-Wall" }
+
+struct A { virtual ~A (); };
+struct B : A {};
+
+bool
+foo (A &a)
+{
+  return dynamic_cast<B *>(&a) == (B *) 0;     // { dg-bogus "nonnull 
argument" }
+}
+
+bool
+bar (A &a)
+{
+  return dynamic_cast<B *>(&a) != (B *) 0;     // { dg-bogus "nonnull 
argument" }
+}

        Jakub