C++ PATCH for c++/87519 - bogus warning with -Wsign-conversion

[Marek Polacek]

This PR complains about bogus -Wsign-conversion warning even with an
explicit cast, which the documentation says should suppress the warning.
Pretty similar to c++/86190 which I fixed here
<https://gcc.gnu.org/ml/gcc-patches/2018-07/msg00115.html>.

The problem only happens when using a typedef, because in cp_build_binary_op
we're comparing the types using ==, which I think only works for canonical
types.  So use same_type_p to avoid this problem.

Bootstrapped/regtested on x86_64-linux, ok for trunk?  I think it makes sense
to fix in in 9.3 too.

2019-08-08  Marek Polacek  <pola...@redhat.com>

        PR c++/87519 - bogus warning with -Wsign-conversion.
        * typeck.c (cp_build_binary_op): Use same_type_p instead of comparing
        the types directly.

        * g++.dg/warn/Wsign-conversion-5.C: New test.

diff --git gcc/cp/typeck.c gcc/cp/typeck.c
index 7e4ea3bee75..4cc0ee0128d 100644
--- gcc/cp/typeck.c
+++ gcc/cp/typeck.c
@@ -5509,9 +5509,9 @@ cp_build_binary_op (const op_location_t &location,
   if (! converted)
     {
       warning_sentinel w (warn_sign_conversion, short_compare);
-      if (TREE_TYPE (op0) != result_type)
+      if (!same_type_p (TREE_TYPE (op0), result_type))
        op0 = cp_convert_and_check (result_type, op0, complain);
-      if (TREE_TYPE (op1) != result_type)
+      if (!same_type_p (TREE_TYPE (op1), result_type))
        op1 = cp_convert_and_check (result_type, op1, complain);
 
       if (op0 == error_mark_node || op1 == error_mark_node)
diff --git gcc/testsuite/g++.dg/warn/Wsign-conversion-5.C 
gcc/testsuite/g++.dg/warn/Wsign-conversion-5.C
new file mode 100644
index 00000000000..ff384164901
--- /dev/null
+++ gcc/testsuite/g++.dg/warn/Wsign-conversion-5.C
@@ -0,0 +1,18 @@
+// PR c++/87519 - bogus warning with -Wsign-conversion.
+// { dg-options "-Wsign-conversion" }
+
+typedef unsigned long int uint64_t;
+
+void f(unsigned long int a, int q)
+{
+  a += a + q; // { dg-warning "may change the sign" }
+
+  // Explicit cast should disable the warning.
+  a = a + static_cast<uint64_t>(q);
+  a = a + (uint64_t) q;
+  a = a + uint64_t(q);
+  a = a + static_cast<const uint64_t>(q);
+  a = a + (const uint64_t) q;
+  a = a + static_cast<unsigned long int>(q);
+  a = a + static_cast<const unsigned long int>(q);
+}