C++ PATCH for c++/86190, bogus -Wsign-conversion warning

[Marek Polacek <polacek at redhat dot com>]

This PR complains about bogus -Wsign-conversion warning even with an
explicit static_cast.  It started with this hunk from the delayed folding
merge:

@@ -5028,20 +5022,12 @@ cp_build_binary_op (location_t location,
 
       if (short_compare)
    {
-     /* Don't write &op0, etc., because that would prevent op0
-        from being kept in a register.
-        Instead, make copies of the our local variables and
-        pass the copies by reference, then copy them back afterward.  */
-     tree xop0 = op0, xop1 = op1, xresult_type = result_type;
+     /* We call shorten_compare only for diagnostic-reason.  */
+     tree xop0 = fold_simple (op0), xop1 = fold_simple (op1),
+          xresult_type = result_type;
      enum tree_code xresultcode = resultcode;
-     tree val
-       = shorten_compare (location, &xop0, &xop1, &xresult_type,
+     shorten_compare (location, &xop0, &xop1, &xresult_type,
                   &xresultcode);
-     if (val != 0)
-       return cp_convert (boolean_type_node, val, complain);
-     op0 = xop0, op1 = xop1;
-     converted = 1;
-     resultcode = xresultcode;
    }
 
       if ((short_compare || code == MIN_EXPR || code == MAX_EXPR)

which means that converted is now unset so we go to

 5350   if (! converted)
 5351     {
 5352       if (TREE_TYPE (op0) != result_type)
 5353         op0 = cp_convert_and_check (result_type, op0, complain);
 5354       if (TREE_TYPE (op1) != result_type)
 5355         op1 = cp_convert_and_check (result_type, op1, complain);

and cp_convert_and_check gives those warning.  The direct comparison
of types instead of same_type_p means we can try to convert same types,
but it still wouldn't fix this PR.  What we should probably do is to
simply disable -Wsign-conversion conversion for comparison, because
-Wsign-compare will warn for those.  With this patch, the C++ FE will
follow what the C FE and clang++ do.

Also fix some formatting that's been bothering me, while at it.

Bootstrapped/regtested on x86_64-linux, ok for trunk/8?

2018-07-03  Marek Polacek  <polacek@redhat.com>

	PR c++/86190 - bogus -Wsign-conversion warning
	* typeck.c (cp_build_binary_op): Fix formatting.  Add a warning
	sentinel.

	* g++.dg/warn/Wsign-conversion-3.C: New test.
	* g++.dg/warn/Wsign-conversion-4.C: New test.

diff --git gcc/cp/typeck.c gcc/cp/typeck.c
index 3a4f1cdf479..cfd1dd8b150 100644
--- gcc/cp/typeck.c
+++ gcc/cp/typeck.c
@@ -5311,12 +5311,13 @@ cp_build_binary_op (location_t location,
 
       if (short_compare)
 	{
-	  /* We call shorten_compare only for diagnostic-reason.  */
-	  tree xop0 = fold_simple (op0), xop1 = fold_simple (op1),
-	       xresult_type = result_type;
+	  /* We call shorten_compare only for diagnostics.  */
+	  tree xop0 = fold_simple (op0);
+	  tree xop1 = fold_simple (op1);
+	  tree xresult_type = result_type;
 	  enum tree_code xresultcode = resultcode;
 	  shorten_compare (location, &xop0, &xop1, &xresult_type,
-			       &xresultcode);
+			   &xresultcode);
 	}
 
       if ((short_compare || code == MIN_EXPR || code == MAX_EXPR)
@@ -5349,6 +5350,7 @@ cp_build_binary_op (location_t location,
      otherwise, it will be given type RESULT_TYPE.  */
   if (! converted)
     {
+      warning_sentinel w (warn_sign_conversion, short_compare);
       if (TREE_TYPE (op0) != result_type)
 	op0 = cp_convert_and_check (result_type, op0, complain);
       if (TREE_TYPE (op1) != result_type)
diff --git gcc/testsuite/g++.dg/warn/Wsign-conversion-3.C gcc/testsuite/g++.dg/warn/Wsign-conversion-3.C
index e69de29bb2d..2c3fef31475 100644
--- gcc/testsuite/g++.dg/warn/Wsign-conversion-3.C
+++ gcc/testsuite/g++.dg/warn/Wsign-conversion-3.C
@@ -0,0 +1,13 @@
+// PR c++/86190
+// { dg-options "-Wsign-conversion -Wsign-compare" }
+
+typedef unsigned long sz_t;
+sz_t s();
+bool f(int i) { return s() < (unsigned long) i; }
+bool f2(int i) { return s() < static_cast<unsigned long>(i); }
+bool f3(int i) { return s() < i; } // { dg-warning "comparison of integer expressions of different signedness" }
+bool f4(int i) { return s() < (long) i; } // { dg-warning "comparison of integer expressions of different signedness" }
+bool f5(short int i) { return s() < (int) i; } // { dg-warning "comparison of integer expressions of different signedness" }
+bool f6(signed char i) { return s() < (int) i; } // { dg-warning "comparison of integer expressions of different signedness" }
+bool f7(unsigned char i) { return s() < i; }
+bool f8(signed char i) { return s() < i; } // { dg-warning "comparison of integer expressions of different signedness" }
diff --git gcc/testsuite/g++.dg/warn/Wsign-conversion-4.C gcc/testsuite/g++.dg/warn/Wsign-conversion-4.C
index e69de29bb2d..40814b95587 100644
--- gcc/testsuite/g++.dg/warn/Wsign-conversion-4.C
+++ gcc/testsuite/g++.dg/warn/Wsign-conversion-4.C
@@ -0,0 +1,14 @@
+// PR c++/86190
+// { dg-options "-Wsign-conversion -Wsign-compare" }
+
+typedef unsigned long size_t;
+
+struct vector {
+  typedef size_t size_type;
+  size_type size();
+};
+
+bool func(vector vec, int var)
+{
+  return vec.size() < static_cast<size_t>(var); // { dg-bogus "may change" }
+}