Fix excess warnings from -Wtype-limits with location wrappers (PR c++/88680)

PR c++/88680 reports excess warnings from -Wtype-limits after the C++
FE's use of location wrappers was extended in r267272 for cases such as:

  const unsigned n = 8;
  static_assert (n >= 0 && n % 2 == 0, "");

t.C:3:18: warning: comparison of unsigned expression >= 0 is always true
  [-Wtype-limits]
    3 | static_assert (n >= 0 && n % 2 == 0, "");
      |                ~~^~~~

The root cause is that the location wrapper around "n" breaks the
suppression of the warning for the "if OP0 is a constant that is >= 0"
case.

This patch fixes it by calling fold_for_warn on OP0, extracting the
constant.

gcc/c-family/ChangeLog:
	PR c++/88680
	* c-common.c (shorten_compare): Call fold_for_warn on op0 when
	implementing -Wtype-limits.

gcc/testsuite/ChangeLog:
	PR c++/88680
	* g++.dg/wrappers/pr88680.C: New test.

From-SVN: r268961

4 files changed, 77 insertions, 3 deletions

diff --git a/gcc/c-family/ChangeLog b/gcc/c-family/ChangeLog
index 7dae7bf..ed8026a 100644
--- a/gcc/c-family/ChangeLog
+++ b/gcc/c-family/ChangeLog
@@ -1,3 +1,9 @@
+2019-02-16  David Malcolm  <dmalcolm@redhat.com>
+
+	PR c++/88680
+	* c-common.c (shorten_compare): Call fold_for_warn on op0 when
+	implementing -Wtype-limits.
+
 2019-02-11  Martin Sebor  <msebor@redhat.com>
 
 	PR c++/87996
diff --git a/gcc/c-family/c-common.c b/gcc/c-family/c-common.c
index ae23e59..c6856c9 100644
--- a/gcc/c-family/c-common.c
+++ b/gcc/c-family/c-common.c
@@ -3117,6 +3117,12 @@ shorten_compare (location_t loc, tree *op0_ptr, tree *op1_ptr,
       primop0 = op0;
       primop1 = op1;
 
+      /* We want to fold unsigned comparisons of >= and < against zero.
+	 For these, we may also issue a warning if we have a non-constant
+	 compared against zero, where the zero was spelled as "0" (rather
+	 than merely folding to it).
+	 If we have at least one constant, then op1 is constant
+	 and we may have a non-constant expression as op0.  */
       if (!real1 && !real2 && integer_zerop (primop1)
 	  && TYPE_UNSIGNED (*restype_ptr))
 	{
@@ -3125,13 +3131,14 @@ shorten_compare (location_t loc, tree *op0_ptr, tree *op1_ptr,
 	     if OP0 is a constant that is >= 0, the signedness of
 	     the comparison isn't an issue, so suppress the
 	     warning.  */
+	  tree folded_op0 = fold_for_warn (op0);
 	  bool warn = 
 	    warn_type_limits && !in_system_header_at (loc)
-	    && !(TREE_CODE (primop0) == INTEGER_CST
+	    && !(TREE_CODE (folded_op0) == INTEGER_CST
 		 && !TREE_OVERFLOW (convert (c_common_signed_type (type),
-					     primop0)))
+					     folded_op0)))
 	    /* Do not warn for enumeration types.  */
-	    && (TREE_CODE (expr_original_type (primop0)) != ENUMERAL_TYPE);
+	    && (TREE_CODE (expr_original_type (folded_op0)) != ENUMERAL_TYPE);
 	  
 	  switch (code)
 	    {
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index e68f969..45fbe7c 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2019-02-16  David Malcolm  <dmalcolm@redhat.com>
+
+	PR c++/88680
+	* g++.dg/wrappers/pr88680.C: New test.
+
 2019-02-17  Thomas Koenig  <tkoenig@gcc.gnu.org>
 
 	PR fortran/71066
diff --git a/gcc/testsuite/g++.dg/wrappers/pr88680.C b/gcc/testsuite/g++.dg/wrappers/pr88680.C
new file mode 100644
index 0000000..5497cda
--- /dev/null
+++ b/gcc/testsuite/g++.dg/wrappers/pr88680.C
@@ -0,0 +1,56 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wtype-limits" }
+
+const unsigned N = 8;
+const unsigned P = 0;
+
+enum { FOO, BAR };
+
+static_assert (N >= 0 && N % 2 == 0, "");
+static_assert (FOO >= 0, "");
+static_assert (FOO >= FOO, "");
+static_assert (FOO >= P, "");
+static_assert (BAR >= P, "");
+static_assert (N >= FOO, "");
+
+void test(unsigned n)
+{
+  if (N >= 0 && N % 2 == 0)
+    return;
+  if (FOO >= 0)
+    return;
+  if (FOO >= FOO)
+    return;
+  if (FOO >= P)
+    return;
+  if (BAR >= P)
+    return;
+  if (N >= FOO)
+    return;
+  if (n >= 0) // { dg-warning ">= 0 is always true" }
+    return;
+  if (n < 0) // { dg-warning "< 0 is always false" }
+    return;
+  if (n >= FOO)
+    return;
+  if (n < FOO)
+    return;
+  if (N >= 0)
+    return;
+  if (N < 0)
+    return;
+  if (N >= FOO)
+    return;
+  if (N < FOO)
+    return;
+  if (0 <= FOO)
+    return;
+  if (0 <= n) // { dg-warning ">= 0 is always true" }
+    return;
+  if (0 > n) // { dg-warning "< 0 is always false" }
+    return;
+  if (N <= FOO)
+    return;
+  if (N <= n)
+    return;
+}