Improve RTC_DCHECK_op so that it won't trigger useless compiler warnings

webrtc/base/checks.h

 /*
  *  Copyright 2006 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 65 matching lines @@
 // TODO(ajm): Ideally, checks.h would be combined with logging.h, but
 // consolidation with system_wrappers/logging.h should happen first.
 
 namespace rtc {
 
 // Helper macro which avoids evaluating the arguments to a stream if
 // the condition doesn't hold.
 #define RTC_LAZY_STREAM(stream, condition)                                    \
   !(condition) ? static_cast<void>(0) : rtc::FatalMessageVoidify() & (stream)
 
-// The actual stream used isn't important. We reference condition in the code
+// The actual stream used isn't important. We reference |ignored| in the code
 // but don't evaluate it; this is to avoid "unused variable" warnings (we do so
 // in a particularly convoluted way with an extra ?: because that appears to be
 // the simplest construct that keeps Visual Studio from complaining about
 // condition being unused).
-#define RTC_EAT_STREAM_PARAMETERS(condition) \
-  (true ? true : !(condition))               \
-      ? static_cast<void>(0)                 \
+#define RTC_EAT_STREAM_PARAMETERS(ignored) \
+  (true ? true : ((void)(ignored), true))  \
+      ? static_cast<void>(0)               \
       : rtc::FatalMessageVoidify() & rtc::FatalMessage("", 0).stream()
 
+// Call RTC_EAT_STREAM_PARAMETERS with an argument that fails to compile if
+// values of the same types as |a| and |b| can't be compared with the given
+// operation, and that would evaluate |a| and |b| if evaluated.
+#define RTC_EAT_STREAM_PARAMETERS_OP(op, a, b)                                 \
+  RTC_EAT_STREAM_PARAMETERS(((void)sizeof(std::declval<decltype(a)>()          \
+                                              op std::declval<decltype(b)>()), \
+                             (void)(a), (void)(b)))
+
 // RTC_CHECK dies with a fatal error if condition is not true. It is *not*
 // controlled by NDEBUG or anything else, so the check will be executed
 // regardless of compilation mode.
 //
 // We make sure RTC_CHECK et al. always evaluates their arguments, as
 // doing RTC_CHECK(FunctionWithSideEffect()) is a common idiom.
 #define RTC_CHECK(condition)                                      \
   RTC_LAZY_STREAM(rtc::FatalMessage(__FILE__, __LINE__).stream(), \
                   !(condition))                                   \
       << "Check failed: " #condition << std::endl << "# "
@@ skipping 80 matching lines @@
 #if RTC_DCHECK_IS_ON
 #define RTC_DCHECK(condition) RTC_CHECK(condition)
 #define RTC_DCHECK_EQ(v1, v2) RTC_CHECK_EQ(v1, v2)
 #define RTC_DCHECK_NE(v1, v2) RTC_CHECK_NE(v1, v2)
 #define RTC_DCHECK_LE(v1, v2) RTC_CHECK_LE(v1, v2)
 #define RTC_DCHECK_LT(v1, v2) RTC_CHECK_LT(v1, v2)
 #define RTC_DCHECK_GE(v1, v2) RTC_CHECK_GE(v1, v2)
 #define RTC_DCHECK_GT(v1, v2) RTC_CHECK_GT(v1, v2)
 #else
 #define RTC_DCHECK(condition) RTC_EAT_STREAM_PARAMETERS(condition)
-#define RTC_DCHECK_EQ(v1, v2) RTC_EAT_STREAM_PARAMETERS((v1) == (v2))
-#define RTC_DCHECK_NE(v1, v2) RTC_EAT_STREAM_PARAMETERS((v1) != (v2))
-#define RTC_DCHECK_LE(v1, v2) RTC_EAT_STREAM_PARAMETERS((v1) <= (v2))
-#define RTC_DCHECK_LT(v1, v2) RTC_EAT_STREAM_PARAMETERS((v1) < (v2))
-#define RTC_DCHECK_GE(v1, v2) RTC_EAT_STREAM_PARAMETERS((v1) >= (v2))
-#define RTC_DCHECK_GT(v1, v2) RTC_EAT_STREAM_PARAMETERS((v1) > (v2))
+#define RTC_DCHECK_EQ(v1, v2) RTC_EAT_STREAM_PARAMETERS_OP(==, v1, v2)
+#define RTC_DCHECK_NE(v1, v2) RTC_EAT_STREAM_PARAMETERS_OP(!=, v1, v2)
+#define RTC_DCHECK_LE(v1, v2) RTC_EAT_STREAM_PARAMETERS_OP(<=, v1, v2)
+#define RTC_DCHECK_LT(v1, v2) RTC_EAT_STREAM_PARAMETERS_OP(<, v1, v2)
+#define RTC_DCHECK_GE(v1, v2) RTC_EAT_STREAM_PARAMETERS_OP(>=, v1, v2)
+#define RTC_DCHECK_GT(v1, v2) RTC_EAT_STREAM_PARAMETERS_OP(>, v1, v2)
 #endif
 
 // This is identical to LogMessageVoidify but in name.
 class FatalMessageVoidify {
  public:
   FatalMessageVoidify() { }
   // This has to be an operator with a precedence lower than << but
   // higher than ?:
   void operator&(std::ostream&) { }
 };
@@ skipping 61 matching lines @@
 #define RTC_DCHECK_EQ(a, b) RTC_DCHECK((a) == (b))
 #define RTC_DCHECK_NE(a, b) RTC_DCHECK((a) != (b))
 #define RTC_DCHECK_LE(a, b) RTC_DCHECK((a) <= (b))
 #define RTC_DCHECK_LT(a, b) RTC_DCHECK((a) < (b))
 #define RTC_DCHECK_GE(a, b) RTC_DCHECK((a) >= (b))
 #define RTC_DCHECK_GT(a, b) RTC_DCHECK((a) > (b))
 
 #endif  // __cplusplus
 
 #endif  // WEBRTC_BASE_CHECKS_H_