Simplify ALLOW_UNUSED_LOCAL to use (void)x directly instead of conditionally.

base/compiler_specific.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_COMPILER_SPECIFIC_H_
 #define BASE_COMPILER_SPECIFIC_H_
 
 #include "build/build_config.h"
 
 #if defined(COMPILER_MSVC)
@@ skipping 53 matching lines @@
 #define MSVC_SUPPRESS_WARNING(n)
 #define MSVC_PUSH_DISABLE_WARNING(n)
 #define MSVC_PUSH_WARNING_LEVEL(n)
 #define MSVC_POP_WARNING()
 #define MSVC_DISABLE_OPTIMIZE()
 #define MSVC_ENABLE_OPTIMIZE()
 #define NON_EXPORTED_BASE(code) code
 
 #endif  // COMPILER_MSVC
 
-
 // Annotate a variable indicating it's ok if the variable is not used.
 // (Typically used to silence a compiler warning when the assignment
 // is important for some other reason.)
 // Use like:
 //   int x = ...;
 //   ALLOW_UNUSED_LOCAL(x);
+//
+// The common-case implementation references |x| in such a way that it's never
+// evaluated.  Static analysis builds still see this as not referencing |x|, so
+// in such cases we reference it directly to make the compiler happy, at the
+// risk of unintended side effects.
+#if defined(__clang_analyzer__)
+
+#define ALLOW_UNUSED_LOCAL(x) (void)x
+
+#else  // !defined(__clang_analyzer__)
+
 #define ALLOW_UNUSED_LOCAL(x) false ? (void)x : (void)0

[Nico, 2017/04/25 15:48:52]

What was the problem with making this use `(void)x` again?

[Kevin Marshall, 2017/04/25 17:17:31]

When I asked pkasting (author of the original #define), he said the ternary
expression guarantees no side effects as a result of storing 'x'. I don't the
specifics about which types are affected.

 
+#endif  // defined(__clang_analyzer__)
+
 // Annotate a typedef or function indicating it's ok if it's not used.
 // Use like:
 //   typedef Foo Bar ALLOW_UNUSED_TYPE;
 #if defined(COMPILER_GCC) || defined(__clang__)
 #define ALLOW_UNUSED_TYPE __attribute__((unused))
 #else
 #define ALLOW_UNUSED_TYPE
 #endif
 
 // Annotate a function indicating it should not be inlined.
@@ skipping 131 matching lines @@
 
 // Compiler feature-detection.
 // clang.llvm.org/docs/LanguageExtensions.html#has-feature-and-has-extension
 #if defined(__has_feature)
 #define HAS_FEATURE(FEATURE) __has_feature(FEATURE)
 #else
 #define HAS_FEATURE(FEATURE) 0
 #endif
 
 #endif  // BASE_COMPILER_SPECIFIC_H_