mojo: Use static_assert for MOJO_COMPILE_ASSERT in C++11 mode.

mojo/public/c/system/macros.h

 // Copyright 2014 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 MOJO_PUBLIC_C_SYSTEM_MACROS_H_
 #define MOJO_PUBLIC_C_SYSTEM_MACROS_H_
 
 #include <stddef.h>
 
 // Annotate a variable indicating it's okay if it's unused.
 // Use like:
 //   int x MOJO_ALLOW_UNUSED = ...;
 #if defined(__GNUC__)
 #define MOJO_ALLOW_UNUSED __attribute__((unused))

[Nico, 2014/05/04 01:32:41]

(Note that base's ignore_result() is a better way to handle this most of the
time.)

 #else
 #define MOJO_ALLOW_UNUSED
 #endif
 
 // Annotate a function indicating that the caller must examine the return value.
 // Use like:
 //   int foo() MOJO_WARN_UNUSED_RESULT;
 // Note that it can only be used on the prototype, and not the definition.
 #if defined(__GNUC__)
 #define MOJO_WARN_UNUSED_RESULT __attribute__((warn_unused_result))
 #else
 #define MOJO_WARN_UNUSED_RESULT
 #endif
 
 // This macro is currently C++-only, but we want to use it in the C core.h.
-#ifdef __cplusplus
 // Used to assert things at compile time.
+#if __cplusplus >= 201103L

[darin (slow to review), 2014/05/04 03:15:15]

this header is intended to be seen by a C compiler as well. __cplusplus would
not be defined in that case.

[Nico, 2014/05/04 04:20:12]

I think that's fine; undefined symbols evaluate to 0 in comparisons as far as I
can tell:

thakis$ cat macrotest.c 
#if __cplusplus >= 201103L
#error c++11
#elif defined(__cplusplus)
#error c++
#else
#error c
#endif

thakis$ clang -c macrotest.c 
macrotest.c:6:2: error: c
#error c
 ^
1 error generated.


If you don't have that yet, you should add a target with a .c file that includes
your c-compatible headers, so that the trybots will notice if someone checks in
something that breaks c compat. (But I think this change should be fine as is.)


(more tests:


thakis$ cat macrotest.c 
#if asdfasfasfasdf >= 0
#error yes
#else
#error no
#endif
thakis$ clang -c macrotest.c 
macrotest.c:2:2: error: yes
#error yes
 ^
1 error generated.

and
thakis$ cat macrotest.c 
#if asdfasfasfasdf > 0
#error yes
#else
#error no
#endif
thakis$ clang -c macrotest.c 
macrotest.c:4:2: error: no
#error no
 ^
1 error generated.
)

+#define MOJO_COMPILE_ASSERT(expr, msg) static_assert(expr, #msg)
+#elif defined(__cplusplus)
 namespace mojo { template <bool> struct CompileAssert {}; }
 #define MOJO_COMPILE_ASSERT(expr, msg) \
     typedef ::mojo::CompileAssert<(bool(expr))> msg[bool(expr) ? 1 : -1]
 #else
 #define MOJO_COMPILE_ASSERT(expr, msg)
 #endif
 
 #endif  // MOJO_PUBLIC_C_SYSTEM_MACROS_H_