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

mojo/public/c/system/macros.h

Index: mojo/public/c/system/macros.h
diff --git a/mojo/public/c/system/macros.h b/mojo/public/c/system/macros.h
index 4be024900aedb48c87de3865161c8338520df0ad..322c3be61f7cb8f7d9cac345770504537964df97 100644
--- a/mojo/public/c/system/macros.h
+++ b/mojo/public/c/system/macros.h
@@ -27,8 +27,10 @@
 #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]