Enable type_traits fallback for all < gcc 5.0 releases.

base/template_util.h

Index: base/template_util.h
diff --git a/base/template_util.h b/base/template_util.h
index 80a24d1722e2c466e31d2e09f483a4a73a45e90b..56da87555d2e28ca34f7841d552faf8472dad777 100644
--- a/base/template_util.h
+++ b/base/template_util.h
@@ -24,13 +24,12 @@
 #endif
 
 // Some versions of libstdc++ have partial support for type_traits, but misses
-// a smaller subset while removing some of the older non-standard stuff.
-#define CR_GLIBCXX_4_8_4 20141219
-#define CR_GLIBCXX_4_9_2 20150426
+// a smaller subset while removing some of the older non-standard stuff. Assume
+// that all versions below 5.0 fall in this category, along with one 5.0
+// experimental release.
 #define CR_GLIBCXX_5_0_0 20150123
-#if defined(__GLIBCXX__) &&                                                \
-    (__GLIBCXX__ == CR_GLIBCXX_4_8_4 || __GLIBCXX__ == CR_GLIBCXX_4_9_2 || \
-     __GLIBCXX__ == CR_GLIBCXX_5_0_0)
+#if (defined(__GNUC__) && __GNUC_ < 5) || \

[danakj, 2017/01/05 16:30:15]

I agree that it's odd to check the compiler version not the library version
here. If some toolchain is using an older gcc but a newer library, then they
will fall back to our own implementation of is_trivially_copyable. But I think
a) using a new lib with older compiler is probably not supported and wouldn't
work anyways b) falling back in this case still does something reasonable..ish.

Can you improve the comment to clarify about the mismatch between checking
compiler vs library versions.

[sof, 2017/01/05 18:59:44]

ok, tried to do so.

ABI skew is one reason why it makes some sense to compile with a consistent
(compiler,library) pairing, esp so with gcc5's changes in that area (
https://gcc.gnu.org/gcc-5/changes.html#libstdcxx )

The version testing here mirrors how we do it on the clang side, btw --

 http://llvm.org/docs/doxygen/html/type__traits_8h_source.html#l00030

 

+    (defined(__GLIBCXX__) && __GLIBCXX__ == CR_GLIBCXX_5_0_0)
 #define CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX
 #endif