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..84e2563e34962819b4a8cca03f1a14af1953c460 100644
--- a/base/template_util.h
+++ b/base/template_util.h
@@ -24,13 +24,11 @@
 #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 _GNUC_VER < 500 || (defined(__GLIBCXX__) && __GLIBCXX__ == CR_GLIBCXX_5_0_0)

[Mostyn Bramley-Moore, 2017/01/05 10:53:54]

Where is _GNUC_VER defined?  It doesn't appear to be predefined in my tests with
g++ 4.8 and 5.4.0.

Also, I think this feature is libstdc++ thing, which is not strictly coupled to
the compiler version (ie you can use a brand new libstdc++ with an old compiler
just fine).

[sof, 2017/01/05 10:57:38]

It's used below in this file, so I assume it is well-defined and safe to use in
this context?

As we're testing against older libstdc++ here, your concern doesn't seem to
hold. You could use a newer g++, but why would you want to use an old libstdc++
with it?

[Mostyn Bramley-Moore, 2017/01/05 12:26:32]

It looks like _GNUC_VER is a libcxx macro, and since it's undefined here then
the value is 0 in libstdc++ builds.  Which would mean that the __GLIBCXX__
checks are possibly never used.
I don't personally use newer g++ with older libstdc++, but I know that Google
does this (or did at some point), in order to produce chrome builds for eg old
debian releases.

[sof, 2017/01/05 12:35:28]

It's been subsequently updated to only rely on __GNUC__; we're only after the
major version here after all.

 #define CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX
 #endif