diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -236,17 +236,33 @@
 #endif
 
 #if !__has_extension(c_static_assert)
-#if (defined(__cplusplus) && __cplusplus >= 201103L) || \
-    __has_extension(cxx_static_assert)
+#if defined(__cplusplus)
+/*
+ * For C++ case, only define _Static_assert for C++ 2011 (or newer) or if the
+ * compiler has the cxx_static_assert extension. This is only to suppert C11+
+ * code in headers that are included with C++ code.  We don't define it for
+ * older versions of C++ because they have no _Static_assert or static_assert
+ * macros in the language and if either are implemented as macros, they break
+ * when used with templates (since it looks to CPP that the macro is invoked
+ * with more than 2 arguments).
+ */
+#if __cplusplus >= 201103L || __has_extension(cxx_static_assert)
 #define	_Static_assert(x, y)	static_assert(x, y)
-#elif __GNUC_PREREQ__(4,6) && !defined(__cplusplus)
+#endif
+#elif __GNUC_PREREQ__(4,6)
 /* Nothing, gcc 4.6 and higher has _Static_assert built-in */
 #elif defined(__COUNTER__)
+/*
+ * Fallback for really old C compilers, using the __COUNTER__ hack.
+ */
 #define	_Static_assert(x, y)	__Static_assert(x, __COUNTER__)
 #define	__Static_assert(x, y)	___Static_assert(x, y)
 #define	___Static_assert(x, y)	typedef char __assert_ ## y[(x) ? 1 : -1] \
 				__unused
 #else
+/*
+ * Fallback for even older C compilers that don't implement that hack.
+ */
 #define	_Static_assert(x, y)	struct __hack
 #endif
 #endif