diff --git a/CMakeLists.txt b/CMakeLists.txt
index 64c6c31c4..d0ed9df5e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -322,10 +322,19 @@ add_compile_flags_if_supported(-nostdinc++)
 # Warning flags ===============================================================
 add_definitions(-D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 add_compile_flags_if_supported(
-    -Wall -W -Wwrite-strings
-    -Wno-unused-parameter -Wno-long-long -Wno-user-defined-literals
-    -Wno-covered-switch-default
+    -Wall -Wextra -W -Wwrite-strings
+    -Wno-unused-parameter -Wno-long-long
     -Werror=return-type)
+if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
+    add_compile_flags_if_supported(
+        -Wno-user-defined-literals
+        -Wno-covered-switch-default)
+elseif("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU")
+    add_compile_flags_if_supported(
+        -Wno-attributes # FIXME: Fix -Wattribute warnings.
+        -Wno-literal-suffix
+        -Wno-c++14-compat)
+endif()
 if (LIBCXX_ENABLE_WERROR)
   add_compile_flags_if_supported(-Werror)
   add_compile_flags_if_supported(-WX)
diff --git a/include/__config b/include/__config
index 353ca10e4..bd3f59a27 100644
--- a/include/__config
+++ b/include/__config
@@ -691,6 +691,12 @@ template <unsigned> struct __static_assert_check {};
 #define _NOALIAS
 #endif
 
+#ifdef __GNUC__
+#define _LIBCPP_MAY_ALIAS __attribute__((__may_alias__))
+#else
+#define _LIBCPP_MAY_ALIAS
+#endif
+
 #if __has_feature(cxx_explicit_conversions) || defined(__IBMCPP__)
 #   define _LIBCPP_EXPLICIT explicit
 #else
diff --git a/include/__tuple b/include/__tuple
index 6d978a2dc..caa427aa5 100644
--- a/include/__tuple
+++ b/include/__tuple
@@ -374,7 +374,7 @@ template <bool ..._Preds>
 struct __all_dummy;
 
 template <bool ..._Pred>
-using __all = is_same<__all_dummy<_Pred...>, __all_dummy<(_Pred, true)...>>;
+using __all = is_same<__all_dummy<_Pred...>, __all_dummy<((void)_Pred, true)...>>;
 
 struct __tuple_sfinae_base {
   template <template <class, class...> class _Trait,
diff --git a/include/cstdlib b/include/cstdlib
index 10ed23107..9a775b844 100644
--- a/include/cstdlib
+++ b/include/cstdlib
@@ -89,6 +89,12 @@ void *aligned_alloc(size_t alignment, size_t size);                       // C11
 #pragma GCC system_header
 #endif
 
+#ifdef __GNUC__
+#define _LIBCPP_UNREACHABLE() __builtin_unreachable()
+#else
+#define _LIBCPP_UNREACHABLE() _VSTD::abort()
+#endif
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 using ::size_t;
diff --git a/include/typeinfo b/include/typeinfo
index fe9123e82..d3155e459 100644
--- a/include/typeinfo
+++ b/include/typeinfo
@@ -73,6 +73,9 @@ class _LIBCPP_EXCEPTION_ABI type_info
 {
     type_info& operator=(const type_info&);
     type_info(const type_info&);
+
+    typedef size_t _LIBCPP_MAY_ALIAS _ASizeT; // Avoid strict-aliasing issues.
+
 protected:
 #ifndef _LIBCPP_NONUNIQUE_RTTI_BIT
     const char* __type_name;
@@ -113,7 +116,7 @@ public:
     _LIBCPP_INLINE_VISIBILITY
     size_t hash_code() const _NOEXCEPT
 #ifndef _LIBCPP_NONUNIQUE_RTTI_BIT
-        {return *reinterpret_cast<const size_t*>(&__type_name);}
+        {return *reinterpret_cast<const _ASizeT *>(&__type_name);}
 #else
         {if (!(__type_name & _LIBCPP_NONUNIQUE_RTTI_BIT)) return __type_name;
          const char *__ptr = name();
diff --git a/src/strstream.cpp b/src/strstream.cpp
index 0e2d7ff21..be94f9c89 100644
--- a/src/strstream.cpp
+++ b/src/strstream.cpp
@@ -11,6 +11,7 @@
 #include "algorithm"
 #include "climits"
 #include "cstring"
+#include "cstdlib"
 #include "__debug"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
@@ -266,6 +267,8 @@ strstreambuf::seekoff(off_type __off, ios_base::seekdir __way, ios_base::openmod
         case ios::end:
             newoff = seekhigh - eback();
             break;
+        default:
+            _LIBCPP_UNREACHABLE();
         }
         newoff += __off;
         if (0 <= newoff && newoff <= seekhigh - eback())