Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -849,9 +849,11 @@
     _LIBCPP_ALWAYS_INLINE explicit x(int __v) : __v_(static_cast<__lx>(__v)) {} \
     _LIBCPP_ALWAYS_INLINE operator int() const {return __v_;} \
     };
+#define _LIBCPP_UNDERLYING_ENUM_TYPE(t)
 #else  // _LIBCPP_HAS_NO_STRONG_ENUMS
 #define _LIBCPP_DECLARE_STRONG_ENUM(x) enum class _LIBCPP_ENUM_VIS x
 #define _LIBCPP_DECLARE_STRONG_ENUM_EPILOG(x)
+#define _LIBCPP_UNDERLYING_ENUM_TYPE(t) : t
 #endif  // _LIBCPP_HAS_NO_STRONG_ENUMS
 
 #ifdef _LIBCPP_DEBUG
Index: include/string
===================================================================
--- include/string
+++ include/string
@@ -680,7 +680,7 @@
     enum {__long_mask  = 0x1ul};
 #else  // _LIBCPP_BIG_ENDIAN
     enum {__short_mask = 0x80};
-    enum {__long_mask  = ~(size_type(~0) >> 1)};
+    enum _LIBCPP_UNDERLYING_ENUM_TYPE(size_type) {__long_mask  = ~(size_type(~0) >> 1)};
 #endif  // _LIBCPP_BIG_ENDIAN
 
     enum {__min_cap = (sizeof(__long) - 1)/sizeof(value_type) > 2 ?
@@ -707,7 +707,7 @@
 
 #if _LIBCPP_BIG_ENDIAN
     enum {__short_mask = 0x80};
-    enum {__long_mask  = ~(size_type(~0) >> 1)};
+    enum _LIBCPP_UNDERLYING_ENUM_TYPE(size_type) {__long_mask  = ~(size_type(~0) >> 1)};
 #else  // _LIBCPP_BIG_ENDIAN
     enum {__short_mask = 0x01};
     enum {__long_mask  = 0x1ul};