Index: libcxx/include/__std_stream
===================================================================
--- libcxx/include/__std_stream
+++ libcxx/include/__std_stream
@@ -83,6 +83,12 @@
     __cv_ = &use_facet<codecvt<char_type, char, state_type> >(__loc);
     __encoding_ = __cv_->encoding();
     __always_noconv_ = __cv_->always_noconv();
+#ifdef _WIN32
+    if constexpr (sizeof(char_type) == sizeof(wchar_t)) {
+        __encoding_ = 1;
+        __always_noconv_ = true;
+    }
+#endif
     if (__encoding_ > __limit)
         __throw_runtime_error("unsupported locale for standard input");
 }
@@ -115,19 +121,39 @@
         }
         return __result;
     }
+#ifdef _WIN32
+    char_type __extbuf[__limit];
+#else
     char __extbuf[__limit];
+#endif
     int __nread = _VSTD::max(1, __encoding_);
     for (int __i = 0; __i < __nread; ++__i)
     {
-        int __c = getc(__file_);
-        if (__c == EOF)
-            return traits_type::eof();
-        __extbuf[__i] = static_cast<char>(__c);
+#ifdef _WIN32
+        if constexpr (sizeof(char_type) == sizeof(wchar_t)) {
+            int __c = getwc(__file_);
+            if (__c == WEOF)
+                return traits_type::eof();
+            __extbuf[__i] = static_cast<char_type>(__c);
+        } else
+#endif
+        {
+            int __c = getc(__file_);
+            if (__c == EOF)
+                return traits_type::eof();
+            __extbuf[__i] = static_cast<char>(__c);
+        }
     }
     char_type __1buf;
     if (__always_noconv_)
         __1buf = static_cast<char_type>(__extbuf[0]);
     else
+#ifdef _WIN32
+          // In wchar_t mode, __extbuf is the wrong type for __cv_->in below.
+          // In this mode, __always_noconv_ is true (but the compiler doesn't
+          // know), let it know that it doesn't need to instantiate this below.
+          if constexpr (sizeof(char_type) == sizeof(char))
+#endif
     {
         const char* __enxt;
         char_type* __inxt;
@@ -165,6 +191,12 @@
     {
         for (int __i = __nread; __i > 0;)
         {
+#ifdef _WIN32
+            if constexpr (sizeof(char_type) == sizeof(wchar_t)) {
+                if (ungetwc(traits_type::to_int_type(__extbuf[--__i]), __file_) == WEOF)
+                    return traits_type::eof();
+            } else
+#endif
             if (ungetc(traits_type::to_int_type(__extbuf[--__i]), __file_) == EOF)
                 return traits_type::eof();
         }
@@ -188,6 +220,13 @@
         }
         return __c;
     }
+#ifdef _WIN32
+    if (sizeof(char_type) == sizeof(wchar_t) && __last_consumed_is_next_) {
+        const char_type __ci = traits_type::to_char_type(__last_consumed_);
+        if (ungetwc(__ci, __file_) == WEOF)
+            return traits_type::eof();
+    } else
+#endif
     if (__last_consumed_is_next_)
     {
         char __extbuf[__limit];
@@ -255,6 +294,10 @@
       __st_(__st),
       __always_noconv_(__cv_->always_noconv())
 {
+#ifdef _WIN32
+    if constexpr (sizeof(char_type) == sizeof(wchar_t))
+        __always_noconv_ = true;
+#endif
 }
 
 template <class _CharT>
@@ -268,6 +311,13 @@
         __1buf = traits_type::to_char_type(__c);
         if (__always_noconv_)
         {
+#ifdef _WIN32
+            if constexpr (sizeof(char_type) == sizeof(wchar_t)) {
+                // fputwc works regardless of wide/narrow mode of stdout.
+                if (fputwc(__1buf, __file_) == WEOF)
+                    return traits_type::eof();
+            } else
+#endif
             if (fwrite(&__1buf, sizeof(char_type), 1, __file_) != 1)
                 return traits_type::eof();
         }
@@ -313,7 +363,13 @@
 streamsize
 __stdoutbuf<_CharT>::xsputn(const char_type* __s, streamsize __n)
 {
-    if (__always_noconv_)
+    if (__always_noconv_
+#ifdef _WIN32
+        // for wchar_t, prefer writing it element by element with fputwc,
+        // since that works when stdout is in narrow mode too.
+        && sizeof(char_type) != sizeof(wchar_t)
+#endif
+    )
         return fwrite(__s, sizeof(char_type), __n, __file_);
     streamsize __i = 0;
     for (; __i < __n; ++__i, ++__s)
@@ -352,6 +408,10 @@
     sync();
     __cv_ = &use_facet<codecvt<char_type, char, state_type> >(__loc);
     __always_noconv_ = __cv_->always_noconv();
+#ifdef _WIN32
+    if constexpr (sizeof(char_type) == sizeof(wchar_t))
+        __always_noconv_ = true;
+#endif
 }
 
 _LIBCPP_END_NAMESPACE_STD