Index: libcxx/trunk/include/__config
===================================================================
--- libcxx/trunk/include/__config
+++ libcxx/trunk/include/__config
@@ -307,7 +307,7 @@
    // random data even when using sandboxing mechanisms such as chroots,
    // Capsicum, etc.
 #  define _LIBCPP_USING_ARC4_RANDOM
-#elif defined(__Fuchsia__)
+#elif defined(__Fuchsia__) || defined(__wasi__)
 #  define _LIBCPP_USING_GETENTROPY
 #elif defined(__native_client__)
    // NaCl's sandbox (which PNaCl also runs in) doesn't allow filesystem access,
@@ -341,7 +341,7 @@
 #  if defined(__FreeBSD__)
 #    define _LIBCPP_HAS_QUICK_EXIT
 #    define _LIBCPP_HAS_C11_FEATURES
-#  elif defined(__Fuchsia__)
+#  elif defined(__Fuchsia__) || defined(__wasi__)
 #    define _LIBCPP_HAS_QUICK_EXIT
 #    define _LIBCPP_HAS_TIMESPEC_GET
 #    define _LIBCPP_HAS_C11_FEATURES
@@ -1133,6 +1133,7 @@
     !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
 #  if defined(__FreeBSD__) || \
       defined(__Fuchsia__) || \
+      defined(__wasi__) || \
       defined(__NetBSD__) || \
       defined(__linux__) || \
       defined(__GNU__) || \
@@ -1179,7 +1180,7 @@
 #endif
 
 #if defined(__BIONIC__) || defined(__CloudABI__) ||                            \
-    defined(__Fuchsia__) || defined(_LIBCPP_HAS_MUSL_LIBC)
+    defined(__Fuchsia__) || defined(__wasi__) || defined(_LIBCPP_HAS_MUSL_LIBC)
 #define _LIBCPP_PROVIDES_DEFAULT_RUNE_TABLE
 #endif
 
Index: libcxx/trunk/include/__locale
===================================================================
--- libcxx/trunk/include/__locale
+++ libcxx/trunk/include/__locale
@@ -35,6 +35,9 @@
 # include <xlocale.h>
 #elif defined(__Fuchsia__)
 # include <support/fuchsia/xlocale.h>
+#elif defined(__wasi__)
+// WASI libc uses musl's locales support.
+# include <support/musl/xlocale.h>
 #elif defined(_LIBCPP_HAS_MUSL_LIBC)
 # include <support/musl/xlocale.h>
 #endif
Index: libcxx/trunk/src/include/config_elast.h
===================================================================
--- libcxx/trunk/src/include/config_elast.h
+++ libcxx/trunk/src/include/config_elast.h
@@ -23,6 +23,8 @@
 #define _LIBCPP_ELAST __ELASTERROR
 #elif defined(__Fuchsia__)
 // No _LIBCPP_ELAST needed on Fuchsia
+#elif defined(__wasi__)
+// No _LIBCPP_ELAST needed on WASI
 #elif defined(__linux__) || defined(_LIBCPP_HAS_MUSL_LIBC)
 #define _LIBCPP_ELAST 4095
 #elif defined(__APPLE__)
Index: libcxx/trunk/src/thread.cpp
===================================================================
--- libcxx/trunk/src/thread.cpp
+++ libcxx/trunk/src/thread.cpp
@@ -23,9 +23,9 @@
 # endif
 #endif // defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 
-#if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__)) || defined(__CloudABI__) || defined(__Fuchsia__)
+#if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__)) || defined(__CloudABI__) || defined(__Fuchsia__) || defined(__wasi__)
 # include <unistd.h>
-#endif // defined(__unix__) || (defined(__APPLE__) && defined(__MACH__)) || defined(__CloudABI__) || defined(__Fuchsia__)
+#endif // defined(__unix__) || (defined(__APPLE__) && defined(__MACH__)) || defined(__CloudABI__) || defined(__Fuchsia__) || defined(__wasi__)
 
 #if defined(__NetBSD__)
 #pragma weak pthread_create // Do not create libpthread dependency
Index: libcxx/trunk/test/support/test_macros.h
===================================================================
--- libcxx/trunk/test/support/test_macros.h
+++ libcxx/trunk/test/support/test_macros.h
@@ -148,7 +148,7 @@
 //  Specifically, FreeBSD does NOT have timespec_get, even though they have all
 //  the rest of C11 - this is PR#38495
 #    define TEST_HAS_C11_FEATURES
-#  elif defined(__Fuchsia__)
+#  elif defined(__Fuchsia__) || defined(__wasi__)
 #    define TEST_HAS_C11_FEATURES
 #    define TEST_HAS_TIMESPEC_GET
 #  elif defined(__linux__)