diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -1118,7 +1118,7 @@
       // Also reject wide string literals.
       if (StringLiteral *MessageStringLiteral =
               cast_or_null<StringLiteral>(MessageExpr.get())) {
-        if (MessageStringLiteral->getCharByteWidth() != 1) {
+        if (!MessageStringLiteral->isAscii()) {
           Diag(MessageStringLiteral->getSourceRange().getBegin(),
                diag::err_expected_string_literal)
             << /*Source='availability attribute'*/ 2;
diff --git a/clang/test/Parser/attr-availability-xcore.c b/clang/test/Parser/attr-availability-xcore.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Parser/attr-availability-xcore.c
@@ -0,0 +1,11 @@
+// Test availability message string type when wide characters are 1 byte.
+// REQUIRES: xcore-registered-target
+// RUN: %clang_cc1 -triple xcore -fsyntax-only -verify %s
+
+#if !__has_feature(attribute_availability)
+#  error 'availability' attribute is not available
+#endif
+
+void f7() __attribute__((availability(macosx,message=L"wide"))); // expected-error {{expected string literal for optional message in 'availability' attribute}}
+
+void f8() __attribute__((availability(macosx,message="a" L"b"))); // expected-error {{expected string literal for optional message in 'availability' attribute}}
diff --git a/clang/test/Parser/attr-availability.c b/clang/test/Parser/attr-availability.c
--- a/clang/test/Parser/attr-availability.c
+++ b/clang/test/Parser/attr-availability.c
@@ -22,6 +22,14 @@
 
 void f8() __attribute__((availability(macosx,message="a" L"b"))); // expected-error {{expected string literal for optional message in 'availability' attribute}}
 
+void f9() __attribute__((availability(macosx,message=u8"b"))); // expected-error {{expected string literal for optional message in 'availability' attribute}}
+
+void f10() __attribute__((availability(macosx,message="a" u8"b"))); // expected-error {{expected string literal for optional message in 'availability' attribute}}
+
+void f11() __attribute__((availability(macosx,message=u"b"))); // expected-error {{expected string literal for optional message in 'availability' attribute}}
+
+void f12() __attribute__((availability(macosx,message="a" u"b"))); // expected-error {{expected string literal for optional message in 'availability' attribute}}
+
 // rdar://10095131
 enum E{
     gorf __attribute__((availability(macosx,introduced=8.5, message = 10.0))), // expected-error {{expected string literal for optional message in 'availability' attribute}}