Index: include/clang/Lex/LiteralSupport.h
===================================================================
--- include/clang/Lex/LiteralSupport.h
+++ include/clang/Lex/LiteralSupport.h
@@ -259,6 +259,8 @@
     return UDSuffixOffset;
   }
 
+  static bool isValidUDSuffix(const LangOptions &LangOpts, StringRef Suffix);
+
 private:
   void init(ArrayRef<Token> StringToks);
   bool CopyStringFragment(const Token &Tok, const char *TokBegin,
Index: lib/Lex/Lexer.cpp
===================================================================
--- lib/Lex/Lexer.cpp
+++ lib/Lex/Lexer.cpp
@@ -1713,9 +1713,9 @@
                                          getLangOpts());
         if (!isIdentifierBody(Next)) {
           // End of suffix. Check whether this is on the whitelist.
-          IsUDSuffix = (Chars == 1 && Buffer[0] == 's') ||
-                       NumericLiteralParser::isValidUDSuffix(
-                           getLangOpts(), StringRef(Buffer, Chars));
+          const StringRef CompleteSuffix(Buffer, Chars);
+          IsUDSuffix = StringLiteralParser::isValidUDSuffix(getLangOpts(),
+                                                            CompleteSuffix);
           break;
         }
 
Index: lib/Lex/LiteralSupport.cpp
===================================================================
--- lib/Lex/LiteralSupport.cpp
+++ lib/Lex/LiteralSupport.cpp
@@ -1708,3 +1708,12 @@
 
   return SpellingPtr-SpellingStart;
 }
+
+/// Determine whether a suffix is a valid ud-suffix. We avoid treating reserved
+/// suffixes as ud-suffixes, because the diagnostic experience is better if we
+/// treat it as an invalid suffix.
+bool StringLiteralParser::isValidUDSuffix(const LangOptions &LangOpts,
+                                          StringRef Suffix) {
+  return NumericLiteralParser::isValidUDSuffix(LangOpts, Suffix) ||
+         Suffix == "sv";
+}
Index: lib/Sema/SemaDeclCXX.cpp
===================================================================
--- lib/Sema/SemaDeclCXX.cpp
+++ lib/Sema/SemaDeclCXX.cpp
@@ -12908,7 +12908,7 @@
     //   Literal suffix identifiers that do not start with an underscore
     //   are reserved for future standardization.
     Diag(FnDecl->getLocation(), diag::warn_user_literal_reserved)
-      << NumericLiteralParser::isValidUDSuffix(getLangOpts(), LiteralName);
+      << StringLiteralParser::isValidUDSuffix(getLangOpts(), LiteralName);
   }
 
   return false;
Index: test/SemaCXX/cxx1z-user-defined-literals.cpp
===================================================================
--- test/SemaCXX/cxx1z-user-defined-literals.cpp
+++ test/SemaCXX/cxx1z-user-defined-literals.cpp
@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -std=c++1z %s -include %s -verify
+
+#ifndef INCLUDED
+#define INCLUDED
+
+#pragma clang system_header
+namespace std {
+  using size_t = decltype(sizeof(0));
+
+  struct string_view {};
+  string_view operator""sv(const char*, size_t);
+}
+
+#else
+
+using namespace std;
+string_view s = "foo"sv;
+const char* p = "bar"sv; // expected-error {{no viable conversion}}
+char error = 'x'sv; // expected-error {{invalid suffix}} expected-error {{expected ';'}}
+
+#endif