Index: clang/include/clang/Sema/Sema.h
===================================================================
--- clang/include/clang/Sema/Sema.h
+++ clang/include/clang/Sema/Sema.h
@@ -4128,7 +4128,8 @@
                                         bool RValueThis, unsigned ThisQuals);
   CXXDestructorDecl *LookupDestructor(CXXRecordDecl *Class);
 
-  bool checkLiteralOperatorId(const CXXScopeSpec &SS, const UnqualifiedId &Id);
+  bool checkLiteralOperatorId(const CXXScopeSpec &SS, const UnqualifiedId &Id,
+                              bool isUDSuffix);
   LiteralOperatorLookupResult
   LookupLiteralOperator(Scope *S, LookupResult &R, ArrayRef<QualType> ArgTys,
                         bool AllowRaw, bool AllowTemplate,
Index: clang/lib/AST/Decl.cpp
===================================================================
--- clang/lib/AST/Decl.cpp
+++ clang/lib/AST/Decl.cpp
@@ -1088,6 +1088,10 @@
   if (!II)
     return ReservedIdentifierStatus::NotReserved;
 
+  // already checked at lexing time
+  if (getDeclName().getCXXLiteralIdentifier())
+    return ReservedIdentifierStatus::NotReserved;
+
   ReservedIdentifierStatus Status = II->isReserved(LangOpts);
   if (Status == ReservedIdentifierStatus::StartsWithUnderscoreAtGlobalScope) {
     // Check if we're at TU level or not.
Index: clang/lib/Parse/ParseExprCXX.cpp
===================================================================
--- clang/lib/Parse/ParseExprCXX.cpp
+++ clang/lib/Parse/ParseExprCXX.cpp
@@ -9,7 +9,6 @@
 // This file implements the Expression parsing implementation for C++.
 //
 //===----------------------------------------------------------------------===//
-#include "clang/Parse/Parser.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclTemplate.h"
@@ -17,6 +16,7 @@
 #include "clang/Basic/PrettyStackTrace.h"
 #include "clang/Lex/LiteralSupport.h"
 #include "clang/Parse/ParseDiagnostic.h"
+#include "clang/Parse/Parser.h"
 #include "clang/Parse/RAIIObjectsForParser.h"
 #include "clang/Sema/DeclSpec.h"
 #include "clang/Sema/ParsedTemplate.h"
@@ -2636,9 +2636,10 @@
 
     // Grab the literal operator's suffix, which will be either the next token
     // or a ud-suffix from the string literal.
+    const bool isUDSuffix = !Literal.getUDSuffix().empty();
     IdentifierInfo *II = nullptr;
     SourceLocation SuffixLoc;
-    if (!Literal.getUDSuffix().empty()) {
+    if (isUDSuffix) {
       II = &PP.getIdentifierTable().get(Literal.getUDSuffix());
       SuffixLoc =
         Lexer::AdvanceToTokenCharacter(TokLocs[Literal.getUDSuffixToken()],
@@ -2675,7 +2676,7 @@
 
     Result.setLiteralOperatorId(II, KeywordLoc, SuffixLoc);
 
-    return Actions.checkLiteralOperatorId(SS, Result);
+    return Actions.checkLiteralOperatorId(SS, Result, isUDSuffix);
   }
 
   // Parse a conversion-function-id.
Index: clang/lib/Sema/SemaExprCXX.cpp
===================================================================
--- clang/lib/Sema/SemaExprCXX.cpp
+++ clang/lib/Sema/SemaExprCXX.cpp
@@ -484,8 +484,19 @@
 }
 
 bool Sema::checkLiteralOperatorId(const CXXScopeSpec &SS,
-                                  const UnqualifiedId &Name) {
+                                  const UnqualifiedId &Name, bool isUDSuffix) {
   assert(Name.getKind() == UnqualifiedIdKind::IK_LiteralOperatorId);
+  if (!isUDSuffix) {
+    // [over.literal] p8
+    IdentifierInfo *II = Name.Identifier;
+    auto Status = II->isReserved(PP.getLangOpts());
+    auto Loc = Name.getBeginLoc();
+    if (Status != ReservedIdentifierStatus::NotReserved &&
+        !PP.getSourceManager().isInSystemHeader(Loc)) {
+      Diag(Loc, diag::warn_reserved_extern_symbol)
+          << II << static_cast<int>(Status);
+    }
+  }
 
   if (!SS.isValid())
     return false;
Index: clang/test/Sema/reserved-identifier.cpp
===================================================================
--- clang/test/Sema/reserved-identifier.cpp
+++ clang/test/Sema/reserved-identifier.cpp
@@ -76,7 +76,12 @@
 int _barbatruc; // no-warning
 }
 
-long double operator"" _BarbeBleue(long double) // expected-warning {{identifier 'operator""_BarbeBleue' is reserved because it starts with '_' followed by a capital letter}}
+long double operator"" _BarbeBleue(long double) // expected-warning {{identifier '_BarbeBleue' is reserved because it starts with '_' followed by a capital letter}}
+{
+  return 0.;
+}
+
+long double operator""_SacreBleue(long double) // no-warning
 {
   return 0.;
 }