[Clang] Add warnings for CWG2521

1. Teach -Wuser-defined-literals to warn on using double underscores in
   literal suffix identifiers.
2. Add -Wdeprecated-literal-operator to warn about the use of the first
   grammar production of literal-operator-id, which defaults to off for now.

Differential Revision: https://reviews.llvm.org/D152632

3 files changed, 33 insertions, 14 deletions

diff --git a/clang/lib/Basic/IdentifierTable.cpp b/clang/lib/Basic/IdentifierTable.cpp
index 661e9f7..0065a61 100644
--- a/clang/lib/Basic/IdentifierTable.cpp
+++ b/clang/lib/Basic/IdentifierTable.cpp
@@ -397,6 +397,19 @@ IdentifierInfo::isReserved(const LangOptions &LangOpts) const {
   return ReservedIdentifierStatus::NotReserved;
 }
 
+ReservedLiteralSuffixIdStatus
+IdentifierInfo::isReservedLiteralSuffixId() const {
+  StringRef Name = getName();
+
+  if (Name[0] != '_')
+    return ReservedLiteralSuffixIdStatus::NotStartsWithUnderscore;
+
+  if (Name.contains("__"))
+    return ReservedLiteralSuffixIdStatus::ContainsDoubleUnderscore;
+
+  return ReservedLiteralSuffixIdStatus::NotReserved;
+}
+
 StringRef IdentifierInfo::deuglifiedName() const {
   StringRef Name = getName();
   if (Name.size() >= 2 && Name.front() == '_' &&
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 5c68642..4848af9 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -16447,15 +16447,18 @@ bool Sema::CheckLiteralOperatorDeclaration(FunctionDecl *FnDecl) {
     }
   }
 
-  StringRef LiteralName
-    = FnDecl->getDeclName().getCXXLiteralIdentifier()->getName();
-  if (LiteralName[0] != '_' &&
+  const IdentifierInfo *II = FnDecl->getDeclName().getCXXLiteralIdentifier();
+  ReservedLiteralSuffixIdStatus Status = II->isReservedLiteralSuffixId();
+  if (Status != ReservedLiteralSuffixIdStatus::NotReserved &&
       !getSourceManager().isInSystemHeader(FnDecl->getLocation())) {
-    // C++11 [usrlit.suffix]p1:
-    //   Literal suffix identifiers that do not start with an underscore
-    //   are reserved for future standardization.
+    // C++23 [usrlit.suffix]p1:
+    //   Literal suffix identifiers that do not start with an underscore are
+    //   reserved for future standardization. Literal suffix identifiers that
+    //   contain a double underscore __ are reserved for use by C++
+    //   implementations.
     Diag(FnDecl->getLocation(), diag::warn_user_literal_reserved)
-      << StringLiteralParser::isValidUDSuffix(getLangOpts(), LiteralName);
+        << static_cast<int>(Status)
+        << StringLiteralParser::isValidUDSuffix(getLangOpts(), II->getName());
   }
 
   return false;
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index 168a0f3..3e43b69 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -502,13 +502,16 @@ bool Sema::checkLiteralOperatorId(const CXXScopeSpec &SS,
     IdentifierInfo *II = Name.Identifier;
     ReservedIdentifierStatus Status = II->isReserved(PP.getLangOpts());
     SourceLocation Loc = Name.getEndLoc();
-    if (isReservedInAllContexts(Status) &&
-        !PP.getSourceManager().isInSystemHeader(Loc)) {
-      Diag(Loc, diag::warn_reserved_extern_symbol)
-          << II << static_cast<int>(Status)
-          << FixItHint::CreateReplacement(
-                 Name.getSourceRange(),
-                 (StringRef("operator\"\"") + II->getName()).str());
+    if (!PP.getSourceManager().isInSystemHeader(Loc)) {
+      if (auto Hint = FixItHint::CreateReplacement(
+              Name.getSourceRange(),
+              (StringRef("operator\"\"") + II->getName()).str());
+          isReservedInAllContexts(Status)) {
+        Diag(Loc, diag::warn_reserved_extern_symbol)
+            << II << static_cast<int>(Status) << Hint;
+      } else {
+        Diag(Loc, diag::warn_deprecated_literal_operator_id) << II << Hint;
+      }
     }
   }