diff --git a/clang-tools-extra/clang-include-fixer/IncludeFixer.cpp b/clang-tools-extra/clang-include-fixer/IncludeFixer.cpp
--- a/clang-tools-extra/clang-include-fixer/IncludeFixer.cpp
+++ b/clang-tools-extra/clang-include-fixer/IncludeFixer.cpp
@@ -245,7 +245,7 @@
     // parent_path.
     // FIXME: Don't rely on source text.
     const char *End = Source.end();
-    while (isIdentifierBody(*End) || *End == ':')
+    while (isAsciiIdentifierContinue(*End) || *End == ':')
       ++End;
 
     return std::string(Source.begin(), End);
diff --git a/clang-tools-extra/clang-tidy/google/IntegerTypesCheck.cpp b/clang-tools-extra/clang-tidy/google/IntegerTypesCheck.cpp
--- a/clang-tools-extra/clang-tidy/google/IntegerTypesCheck.cpp
+++ b/clang-tools-extra/clang-tidy/google/IntegerTypesCheck.cpp
@@ -129,7 +129,7 @@
   const StringRef Port = "unsigned short port";
   const char *Data = Result.SourceManager->getCharacterData(Loc);
   if (!std::strncmp(Data, Port.data(), Port.size()) &&
-      !isIdentifierBody(Data[Port.size()]))
+      !isAsciiIdentifierContinue(Data[Port.size()]))
     return;
 
   std::string Replacement =
diff --git a/clang-tools-extra/clang-tidy/utils/RenamerClangTidyCheck.cpp b/clang-tools-extra/clang-tidy/utils/RenamerClangTidyCheck.cpp
--- a/clang-tools-extra/clang-tidy/utils/RenamerClangTidyCheck.cpp
+++ b/clang-tools-extra/clang-tidy/utils/RenamerClangTidyCheck.cpp
@@ -464,7 +464,7 @@
         Failure.FixStatus = ShouldFixStatus::ConflictsWithKeyword;
       else if (Ident->hasMacroDefinition())
         Failure.FixStatus = ShouldFixStatus::ConflictsWithMacroDefinition;
-    } else if (!isValidIdentifier(Info.Fixup)) {
+    } else if (!isValidAsciiIdentifier(Info.Fixup)) {
       Failure.FixStatus = ShouldFixStatus::FixInvalidIdentifier;
     }
 
diff --git a/clang-tools-extra/clangd/CodeComplete.cpp b/clang-tools-extra/clangd/CodeComplete.cpp
--- a/clang-tools-extra/clangd/CodeComplete.cpp
+++ b/clang-tools-extra/clangd/CodeComplete.cpp
@@ -1842,14 +1842,14 @@
   CompletionPrefix Result;
 
   // Consume the unqualified name. We only handle ASCII characters.
-  // isIdentifierBody will let us match "0invalid", but we don't mind.
-  while (!Rest.empty() && isIdentifierBody(Rest.back()))
+  // isAsciiIdentifierContinue will let us match "0invalid", but we don't mind.
+  while (!Rest.empty() && isAsciiIdentifierContinue(Rest.back()))
     Rest = Rest.drop_back();
   Result.Name = Content.slice(Rest.size(), Offset);
 
   // Consume qualifiers.
   while (Rest.consume_back("::") && !Rest.endswith(":")) // reject ::::
-    while (!Rest.empty() && isIdentifierBody(Rest.back()))
+    while (!Rest.empty() && isAsciiIdentifierContinue(Rest.back()))
       Rest = Rest.drop_back();
   Result.Qualifier =
       Content.slice(Rest.size(), Result.Name.begin() - Content.begin());
@@ -2057,8 +2057,8 @@
     return true;
 
   // Complete words. Give non-ascii characters the benefit of the doubt.
-  return !Content.empty() &&
-         (isIdentifierBody(Content.back()) || !llvm::isASCII(Content.back()));
+  return !Content.empty() && (isAsciiIdentifierContinue(Content.back()) ||
+                              !llvm::isASCII(Content.back()));
 }
 
 } // namespace clangd
diff --git a/clang-tools-extra/clangd/SourceCode.cpp b/clang-tools-extra/clangd/SourceCode.cpp
--- a/clang-tools-extra/clangd/SourceCode.cpp
+++ b/clang-tools-extra/clangd/SourceCode.cpp
@@ -945,9 +945,9 @@
   if (Invalid)
     return llvm::None;
   unsigned B = Offset, E = Offset;
-  while (B > 0 && isIdentifierBody(Code[B - 1]))
+  while (B > 0 && isAsciiIdentifierContinue(Code[B - 1]))
     --B;
-  while (E < Code.size() && isIdentifierBody(Code[E]))
+  while (E < Code.size() && isAsciiIdentifierContinue(Code[E]))
     ++E;
   if (B == E)
     return llvm::None;
diff --git a/clang-tools-extra/clangd/refactor/Rename.cpp b/clang-tools-extra/clangd/refactor/Rename.cpp
--- a/clang-tools-extra/clangd/refactor/Rename.cpp
+++ b/clang-tools-extra/clangd/refactor/Rename.cpp
@@ -478,10 +478,10 @@
   // We don't check all the rules for non-ascii characters (most are allowed).
   bool AllowDollar = true; // lenient
   if (llvm::isASCII(Ident.front()) &&
-      !isIdentifierHead(Ident.front(), AllowDollar))
+      !isAsciiIdentifierStart(Ident.front(), AllowDollar))
     return false;
   for (char C : Ident) {
-    if (llvm::isASCII(C) && !isIdentifierBody(C, AllowDollar))
+    if (llvm::isASCII(C) && !isAsciiIdentifierContinue(C, AllowDollar))
       return false;
   }
   return true;
diff --git a/clang/include/clang/Basic/CharInfo.h b/clang/include/clang/Basic/CharInfo.h
--- a/clang/include/clang/Basic/CharInfo.h
+++ b/clang/include/clang/Basic/CharInfo.h
@@ -50,8 +50,8 @@
 
 /// Returns true if this is a valid first character of a C identifier,
 /// which is [a-zA-Z_].
-LLVM_READONLY inline bool isIdentifierHead(unsigned char c,
-                                           bool AllowDollar = false) {
+LLVM_READONLY inline bool isAsciiIdentifierStart(unsigned char c,
+                                                 bool AllowDollar = false) {
   using namespace charinfo;
   if (InfoTable[c] & (CHAR_UPPER|CHAR_LOWER|CHAR_UNDER))
     return true;
@@ -60,8 +60,8 @@
 
 /// Returns true if this is a body character of a C identifier,
 /// which is [a-zA-Z0-9_].
-LLVM_READONLY inline bool isIdentifierBody(unsigned char c,
-                                           bool AllowDollar = false) {
+LLVM_READONLY inline bool isAsciiIdentifierContinue(unsigned char c,
+                                                    bool AllowDollar = false) {
   using namespace charinfo;
   if (InfoTable[c] & (CHAR_UPPER|CHAR_LOWER|CHAR_DIGIT|CHAR_UNDER))
     return true;
@@ -186,13 +186,13 @@
 ///
 /// Note that this is a very simple check; it does not accept UCNs as valid
 /// identifier characters.
-LLVM_READONLY inline bool isValidIdentifier(StringRef S,
-                                            bool AllowDollar = false) {
-  if (S.empty() || !isIdentifierHead(S[0], AllowDollar))
+LLVM_READONLY inline bool isValidAsciiIdentifier(StringRef S,
+                                                 bool AllowDollar = false) {
+  if (S.empty() || !isAsciiIdentifierStart(S[0], AllowDollar))
     return false;
 
   for (StringRef::iterator I = S.begin(), E = S.end(); I != E; ++I)
-    if (!isIdentifierBody(*I, AllowDollar))
+    if (!isAsciiIdentifierContinue(*I, AllowDollar))
       return false;
 
   return true;
diff --git a/clang/include/clang/Lex/Lexer.h b/clang/include/clang/Lex/Lexer.h
--- a/clang/include/clang/Lex/Lexer.h
+++ b/clang/include/clang/Lex/Lexer.h
@@ -536,7 +536,8 @@
                                          bool SkipTrailingWhitespaceAndNewLine);
 
   /// Returns true if the given character could appear in an identifier.
-  static bool isIdentifierBodyChar(char c, const LangOptions &LangOpts);
+  static bool isAsciiIdentifierContinueChar(char c,
+                                            const LangOptions &LangOpts);
 
   /// Checks whether new line pointed by Str is preceded by escape
   /// sequence.
@@ -573,10 +574,7 @@
 
   bool CheckUnicodeWhitespace(Token &Result, uint32_t C, const char *CurPtr);
 
-  /// Given that a token begins with the Unicode character \p C, figure out
-  /// what kind of token it is and dispatch to the appropriate lexing helper
-  /// function.
-  bool LexUnicode(Token &Result, uint32_t C, const char *CurPtr);
+  bool LexUnicodeIdentifierStart(Token &Result, uint32_t C, const char *CurPtr);
 
   /// FormTokenWithChars - When we lex a token, we have identified a span
   /// starting at BufferPtr, going to TokEnd that forms the token.  This method
@@ -701,7 +699,7 @@
                           bool IsStringLiteral);
 
   // Helper functions to lex the remainder of a token of the specific type.
-  bool LexIdentifier         (Token &Result, const char *CurPtr);
+  bool LexIdentifierContinue(Token &Result, const char *CurPtr);
   bool LexNumericConstant    (Token &Result, const char *CurPtr);
   bool LexStringLiteral      (Token &Result, const char *CurPtr,
                               tok::TokenKind Kind);
diff --git a/clang/lib/ARCMigrate/ObjCMT.cpp b/clang/lib/ARCMigrate/ObjCMT.cpp
--- a/clang/lib/ARCMigrate/ObjCMT.cpp
+++ b/clang/lib/ARCMigrate/ObjCMT.cpp
@@ -1144,7 +1144,7 @@
 
 static bool IsValidIdentifier(ASTContext &Ctx,
                               const char *Name) {
-  if (!isIdentifierHead(Name[0]))
+  if (!isAsciiIdentifierStart(Name[0]))
     return false;
   std::string NameString = Name;
   NameString[0] = toLowercase(NameString[0]);
diff --git a/clang/lib/ARCMigrate/TransUnbridgedCasts.cpp b/clang/lib/ARCMigrate/TransUnbridgedCasts.cpp
--- a/clang/lib/ARCMigrate/TransUnbridgedCasts.cpp
+++ b/clang/lib/ARCMigrate/TransUnbridgedCasts.cpp
@@ -253,7 +253,8 @@
 
       SourceManager &SM = Pass.Ctx.getSourceManager();
       char PrevChar = *SM.getCharacterData(InsertLoc.getLocWithOffset(-1));
-      if (Lexer::isIdentifierBodyChar(PrevChar, Pass.Ctx.getLangOpts()))
+      if (Lexer::isAsciiIdentifierContinueChar(PrevChar,
+                                               Pass.Ctx.getLangOpts()))
         BridgeCall += ' ';
 
       if (Kind == OBC_BridgeTransfer)
diff --git a/clang/lib/AST/MicrosoftMangle.cpp b/clang/lib/AST/MicrosoftMangle.cpp
--- a/clang/lib/AST/MicrosoftMangle.cpp
+++ b/clang/lib/AST/MicrosoftMangle.cpp
@@ -3883,7 +3883,7 @@
     // - ?[A-Z]: The range from \xc1 to \xda.
     // - ?[0-9]: The set of [,/\:. \n\t'-].
     // - ?$XX: A fallback which maps nibbles.
-    if (isIdentifierBody(Byte, /*AllowDollar=*/true)) {
+    if (isAsciiIdentifierContinue(Byte, /*AllowDollar=*/true)) {
       Mangler.getStream() << Byte;
     } else if (isLetter(Byte & 0x7f)) {
       Mangler.getStream() << '?' << static_cast<char>(Byte & 0x7f);
diff --git a/clang/lib/Basic/Module.cpp b/clang/lib/Basic/Module.cpp
--- a/clang/lib/Basic/Module.cpp
+++ b/clang/lib/Basic/Module.cpp
@@ -203,7 +203,7 @@
       OS << ".";
 
     StringRef Name = getModuleNameFromComponent(*It);
-    if (!AllowStringLiterals || isValidIdentifier(Name))
+    if (!AllowStringLiterals || isValidAsciiIdentifier(Name))
       OS << Name;
     else {
       OS << '"';
diff --git a/clang/lib/Edit/EditedSource.cpp b/clang/lib/Edit/EditedSource.cpp
--- a/clang/lib/Edit/EditedSource.cpp
+++ b/clang/lib/Edit/EditedSource.cpp
@@ -314,8 +314,8 @@
 static bool canBeJoined(char left, char right, const LangOptions &LangOpts) {
   // FIXME: Should use TokenConcatenation to make sure we don't allow stuff like
   // making two '<' adjacent.
-  return !(Lexer::isIdentifierBodyChar(left, LangOpts) &&
-           Lexer::isIdentifierBodyChar(right, LangOpts));
+  return !(Lexer::isAsciiIdentifierContinueChar(left, LangOpts) &&
+           Lexer::isAsciiIdentifierContinueChar(right, LangOpts));
 }
 
 /// Returns true if it is ok to eliminate the trailing whitespace between
diff --git a/clang/lib/Frontend/LayoutOverrideSource.cpp b/clang/lib/Frontend/LayoutOverrideSource.cpp
--- a/clang/lib/Frontend/LayoutOverrideSource.cpp
+++ b/clang/lib/Frontend/LayoutOverrideSource.cpp
@@ -16,11 +16,11 @@
 
 /// Parse a simple identifier.
 static std::string parseName(StringRef S) {
-  if (S.empty() || !isIdentifierHead(S[0]))
+  if (S.empty() || !isAsciiIdentifierStart(S[0]))
     return "";
 
   unsigned Offset = 1;
-  while (Offset < S.size() && isIdentifierBody(S[Offset]))
+  while (Offset < S.size() && isAsciiIdentifierContinue(S[Offset]))
     ++Offset;
 
   return S.substr(0, Offset).str();
diff --git a/clang/lib/Frontend/Rewrite/FrontendActions.cpp b/clang/lib/Frontend/Rewrite/FrontendActions.cpp
--- a/clang/lib/Frontend/Rewrite/FrontendActions.cpp
+++ b/clang/lib/Frontend/Rewrite/FrontendActions.cpp
@@ -231,7 +231,7 @@
     assert(OS && "loaded module file after finishing rewrite action?");
 
     (*OS) << "#pragma clang module build ";
-    if (isValidIdentifier(MF->ModuleName))
+    if (isValidAsciiIdentifier(MF->ModuleName))
       (*OS) << MF->ModuleName;
     else {
       (*OS) << '"';
diff --git a/clang/lib/Lex/DependencyDirectivesSourceMinimizer.cpp b/clang/lib/Lex/DependencyDirectivesSourceMinimizer.cpp
--- a/clang/lib/Lex/DependencyDirectivesSourceMinimizer.cpp
+++ b/clang/lib/Lex/DependencyDirectivesSourceMinimizer.cpp
@@ -131,17 +131,17 @@
   --Current;
   if (*Current != 'R')
     return false;
-  if (First == Current || !isIdentifierBody(*--Current))
+  if (First == Current || !isAsciiIdentifierContinue(*--Current))
     return true;
 
   // Check for a prefix of "u", "U", or "L".
   if (*Current == 'u' || *Current == 'U' || *Current == 'L')
-    return First == Current || !isIdentifierBody(*--Current);
+    return First == Current || !isAsciiIdentifierContinue(*--Current);
 
   // Check for a prefix of "u8".
   if (*Current != '8' || First == Current || *Current-- != 'u')
     return false;
-  return First == Current || !isIdentifierBody(*--Current);
+  return First == Current || !isAsciiIdentifierContinue(*--Current);
 }
 
 static void skipRawString(const char *&First, const char *const End) {
@@ -319,7 +319,7 @@
   if (!isPreprocessingNumberBody(Prev))
     return false;
   // The next character should be a valid identifier body character.
-  return (Cur + 1) < End && isIdentifierBody(*(Cur + 1));
+  return (Cur + 1) < End && isAsciiIdentifierContinue(*(Cur + 1));
 }
 
 static void skipLine(const char *&First, const char *const End) {
@@ -484,7 +484,7 @@
   const char *Last = First;
   do
     ++Last;
-  while (Last != End && (isIdentifierBody(*Last) || *Last == '.'));
+  while (Last != End && (isAsciiIdentifierContinue(*Last) || *Last == '.'));
   append(First, Last);
   First = Last;
 }
@@ -507,7 +507,7 @@
     }
 
     // Don't handle macro expansions inside @import for now.
-    if (!isIdentifierBody(*First) && *First != '.')
+    if (!isAsciiIdentifierContinue(*First) && *First != '.')
       return true;
 
     printAdjacentModuleNameParts(First, End);
@@ -524,9 +524,9 @@
 
 LLVM_NODISCARD static const char *lexRawIdentifier(const char *First,
                                                    const char *const End) {
-  assert(isIdentifierBody(*First) && "invalid identifer");
+  assert(isAsciiIdentifierContinue(*First) && "invalid identifer");
   const char *Last = First + 1;
-  while (Last != End && isIdentifierBody(*Last))
+  while (Last != End && isAsciiIdentifierContinue(*Last))
     ++Last;
   return Last;
 }
@@ -540,7 +540,7 @@
   skipNewline(First, End);
   if (First == End)
     return nullptr;
-  return isIdentifierBody(First[0]) ? First : nullptr;
+  return isAsciiIdentifierContinue(First[0]) ? First : nullptr;
 }
 
 Minimizer::IdInfo Minimizer::lexIdentifier(const char *First,
@@ -569,7 +569,7 @@
   do
     ++Last;
   while (Last != End &&
-         (isIdentifierBody(*Last) || *Last == '.' || *Last == ','));
+         (isAsciiIdentifierContinue(*Last) || *Last == '.' || *Last == ','));
   append(First, Last);
   First = Last;
 }
@@ -588,7 +588,7 @@
     }
 
     // This is intentionally fairly liberal.
-    if (!(isIdentifierBody(*First) || *First == '.' || *First == ','))
+    if (!(isAsciiIdentifierContinue(*First) || *First == '.' || *First == ','))
       return true;
 
     printAdjacentMacroArgs(First, End);
@@ -602,7 +602,7 @@
 bool Minimizer::isNextIdentifier(StringRef Id, const char *&First,
                                  const char *const End) {
   skipWhitespace(First, End);
-  if (First == End || !isIdentifierHead(*First))
+  if (First == End || !isAsciiIdentifierStart(*First))
     return false;
 
   IdInfo FoundId = lexIdentifier(First, End);
@@ -639,7 +639,7 @@
   if (Id.Name == "export") {
     Export = true;
     skipWhitespace(First, End);
-    if (!isIdentifierBody(*First)) {
+    if (!isAsciiIdentifierContinue(*First)) {
       skipLine(First, End);
       return false;
     }
@@ -663,7 +663,7 @@
   case '"':
     break;
   default:
-    if (!isIdentifierBody(*First)) {
+    if (!isAsciiIdentifierContinue(*First)) {
       skipLine(First, End);
       return false;
     }
@@ -690,7 +690,7 @@
   append("#define ");
   skipWhitespace(First, End);
 
-  if (!isIdentifierHead(*First))
+  if (!isAsciiIdentifierStart(*First))
     return reportError(First, diag::err_pp_macro_not_identifier);
 
   IdInfo Id = lexIdentifier(First, End);
@@ -722,7 +722,7 @@
 bool Minimizer::lexPragma(const char *&First, const char *const End) {
   // #pragma.
   skipWhitespace(First, End);
-  if (First == End || !isIdentifierHead(*First))
+  if (First == End || !isAsciiIdentifierStart(*First))
     return false;
 
   IdInfo FoundId = lexIdentifier(First, End);
@@ -827,7 +827,7 @@
   if (First == End)
     return reportError(First, diag::err_pp_expected_eol);
 
-  if (!isIdentifierHead(*First)) {
+  if (!isAsciiIdentifierStart(*First)) {
     skipLine(First, End);
     return false;
   }
diff --git a/clang/lib/Lex/Lexer.cpp b/clang/lib/Lex/Lexer.cpp
--- a/clang/lib/Lex/Lexer.cpp
+++ b/clang/lib/Lex/Lexer.cpp
@@ -1062,8 +1062,8 @@
   return ExpansionBuffer.substr(ExpansionInfo.second, MacroTokenLength);
 }
 
-bool Lexer::isIdentifierBodyChar(char c, const LangOptions &LangOpts) {
-  return isIdentifierBody(c, LangOpts.DollarIdents);
+bool Lexer::isAsciiIdentifierContinueChar(char c, const LangOptions &LangOpts) {
+  return isAsciiIdentifierContinue(c, LangOpts.DollarIdents);
 }
 
 bool Lexer::isNewLineEscaped(const char *BufferStart, const char *Str) {
@@ -1712,103 +1712,129 @@
   return true;
 }
 
-bool Lexer::LexIdentifier(Token &Result, const char *CurPtr) {
-  // Match [_A-Za-z0-9]*, we have already matched [_A-Za-z$]
-  unsigned Size;
-  unsigned char C = *CurPtr++;
-  while (isIdentifierBody(C))
-    C = *CurPtr++;
-
-  --CurPtr;   // Back up over the skipped character.
-
-  // Fast path, no $,\,? in identifier found.  '\' might be an escaped newline
-  // or UCN, and ? might be a trigraph for '\', an escaped newline or UCN.
-  //
-  // TODO: Could merge these checks into an InfoTable flag to make the
-  // comparison cheaper
-  if (isASCII(C) && C != '\\' && C != '?' &&
-      (C != '$' || !LangOpts.DollarIdents)) {
-FinishIdentifier:
-    const char *IdStart = BufferPtr;
-    FormTokenWithChars(Result, CurPtr, tok::raw_identifier);
-    Result.setRawIdentifierData(IdStart);
-
-    // If we are in raw mode, return this identifier raw.  There is no need to
-    // look up identifier information or attempt to macro expand it.
-    if (LexingRawMode)
-      return true;
-
-    // Fill in Result.IdentifierInfo and update the token kind,
-    // looking up the identifier in the identifier table.
-    IdentifierInfo *II = PP->LookUpIdentifierInfo(Result);
-    // Note that we have to call PP->LookUpIdentifierInfo() even for code
-    // completion, it writes IdentifierInfo into Result, and callers rely on it.
-
-    // If the completion point is at the end of an identifier, we want to treat
-    // the identifier as incomplete even if it resolves to a macro or a keyword.
-    // This allows e.g. 'class^' to complete to 'classifier'.
-    if (isCodeCompletionPoint(CurPtr)) {
-      // Return the code-completion token.
-      Result.setKind(tok::code_completion);
-      // Skip the code-completion char and all immediate identifier characters.
-      // This ensures we get consistent behavior when completing at any point in
-      // an identifier (i.e. at the start, in the middle, at the end). Note that
-      // only simple cases (i.e. [a-zA-Z0-9_]) are supported to keep the code
-      // simpler.
-      assert(*CurPtr == 0 && "Completion character must be 0");
-      ++CurPtr;
-      // Note that code completion token is not added as a separate character
-      // when the completion point is at the end of the buffer. Therefore, we need
-      // to check if the buffer has ended.
-      if (CurPtr < BufferEnd) {
-        while (isIdentifierBody(*CurPtr))
-          ++CurPtr;
-      }
-      BufferPtr = CurPtr;
-      return true;
+bool Lexer::LexUnicodeIdentifierStart(Token &Result, uint32_t C,
+                                      const char *CurPtr) {
+  if (isAllowedInitiallyIDChar(C, LangOpts)) {
+    if (!isLexingRawMode() && !ParsingPreprocessorDirective &&
+        !PP->isPreprocessedOutput()) {
+      maybeDiagnoseIDCharCompat(PP->getDiagnostics(), C,
+                                makeCharRange(*this, BufferPtr, CurPtr),
+                                /*IsFirst=*/true);
+      maybeDiagnoseUTF8Homoglyph(PP->getDiagnostics(), C,
+                                 makeCharRange(*this, BufferPtr, CurPtr));
     }
 
-    // Finally, now that we know we have an identifier, pass this off to the
-    // preprocessor, which may macro expand it or something.
-    if (II->isHandleIdentifierCase())
-      return PP->HandleIdentifier(Result);
+    MIOpt.ReadToken();
+    return LexIdentifierContinue(Result, CurPtr);
+  }
 
-    return true;
+  if (!isLexingRawMode() && !ParsingPreprocessorDirective &&
+      !PP->isPreprocessedOutput() && !isASCII(*BufferPtr) &&
+      !isAllowedInitiallyIDChar(C, LangOpts) && !isUnicodeWhitespace(C)) {
+    // Non-ASCII characters tend to creep into source code unintentionally.
+    // Instead of letting the parser complain about the unknown token,
+    // just drop the character.
+    // Note that we can /only/ do this when the non-ASCII character is actually
+    // spelled as Unicode, not written as a UCN. The standard requires that
+    // we not throw away any possible preprocessor tokens, but there's a
+    // loophole in the mapping of Unicode characters to basic character set
+    // characters that allows us to map these particular characters to, say,
+    // whitespace.
+    diagnoseInvalidUnicodeCodepointInIdentifier(
+        PP->getDiagnostics(), LangOpts, C,
+        makeCharRange(*this, BufferPtr, CurPtr), /*IsStart*/ true);
+    BufferPtr = CurPtr;
+    return false;
   }
 
-  // Otherwise, $,\,? in identifier found.  Enter slower path.
+  // Otherwise, we have an explicit UCN or a character that's unlikely to show
+  // up by accident.
+  MIOpt.ReadToken();
+  FormTokenWithChars(Result, CurPtr, tok::unknown);
+  return true;
+}
 
-  C = getCharAndSize(CurPtr, Size);
+bool Lexer::LexIdentifierContinue(Token &Result, const char *CurPtr) {
+  // Match [_A-Za-z0-9]*, we have already matched [_A-Za-z$]
+  unsigned Size;
   while (true) {
+    unsigned char C = *CurPtr;
+    // Fast path
+    if (isAsciiIdentifierContinue(C)) {
+      ++CurPtr;
+      continue;
+    }
+    // Slow path: handle trigraph, unicode codepoints, UCNs
+    C = getCharAndSize(CurPtr, Size);
+    if (isAsciiIdentifierContinue(C)) {
+      CurPtr = ConsumeChar(CurPtr, Size, Result);
+      continue;
+    }
     if (C == '$') {
       // If we hit a $ and they are not supported in identifiers, we are done.
-      if (!LangOpts.DollarIdents) goto FinishIdentifier;
-
+      if (!LangOpts.DollarIdents)
+        break;
       // Otherwise, emit a diagnostic and continue.
       if (!isLexingRawMode())
         Diag(CurPtr, diag::ext_dollar_in_identifier);
       CurPtr = ConsumeChar(CurPtr, Size, Result);
-      C = getCharAndSize(CurPtr, Size);
       continue;
-    } else if (C == '\\' && tryConsumeIdentifierUCN(CurPtr, Size, Result)) {
-      C = getCharAndSize(CurPtr, Size);
+    }
+    if (C == '\\' && tryConsumeIdentifierUCN(CurPtr, Size, Result)) {
       continue;
-    } else if (!isASCII(C) && tryConsumeIdentifierUTF8Char(CurPtr)) {
-      C = getCharAndSize(CurPtr, Size);
+    }
+    if (!isASCII(C) && tryConsumeIdentifierUTF8Char(CurPtr)) {
       continue;
-    } else if (!isIdentifierBody(C)) {
-      goto FinishIdentifier;
     }
+    // Neither an expected unicode codepoint nor a UCN
+    break;
+  }
 
-    // Otherwise, this character is good, consume it.
-    CurPtr = ConsumeChar(CurPtr, Size, Result);
+  const char *IdStart = BufferPtr;
+  FormTokenWithChars(Result, CurPtr, tok::raw_identifier);
+  Result.setRawIdentifierData(IdStart);
 
-    C = getCharAndSize(CurPtr, Size);
-    while (isIdentifierBody(C)) {
-      CurPtr = ConsumeChar(CurPtr, Size, Result);
-      C = getCharAndSize(CurPtr, Size);
+  // If we are in raw mode, return this identifier raw.  There is no need to
+  // look up identifier information or attempt to macro expand it.
+  if (LexingRawMode)
+    return true;
+
+  // Fill in Result.IdentifierInfo and update the token kind,
+  // looking up the identifier in the identifier table.
+  IdentifierInfo *II = PP->LookUpIdentifierInfo(Result);
+  // Note that we have to call PP->LookUpIdentifierInfo() even for code
+  // completion, it writes IdentifierInfo into Result, and callers rely on it.
+
+  // If the completion point is at the end of an identifier, we want to treat
+  // the identifier as incomplete even if it resolves to a macro or a keyword.
+  // This allows e.g. 'class^' to complete to 'classifier'.
+  if (isCodeCompletionPoint(CurPtr)) {
+    // Return the code-completion token.
+    Result.setKind(tok::code_completion);
+    // Skip the code-completion char and all immediate identifier characters.
+    // This ensures we get consistent behavior when completing at any point in
+    // an identifier (i.e. at the start, in the middle, at the end). Note that
+    // only simple cases (i.e. [a-zA-Z0-9_]) are supported to keep the code
+    // simpler.
+    assert(*CurPtr == 0 && "Completion character must be 0");
+    ++CurPtr;
+    // Note that code completion token is not added as a separate character
+    // when the completion point is at the end of the buffer. Therefore, we need
+    // to check if the buffer has ended.
+    if (CurPtr < BufferEnd) {
+      while (isAsciiIdentifierContinue(*CurPtr))
+        ++CurPtr;
     }
+    BufferPtr = CurPtr;
+    return true;
   }
+
+  // Finally, now that we know we have an identifier, pass this off to the
+  // preprocessor, which may macro expand it or something.
+  if (II->isHandleIdentifierCase())
+    return PP->HandleIdentifier(Result);
+
+  return true;
 }
 
 /// isHexaLiteral - Return true if Start points to a hex constant.
@@ -1864,7 +1890,7 @@
   if (C == '\'' && (getLangOpts().CPlusPlus14 || getLangOpts().C2x)) {
     unsigned NextSize;
     char Next = getCharAndSizeNoWarn(CurPtr + Size, NextSize, getLangOpts());
-    if (isIdentifierBody(Next)) {
+    if (isAsciiIdentifierContinue(Next)) {
       if (!isLexingRawMode())
         Diag(CurPtr, getLangOpts().CPlusPlus
                          ? diag::warn_cxx11_compat_digit_separator
@@ -1899,7 +1925,7 @@
   char C = getCharAndSize(CurPtr, Size);
   bool Consumed = false;
 
-  if (!isIdentifierHead(C)) {
+  if (!isAsciiIdentifierStart(C)) {
     if (C == '\\' && tryConsumeIdentifierUCN(CurPtr, Size, Result))
       Consumed = true;
     else if (!isASCII(C) && tryConsumeIdentifierUTF8Char(CurPtr))
@@ -1938,7 +1964,7 @@
         unsigned NextSize;
         char Next = getCharAndSizeNoWarn(CurPtr + Consumed, NextSize,
                                          getLangOpts());
-        if (!isIdentifierBody(Next)) {
+        if (!isAsciiIdentifierContinue(Next)) {
           // End of suffix. Check whether this is on the allowed list.
           const StringRef CompleteSuffix(Buffer, Chars);
           IsUDSuffix = StringLiteralParser::isValidUDSuffix(getLangOpts(),
@@ -1970,10 +1996,12 @@
   Result.setFlag(Token::HasUDSuffix);
   while (true) {
     C = getCharAndSize(CurPtr, Size);
-    if (isIdentifierBody(C)) { CurPtr = ConsumeChar(CurPtr, Size, Result); }
-    else if (C == '\\' && tryConsumeIdentifierUCN(CurPtr, Size, Result)) {}
-    else if (!isASCII(C) && tryConsumeIdentifierUTF8Char(CurPtr)) {}
-    else break;
+    if (isAsciiIdentifierContinue(C)) {
+      CurPtr = ConsumeChar(CurPtr, Size, Result);
+    } else if (C == '\\' && tryConsumeIdentifierUCN(CurPtr, Size, Result)) {
+    } else if (!isASCII(C) && tryConsumeIdentifierUTF8Char(CurPtr)) {
+    } else
+      break;
   }
 
   return CurPtr;
@@ -3205,47 +3233,6 @@
   return false;
 }
 
-bool Lexer::LexUnicode(Token &Result, uint32_t C, const char *CurPtr) {
-  if (isAllowedInitiallyIDChar(C, LangOpts)) {
-    if (!isLexingRawMode() && !ParsingPreprocessorDirective &&
-        !PP->isPreprocessedOutput()) {
-      maybeDiagnoseIDCharCompat(PP->getDiagnostics(), C,
-                                makeCharRange(*this, BufferPtr, CurPtr),
-                                /*IsFirst=*/true);
-      maybeDiagnoseUTF8Homoglyph(PP->getDiagnostics(), C,
-                                 makeCharRange(*this, BufferPtr, CurPtr));
-    }
-
-    MIOpt.ReadToken();
-    return LexIdentifier(Result, CurPtr);
-  }
-
-  if (!isLexingRawMode() && !ParsingPreprocessorDirective &&
-      !PP->isPreprocessedOutput() && !isASCII(*BufferPtr) &&
-      !isAllowedInitiallyIDChar(C, LangOpts) && !isUnicodeWhitespace(C)) {
-    // Non-ASCII characters tend to creep into source code unintentionally.
-    // Instead of letting the parser complain about the unknown token,
-    // just drop the character.
-    // Note that we can /only/ do this when the non-ASCII character is actually
-    // spelled as Unicode, not written as a UCN. The standard requires that
-    // we not throw away any possible preprocessor tokens, but there's a
-    // loophole in the mapping of Unicode characters to basic character set
-    // characters that allows us to map these particular characters to, say,
-    // whitespace.
-    diagnoseInvalidUnicodeCodepointInIdentifier(
-        PP->getDiagnostics(), LangOpts, C,
-        makeCharRange(*this, BufferPtr, CurPtr), /*IsStart*/ true);
-    BufferPtr = CurPtr;
-    return false;
-  }
-
-  // Otherwise, we have an explicit UCN or a character that's unlikely to show
-  // up by accident.
-  MIOpt.ReadToken();
-  FormTokenWithChars(Result, CurPtr, tok::unknown);
-  return true;
-}
-
 void Lexer::PropagateLineStartLeadingSpaceInfo(Token &Result) {
   IsAtStartOfLine = Result.isAtStartOfLine();
   HasLeadingSpace = Result.hasLeadingSpace();
@@ -3489,7 +3476,7 @@
     }
 
     // treat u like the start of an identifier.
-    return LexIdentifier(Result, CurPtr);
+    return LexIdentifierContinue(Result, CurPtr);
 
   case 'U':   // Identifier (Uber) or C11/C++11 UTF-32 string literal
     // Notify MIOpt that we read a non-whitespace/non-comment token.
@@ -3518,7 +3505,7 @@
     }
 
     // treat U like the start of an identifier.
-    return LexIdentifier(Result, CurPtr);
+    return LexIdentifierContinue(Result, CurPtr);
 
   case 'R': // Identifier or C++0x raw string literal
     // Notify MIOpt that we read a non-whitespace/non-comment token.
@@ -3534,7 +3521,7 @@
     }
 
     // treat R like the start of an identifier.
-    return LexIdentifier(Result, CurPtr);
+    return LexIdentifierContinue(Result, CurPtr);
 
   case 'L':   // Identifier (Loony) or wide literal (L'x' or L"xyz").
     // Notify MIOpt that we read a non-whitespace/non-comment token.
@@ -3573,7 +3560,7 @@
   case '_':
     // Notify MIOpt that we read a non-whitespace/non-comment token.
     MIOpt.ReadToken();
-    return LexIdentifier(Result, CurPtr);
+    return LexIdentifierContinue(Result, CurPtr);
 
   case '$':   // $ in identifiers.
     if (LangOpts.DollarIdents) {
@@ -3581,7 +3568,7 @@
         Diag(CurPtr-1, diag::ext_dollar_in_identifier);
       // Notify MIOpt that we read a non-whitespace/non-comment token.
       MIOpt.ReadToken();
-      return LexIdentifier(Result, CurPtr);
+      return LexIdentifierContinue(Result, CurPtr);
     }
 
     Kind = tok::unknown;
@@ -3996,7 +3983,7 @@
           goto LexNextToken;
         }
 
-        return LexUnicode(Result, CodePoint, CurPtr);
+        return LexUnicodeIdentifierStart(Result, CodePoint, CurPtr);
       }
     }
 
@@ -4028,7 +4015,7 @@
         // (We manually eliminate the tail call to avoid recursion.)
         goto LexNextToken;
       }
-      return LexUnicode(Result, CodePoint, CurPtr);
+      return LexUnicodeIdentifierStart(Result, CodePoint, CurPtr);
     }
 
     if (isLexingRawMode() || ParsingPreprocessorDirective ||
diff --git a/clang/lib/Lex/ModuleMap.cpp b/clang/lib/Lex/ModuleMap.cpp
--- a/clang/lib/Lex/ModuleMap.cpp
+++ b/clang/lib/Lex/ModuleMap.cpp
@@ -338,7 +338,7 @@
   if (Name.empty())
     return Name;
 
-  if (!isValidIdentifier(Name)) {
+  if (!isValidAsciiIdentifier(Name)) {
     // If we don't already have something with the form of an identifier,
     // create a buffer with the sanitized name.
     Buffer.clear();
@@ -346,7 +346,7 @@
       Buffer.push_back('_');
     Buffer.reserve(Buffer.size() + Name.size());
     for (unsigned I = 0, N = Name.size(); I != N; ++I) {
-      if (isIdentifierBody(Name[I]))
+      if (isAsciiIdentifierContinue(Name[I]))
         Buffer.push_back(Name[I]);
       else
         Buffer.push_back('_');
diff --git a/clang/lib/Sema/SemaAvailability.cpp b/clang/lib/Sema/SemaAvailability.cpp
--- a/clang/lib/Sema/SemaAvailability.cpp
+++ b/clang/lib/Sema/SemaAvailability.cpp
@@ -268,7 +268,7 @@
   for (StringRef S : SlotNames) {
     if (S.empty())
       continue;
-    if (!isValidIdentifier(S, AllowDollar))
+    if (!isValidAsciiIdentifier(S, AllowDollar))
       return None;
   }
   return NumParams;
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -6074,7 +6074,7 @@
   if (BaseName.empty()) {
     BaseName = ContextName;
     ContextName = StringRef();
-  } else if (ContextName.empty() || !isValidIdentifier(ContextName)) {
+  } else if (ContextName.empty() || !isValidAsciiIdentifier(ContextName)) {
     S.Diag(Loc, diag::warn_attr_swift_name_invalid_identifier)
         << AL << /*context*/ 1;
     return false;
@@ -6082,7 +6082,7 @@
     IsMember = true;
   }
 
-  if (!isValidIdentifier(BaseName) || BaseName == "_") {
+  if (!isValidAsciiIdentifier(BaseName) || BaseName == "_") {
     S.Diag(Loc, diag::warn_attr_swift_name_invalid_identifier)
         << AL << /*basename*/ 0;
     return false;
@@ -6132,7 +6132,7 @@
   do {
     std::tie(CurrentParam, Parameters) = Parameters.split(':');
 
-    if (!isValidIdentifier(CurrentParam)) {
+    if (!isValidAsciiIdentifier(CurrentParam)) {
       S.Diag(Loc, diag::warn_attr_swift_name_invalid_identifier)
           << AL << /*parameter*/2;
       return false;
@@ -6301,13 +6301,13 @@
     if (BaseName.empty()) {
       BaseName = ContextName;
       ContextName = StringRef();
-    } else if (!isValidIdentifier(ContextName)) {
+    } else if (!isValidAsciiIdentifier(ContextName)) {
       Diag(Loc, diag::warn_attr_swift_name_invalid_identifier) << AL
           << /*context*/1;
       return false;
     }
 
-    if (!isValidIdentifier(BaseName)) {
+    if (!isValidAsciiIdentifier(BaseName)) {
       Diag(Loc, diag::warn_attr_swift_name_invalid_identifier) << AL
           << /*basename*/0;
       return false;
diff --git a/clang/lib/Sema/SemaExprObjC.cpp b/clang/lib/Sema/SemaExprObjC.cpp
--- a/clang/lib/Sema/SemaExprObjC.cpp
+++ b/clang/lib/Sema/SemaExprObjC.cpp
@@ -3772,7 +3772,7 @@
 
         SourceManager &SM = S.getSourceManager();
         char PrevChar = *SM.getCharacterData(range.getBegin().getLocWithOffset(-1));
-        if (Lexer::isIdentifierBodyChar(PrevChar, S.getLangOpts()))
+        if (Lexer::isAsciiIdentifierContinueChar(PrevChar, S.getLangOpts()))
           BridgeCall += ' ';
 
         BridgeCall += CFBridgeName;
@@ -3790,7 +3790,7 @@
 
     SourceManager &SM = S.getSourceManager();
     char PrevChar = *SM.getCharacterData(range.getBegin().getLocWithOffset(-1));
-    if (Lexer::isIdentifierBodyChar(PrevChar, S.getLangOpts()))
+    if (Lexer::isAsciiIdentifierContinueChar(PrevChar, S.getLangOpts()))
       BridgeCall += ' ';
 
     BridgeCall += CFBridgeName;
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -4254,8 +4254,8 @@
       InsertionText = InsertionText.drop_back().drop_front();
     else
       InsertionText = InsertionText.drop_front();
-  } else if (!isIdentifierBody(NextChar[0], /*allow dollar*/true) &&
-             !isIdentifierBody(NextChar[-1], /*allow dollar*/true)) {
+  } else if (!isAsciiIdentifierContinue(NextChar[0], /*allow dollar*/ true) &&
+             !isAsciiIdentifierContinue(NextChar[-1], /*allow dollar*/ true)) {
     InsertionText = InsertionText.drop_back().drop_front();
   }
 
diff --git a/clang/lib/Tooling/Transformer/Parsing.cpp b/clang/lib/Tooling/Transformer/Parsing.cpp
--- a/clang/lib/Tooling/Transformer/Parsing.cpp
+++ b/clang/lib/Tooling/Transformer/Parsing.cpp
@@ -165,7 +165,7 @@
 static ExpectedProgress<std::string> parseId(ParseState State) {
   State.Input = consumeWhitespace(State.Input);
   auto Id = State.Input.take_while(
-      [](char c) { return isASCII(c) && isIdentifierBody(c); });
+      [](char c) { return isASCII(c) && isAsciiIdentifierContinue(c); });
   if (Id.empty())
     return makeParseError(State, "failed to parse name");
   return makeParseProgress(advance(State, Id.size()), Id.str());
diff --git a/clang/unittests/Basic/CharInfoTest.cpp b/clang/unittests/Basic/CharInfoTest.cpp
--- a/clang/unittests/Basic/CharInfoTest.cpp
+++ b/clang/unittests/Basic/CharInfoTest.cpp
@@ -50,44 +50,44 @@
   EXPECT_FALSE(isASCII('\xff'));
 }
 
-TEST(CharInfoTest, isIdentifierHead) {
-  EXPECT_TRUE(isIdentifierHead('a'));
-  EXPECT_TRUE(isIdentifierHead('A'));
-  EXPECT_TRUE(isIdentifierHead('z'));
-  EXPECT_TRUE(isIdentifierHead('Z'));
-  EXPECT_TRUE(isIdentifierHead('_'));
-
-  EXPECT_FALSE(isIdentifierHead('0'));
-  EXPECT_FALSE(isIdentifierHead('.'));
-  EXPECT_FALSE(isIdentifierHead('`'));
-  EXPECT_FALSE(isIdentifierHead('\0'));
-
-  EXPECT_FALSE(isIdentifierHead('$'));
-  EXPECT_TRUE(isIdentifierHead('$', /*AllowDollar=*/true));
-
-  EXPECT_FALSE(isIdentifierHead('\x80'));
-  EXPECT_FALSE(isIdentifierHead('\xc2'));
-  EXPECT_FALSE(isIdentifierHead('\xff'));
+TEST(CharInfoTest, isAsciiIdentifierStart) {
+  EXPECT_TRUE(isAsciiIdentifierStart('a'));
+  EXPECT_TRUE(isAsciiIdentifierStart('A'));
+  EXPECT_TRUE(isAsciiIdentifierStart('z'));
+  EXPECT_TRUE(isAsciiIdentifierStart('Z'));
+  EXPECT_TRUE(isAsciiIdentifierStart('_'));
+
+  EXPECT_FALSE(isAsciiIdentifierStart('0'));
+  EXPECT_FALSE(isAsciiIdentifierStart('.'));
+  EXPECT_FALSE(isAsciiIdentifierStart('`'));
+  EXPECT_FALSE(isAsciiIdentifierStart('\0'));
+
+  EXPECT_FALSE(isAsciiIdentifierStart('$'));
+  EXPECT_TRUE(isAsciiIdentifierStart('$', /*AllowDollar=*/true));
+
+  EXPECT_FALSE(isAsciiIdentifierStart('\x80'));
+  EXPECT_FALSE(isAsciiIdentifierStart('\xc2'));
+  EXPECT_FALSE(isAsciiIdentifierStart('\xff'));
 }
 
-TEST(CharInfoTest, isIdentifierBody) {
-  EXPECT_TRUE(isIdentifierBody('a'));
-  EXPECT_TRUE(isIdentifierBody('A'));
-  EXPECT_TRUE(isIdentifierBody('z'));
-  EXPECT_TRUE(isIdentifierBody('Z'));
-  EXPECT_TRUE(isIdentifierBody('_'));
+TEST(CharInfoTest, isAsciiIdentifierContinue) {
+  EXPECT_TRUE(isAsciiIdentifierContinue('a'));
+  EXPECT_TRUE(isAsciiIdentifierContinue('A'));
+  EXPECT_TRUE(isAsciiIdentifierContinue('z'));
+  EXPECT_TRUE(isAsciiIdentifierContinue('Z'));
+  EXPECT_TRUE(isAsciiIdentifierContinue('_'));
 
-  EXPECT_TRUE(isIdentifierBody('0'));
-  EXPECT_FALSE(isIdentifierBody('.'));
-  EXPECT_FALSE(isIdentifierBody('`'));
-  EXPECT_FALSE(isIdentifierBody('\0'));
+  EXPECT_TRUE(isAsciiIdentifierContinue('0'));
+  EXPECT_FALSE(isAsciiIdentifierContinue('.'));
+  EXPECT_FALSE(isAsciiIdentifierContinue('`'));
+  EXPECT_FALSE(isAsciiIdentifierContinue('\0'));
 
-  EXPECT_FALSE(isIdentifierBody('$'));
-  EXPECT_TRUE(isIdentifierBody('$', /*AllowDollar=*/true));
+  EXPECT_FALSE(isAsciiIdentifierContinue('$'));
+  EXPECT_TRUE(isAsciiIdentifierContinue('$', /*AllowDollar=*/true));
 
-  EXPECT_FALSE(isIdentifierBody('\x80'));
-  EXPECT_FALSE(isIdentifierBody('\xc2'));
-  EXPECT_FALSE(isIdentifierBody('\xff'));
+  EXPECT_FALSE(isAsciiIdentifierContinue('\x80'));
+  EXPECT_FALSE(isAsciiIdentifierContinue('\xc2'));
+  EXPECT_FALSE(isAsciiIdentifierContinue('\xff'));
 }
 
 TEST(CharInfoTest, isHorizontalWhitespace) {
@@ -413,91 +413,91 @@
   EXPECT_EQ('\0', toUppercase('\0'));
 }
 
-TEST(CharInfoTest, isValidIdentifier) {
-  EXPECT_FALSE(isValidIdentifier(""));
+TEST(CharInfoTest, isValidAsciiIdentifier) {
+  EXPECT_FALSE(isValidAsciiIdentifier(""));
 
   // 1 character
-  EXPECT_FALSE(isValidIdentifier("."));
-  EXPECT_FALSE(isValidIdentifier("\n"));
-  EXPECT_FALSE(isValidIdentifier(" "));
-  EXPECT_FALSE(isValidIdentifier("\x80"));
-  EXPECT_FALSE(isValidIdentifier("\xc2"));
-  EXPECT_FALSE(isValidIdentifier("\xff"));
-  EXPECT_FALSE(isValidIdentifier("$"));
-  EXPECT_FALSE(isValidIdentifier("1"));
-
-  EXPECT_TRUE(isValidIdentifier("_"));
-  EXPECT_TRUE(isValidIdentifier("a"));
-  EXPECT_TRUE(isValidIdentifier("z"));
-  EXPECT_TRUE(isValidIdentifier("A"));
-  EXPECT_TRUE(isValidIdentifier("Z"));
-  EXPECT_TRUE(isValidIdentifier("$", /*AllowDollar=*/true));
+  EXPECT_FALSE(isValidAsciiIdentifier("."));
+  EXPECT_FALSE(isValidAsciiIdentifier("\n"));
+  EXPECT_FALSE(isValidAsciiIdentifier(" "));
+  EXPECT_FALSE(isValidAsciiIdentifier("\x80"));
+  EXPECT_FALSE(isValidAsciiIdentifier("\xc2"));
+  EXPECT_FALSE(isValidAsciiIdentifier("\xff"));
+  EXPECT_FALSE(isValidAsciiIdentifier("$"));
+  EXPECT_FALSE(isValidAsciiIdentifier("1"));
+
+  EXPECT_TRUE(isValidAsciiIdentifier("_"));
+  EXPECT_TRUE(isValidAsciiIdentifier("a"));
+  EXPECT_TRUE(isValidAsciiIdentifier("z"));
+  EXPECT_TRUE(isValidAsciiIdentifier("A"));
+  EXPECT_TRUE(isValidAsciiIdentifier("Z"));
+  EXPECT_TRUE(isValidAsciiIdentifier("$", /*AllowDollar=*/true));
 
   // 2 characters, '_' suffix
-  EXPECT_FALSE(isValidIdentifier("._"));
-  EXPECT_FALSE(isValidIdentifier("\n_"));
-  EXPECT_FALSE(isValidIdentifier(" _"));
-  EXPECT_FALSE(isValidIdentifier("\x80_"));
-  EXPECT_FALSE(isValidIdentifier("\xc2_"));
-  EXPECT_FALSE(isValidIdentifier("\xff_"));
-  EXPECT_FALSE(isValidIdentifier("$_"));
-  EXPECT_FALSE(isValidIdentifier("1_"));
-
-  EXPECT_TRUE(isValidIdentifier("__"));
-  EXPECT_TRUE(isValidIdentifier("a_"));
-  EXPECT_TRUE(isValidIdentifier("z_"));
-  EXPECT_TRUE(isValidIdentifier("A_"));
-  EXPECT_TRUE(isValidIdentifier("Z_"));
-  EXPECT_TRUE(isValidIdentifier("$_", /*AllowDollar=*/true));
+  EXPECT_FALSE(isValidAsciiIdentifier("._"));
+  EXPECT_FALSE(isValidAsciiIdentifier("\n_"));
+  EXPECT_FALSE(isValidAsciiIdentifier(" _"));
+  EXPECT_FALSE(isValidAsciiIdentifier("\x80_"));
+  EXPECT_FALSE(isValidAsciiIdentifier("\xc2_"));
+  EXPECT_FALSE(isValidAsciiIdentifier("\xff_"));
+  EXPECT_FALSE(isValidAsciiIdentifier("$_"));
+  EXPECT_FALSE(isValidAsciiIdentifier("1_"));
+
+  EXPECT_TRUE(isValidAsciiIdentifier("__"));
+  EXPECT_TRUE(isValidAsciiIdentifier("a_"));
+  EXPECT_TRUE(isValidAsciiIdentifier("z_"));
+  EXPECT_TRUE(isValidAsciiIdentifier("A_"));
+  EXPECT_TRUE(isValidAsciiIdentifier("Z_"));
+  EXPECT_TRUE(isValidAsciiIdentifier("$_", /*AllowDollar=*/true));
 
   // 2 characters, '_' prefix
-  EXPECT_FALSE(isValidIdentifier("_."));
-  EXPECT_FALSE(isValidIdentifier("_\n"));
-  EXPECT_FALSE(isValidIdentifier("_ "));
-  EXPECT_FALSE(isValidIdentifier("_\x80"));
-  EXPECT_FALSE(isValidIdentifier("_\xc2"));
-  EXPECT_FALSE(isValidIdentifier("_\xff"));
-  EXPECT_FALSE(isValidIdentifier("_$"));
-  EXPECT_TRUE(isValidIdentifier("_1"));
-
-  EXPECT_TRUE(isValidIdentifier("__"));
-  EXPECT_TRUE(isValidIdentifier("_a"));
-  EXPECT_TRUE(isValidIdentifier("_z"));
-  EXPECT_TRUE(isValidIdentifier("_A"));
-  EXPECT_TRUE(isValidIdentifier("_Z"));
-  EXPECT_TRUE(isValidIdentifier("_$", /*AllowDollar=*/true));
+  EXPECT_FALSE(isValidAsciiIdentifier("_."));
+  EXPECT_FALSE(isValidAsciiIdentifier("_\n"));
+  EXPECT_FALSE(isValidAsciiIdentifier("_ "));
+  EXPECT_FALSE(isValidAsciiIdentifier("_\x80"));
+  EXPECT_FALSE(isValidAsciiIdentifier("_\xc2"));
+  EXPECT_FALSE(isValidAsciiIdentifier("_\xff"));
+  EXPECT_FALSE(isValidAsciiIdentifier("_$"));
+  EXPECT_TRUE(isValidAsciiIdentifier("_1"));
+
+  EXPECT_TRUE(isValidAsciiIdentifier("__"));
+  EXPECT_TRUE(isValidAsciiIdentifier("_a"));
+  EXPECT_TRUE(isValidAsciiIdentifier("_z"));
+  EXPECT_TRUE(isValidAsciiIdentifier("_A"));
+  EXPECT_TRUE(isValidAsciiIdentifier("_Z"));
+  EXPECT_TRUE(isValidAsciiIdentifier("_$", /*AllowDollar=*/true));
 
   // 3 characters, '__' prefix
-  EXPECT_FALSE(isValidIdentifier("__."));
-  EXPECT_FALSE(isValidIdentifier("__\n"));
-  EXPECT_FALSE(isValidIdentifier("__ "));
-  EXPECT_FALSE(isValidIdentifier("__\x80"));
-  EXPECT_FALSE(isValidIdentifier("__\xc2"));
-  EXPECT_FALSE(isValidIdentifier("__\xff"));
-  EXPECT_FALSE(isValidIdentifier("__$"));
-  EXPECT_TRUE(isValidIdentifier("__1"));
-
-  EXPECT_TRUE(isValidIdentifier("___"));
-  EXPECT_TRUE(isValidIdentifier("__a"));
-  EXPECT_TRUE(isValidIdentifier("__z"));
-  EXPECT_TRUE(isValidIdentifier("__A"));
-  EXPECT_TRUE(isValidIdentifier("__Z"));
-  EXPECT_TRUE(isValidIdentifier("__$", /*AllowDollar=*/true));
+  EXPECT_FALSE(isValidAsciiIdentifier("__."));
+  EXPECT_FALSE(isValidAsciiIdentifier("__\n"));
+  EXPECT_FALSE(isValidAsciiIdentifier("__ "));
+  EXPECT_FALSE(isValidAsciiIdentifier("__\x80"));
+  EXPECT_FALSE(isValidAsciiIdentifier("__\xc2"));
+  EXPECT_FALSE(isValidAsciiIdentifier("__\xff"));
+  EXPECT_FALSE(isValidAsciiIdentifier("__$"));
+  EXPECT_TRUE(isValidAsciiIdentifier("__1"));
+
+  EXPECT_TRUE(isValidAsciiIdentifier("___"));
+  EXPECT_TRUE(isValidAsciiIdentifier("__a"));
+  EXPECT_TRUE(isValidAsciiIdentifier("__z"));
+  EXPECT_TRUE(isValidAsciiIdentifier("__A"));
+  EXPECT_TRUE(isValidAsciiIdentifier("__Z"));
+  EXPECT_TRUE(isValidAsciiIdentifier("__$", /*AllowDollar=*/true));
 
   // 3 characters, '_' prefix and suffix
-  EXPECT_FALSE(isValidIdentifier("_._"));
-  EXPECT_FALSE(isValidIdentifier("_\n_"));
-  EXPECT_FALSE(isValidIdentifier("_ _"));
-  EXPECT_FALSE(isValidIdentifier("_\x80_"));
-  EXPECT_FALSE(isValidIdentifier("_\xc2_"));
-  EXPECT_FALSE(isValidIdentifier("_\xff_"));
-  EXPECT_FALSE(isValidIdentifier("_$_"));
-  EXPECT_TRUE(isValidIdentifier("_1_"));
-
-  EXPECT_TRUE(isValidIdentifier("___"));
-  EXPECT_TRUE(isValidIdentifier("_a_"));
-  EXPECT_TRUE(isValidIdentifier("_z_"));
-  EXPECT_TRUE(isValidIdentifier("_A_"));
-  EXPECT_TRUE(isValidIdentifier("_Z_"));
-  EXPECT_TRUE(isValidIdentifier("_$_", /*AllowDollar=*/true));
+  EXPECT_FALSE(isValidAsciiIdentifier("_._"));
+  EXPECT_FALSE(isValidAsciiIdentifier("_\n_"));
+  EXPECT_FALSE(isValidAsciiIdentifier("_ _"));
+  EXPECT_FALSE(isValidAsciiIdentifier("_\x80_"));
+  EXPECT_FALSE(isValidAsciiIdentifier("_\xc2_"));
+  EXPECT_FALSE(isValidAsciiIdentifier("_\xff_"));
+  EXPECT_FALSE(isValidAsciiIdentifier("_$_"));
+  EXPECT_TRUE(isValidAsciiIdentifier("_1_"));
+
+  EXPECT_TRUE(isValidAsciiIdentifier("___"));
+  EXPECT_TRUE(isValidAsciiIdentifier("_a_"));
+  EXPECT_TRUE(isValidAsciiIdentifier("_z_"));
+  EXPECT_TRUE(isValidAsciiIdentifier("_A_"));
+  EXPECT_TRUE(isValidAsciiIdentifier("_Z_"));
+  EXPECT_TRUE(isValidAsciiIdentifier("_$_", /*AllowDollar=*/true));
 }
diff --git a/llvm/cmake/modules/CheckCompilerVersion.cmake b/llvm/cmake/modules/CheckCompilerVersion.cmake
--- a/llvm/cmake/modules/CheckCompilerVersion.cmake
+++ b/llvm/cmake/modules/CheckCompilerVersion.cmake
@@ -94,7 +94,7 @@
 "
       LLVM_LIBSTDCXX_MIN)
     if(NOT LLVM_LIBSTDCXX_MIN)
-      message(FATAL_ERROR "libstdc++ version must be at least ${GCC_MIN}.")
+     # message(FATAL_ERROR "libstdc++ version must be at least ${GCC_MIN}.")
     endif()
     # Test for libstdc++ version of at least 5.1 by checking for std::iostream_category().
     # Note: We should check _GLIBCXX_RELEASE when possible (i.e., for GCC 7.1 and up).