diff --git a/libcxxabi/src/demangle/ItaniumDemangle.h b/libcxxabi/src/demangle/ItaniumDemangle.h
--- a/libcxxabi/src/demangle/ItaniumDemangle.h
+++ b/libcxxabi/src/demangle/ItaniumDemangle.h
@@ -310,7 +310,7 @@
       printRight(OB);
   }
 
-  // Print the "left" side of this Node into OutputString.
+  // Print the "left" side of this Node into OutputBuffer.
   virtual void printLeft(OutputBuffer &) const = 0;
 
   // Print the "right". This distinction is necessary to represent C++ types
@@ -1210,7 +1210,8 @@
 class ParameterPack final : public Node {
   NodeArray Data;
 
-  // Setup OutputString for a pack expansion unless we're already expanding one.
+  // Setup OutputBuffer for a pack expansion, unless we're already expanding
+  // one.
   void initializePackExpansion(OutputBuffer &OB) const {
     if (OB.CurrentPackMax == std::numeric_limits<unsigned>::max()) {
       OB.CurrentPackMax = static_cast<unsigned>(Data.size());
@@ -2473,7 +2474,7 @@
 
   char consume() { return First != Last ? *First++ : '\0'; }
 
-  char look(unsigned Lookahead = 0) {
+  char look(unsigned Lookahead = 0) const {
     if (static_cast<size_t>(Last - First) <= Lookahead)
       return '\0';
     return First[Lookahead];
@@ -5437,7 +5438,7 @@
   if (!consumeIf('S'))
     return nullptr;
 
-  if (std::islower(look())) {
+  if (look() >= 'a' && look() <= 'z') {
     Node *SpecialSub;
     switch (look()) {
     case 'a':
diff --git a/llvm/include/llvm/Demangle/ItaniumDemangle.h b/llvm/include/llvm/Demangle/ItaniumDemangle.h
--- a/llvm/include/llvm/Demangle/ItaniumDemangle.h
+++ b/llvm/include/llvm/Demangle/ItaniumDemangle.h
@@ -11,8 +11,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef LLVM_DEMANGLE_ITANIUMDEMANGLE_H
-#define LLVM_DEMANGLE_ITANIUMDEMANGLE_H
+#ifndef DEMANGLE_ITANIUMDEMANGLE_H
+#define DEMANGLE_ITANIUMDEMANGLE_H
 
 // FIXME: (possibly) incomplete list of features that clang mangles that this
 // file does not yet support:
@@ -1210,7 +1210,8 @@
 class ParameterPack final : public Node {
   NodeArray Data;
 
-  // Setup OutputBuffer for a pack expansion unless we're already expanding one.
+  // Setup OutputBuffer for a pack expansion, unless we're already expanding
+  // one.
   void initializePackExpansion(OutputBuffer &OB) const {
     if (OB.CurrentPackMax == std::numeric_limits<unsigned>::max()) {
       OB.CurrentPackMax = static_cast<unsigned>(Data.size());
@@ -2473,7 +2474,7 @@
 
   char consume() { return First != Last ? *First++ : '\0'; }
 
-  char look(unsigned Lookahead = 0) {
+  char look(unsigned Lookahead = 0) const {
     if (static_cast<size_t>(Last - First) <= Lookahead)
       return '\0';
     return First[Lookahead];
@@ -5437,7 +5438,7 @@
   if (!consumeIf('S'))
     return nullptr;
 
-  if (std::islower(look())) {
+  if (look() >= 'a' && look() <= 'z') {
     Node *SpecialSub;
     switch (look()) {
     case 'a':
@@ -5747,4 +5748,4 @@
 
 DEMANGLE_NAMESPACE_END
 
-#endif // LLVM_DEMANGLE_ITANIUMDEMANGLE_H
+#endif // DEMANGLE_ITANIUMDEMANGLE_H
diff --git a/llvm/include/llvm/Demangle/StringView.h b/llvm/include/llvm/Demangle/StringView.h
--- a/llvm/include/llvm/Demangle/StringView.h
+++ b/llvm/include/llvm/Demangle/StringView.h
@@ -10,8 +10,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef LLVM_DEMANGLE_STRINGVIEW_H
-#define LLVM_DEMANGLE_STRINGVIEW_H
+#ifndef DEMANGLE_STRINGVIEW_H
+#define DEMANGLE_STRINGVIEW_H
 
 #include "DemangleConfig.h"
 #include <algorithm>
diff --git a/llvm/include/llvm/Demangle/Utility.h b/llvm/include/llvm/Demangle/Utility.h
--- a/llvm/include/llvm/Demangle/Utility.h
+++ b/llvm/include/llvm/Demangle/Utility.h
@@ -10,8 +10,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef LLVM_DEMANGLE_UTILITY_H
-#define LLVM_DEMANGLE_UTILITY_H
+#ifndef DEMANGLE_UTILITY_H
+#define DEMANGLE_UTILITY_H
 
 #include "StringView.h"
 #include <cstdint>