Index: libcxxabi/src/demangle/ItaniumDemangle.h
===================================================================
--- libcxxabi/src/demangle/ItaniumDemangle.h
+++ libcxxabi/src/demangle/ItaniumDemangle.h
@@ -2592,34 +2592,38 @@
   if (look() == 'Z')
     return getDerived().parseLocalName(State);
 
-  //        ::= <unscoped-template-name> <template-args>
-  if (look() == 'S' && look(1) != 't') {
-    Node *S = getDerived().parseSubstitution();
-    if (S == nullptr)
-      return nullptr;
-    if (look() != 'I')
-      return nullptr;
-    Node *TA = getDerived().parseTemplateArgs(State != nullptr);
-    if (TA == nullptr)
-      return nullptr;
-    if (State) State->EndsWithTemplateArgs = true;
-    return make<NameWithTemplateArgs>(S, TA);
+  Node *Result = nullptr;
+  bool IsSubst = look() == 'S' && look(1) != 't';
+  if (IsSubst) {
+    // A substitution must lead to:
+    //        ::= <unscoped-template-name> <template-args>
+    Result = getDerived().parseSubstitution();
+  } else {
+    // An unscoped name can be one of:
+    //        ::= <unscoped-name>
+    //        ::= <unscoped-template-name> <template-args>
+    Result = getDerived().parseUnscopedName(State);
   }
-
-  Node *N = getDerived().parseUnscopedName(State);
-  if (N == nullptr)
+  if (Result == nullptr)
     return nullptr;
-  //        ::= <unscoped-template-name> <template-args>
+
   if (look() == 'I') {
-    Subs.push_back(N);
+    //        ::= <unscoped-template-name> <template-args>
+    if (!IsSubst)
+      // An unscoped-template-name is substitutable.
+      Subs.push_back(Result);
     Node *TA = getDerived().parseTemplateArgs(State != nullptr);
     if (TA == nullptr)
       return nullptr;
-    if (State) State->EndsWithTemplateArgs = true;
-    return make<NameWithTemplateArgs>(N, TA);
+    if (State)
+      State->EndsWithTemplateArgs = true;
+    Result = make<NameWithTemplateArgs>(Result, TA);
+  } else if (IsSubst) {
+    // The substitution case must be followed by <template-args>.
+    return nullptr;
   }
-  //        ::= <unscoped-name>
-  return N;
+
+  return Result;
 }
 
 // <local-name> := Z <function encoding> E <entity name> [<discriminator>]
@@ -2664,13 +2668,17 @@
 template <typename Derived, typename Alloc>
 Node *
 AbstractManglingParser<Derived, Alloc>::parseUnscopedName(NameState *State) {
-  if (consumeIf("StL") || consumeIf("St")) {
-    Node *R = getDerived().parseUnqualifiedName(State);
-    if (R == nullptr)
-      return nullptr;
-    return make<StdQualifiedName>(R);
-  }
-  return getDerived().parseUnqualifiedName(State);
+  bool IsStd = consumeIf("St");
+  if (IsStd)
+    consumeIf('L');
+
+  Node *Result = getDerived().parseUnqualifiedName(State);
+  if (Result == nullptr)
+    return nullptr;
+  if (IsStd)
+    Result = make<StdQualifiedName>(Result);
+
+  return Result;
 }
 
 // <unqualified-name> ::= <operator-name> [abi-tags]
@@ -4065,9 +4073,9 @@
   }
   //             ::= <substitution>  # See Compression below
   case 'S': {
-    if (look(1) && look(1) != 't') {
-      Node *Sub = getDerived().parseSubstitution();
-      if (Sub == nullptr)
+    if (look(1) != 't') {
+      Result = getDerived().parseSubstitution();
+      if (Result == nullptr)
         return nullptr;
 
       // Sub could be either of:
@@ -4084,13 +4092,13 @@
         Node *TA = getDerived().parseTemplateArgs();
         if (TA == nullptr)
           return nullptr;
-        Result = make<NameWithTemplateArgs>(Sub, TA);
-        break;
+        Result = make<NameWithTemplateArgs>(Result, TA);
+      } else {
+        // If all we parsed was a substitution, don't re-insert into the
+        // substitution table.
+        return Result;
       }
-
-      // If all we parsed was a substitution, don't re-insert into the
-      // substitution table.
-      return Sub;
+      break;
     }
     DEMANGLE_FALLTHROUGH;
   }
Index: libcxxabi/test/test_demangle.pass.cpp
===================================================================
--- libcxxabi/test/test_demangle.pass.cpp
+++ libcxxabi/test/test_demangle.pass.cpp
@@ -29855,6 +29855,8 @@
     // This should be invalid, but it is currently not recognized as such
     // See https://llvm.org/PR51407
     {"_Zcv1BIRT_EIS1_E", "operator B<><>"},
+
+    {"_Z3TPLIiET_S0_", "int TPL<int>(int)"},
 };
 
 const unsigned N = sizeof(cases) / sizeof(cases[0]);
Index: llvm/include/llvm/Demangle/ItaniumDemangle.h
===================================================================
--- llvm/include/llvm/Demangle/ItaniumDemangle.h
+++ llvm/include/llvm/Demangle/ItaniumDemangle.h
@@ -21,13 +21,12 @@
 #include "DemangleConfig.h"
 #include "StringView.h"
 #include "Utility.h"
-#include <algorithm>
 #include <cassert>
 #include <cctype>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
-#include <limits>
+#include <numeric>
 #include <utility>
 
 #define FOR_EACH_NODE_KIND(X) \
@@ -2593,34 +2592,38 @@
   if (look() == 'Z')
     return getDerived().parseLocalName(State);
 
-  //        ::= <unscoped-template-name> <template-args>
-  if (look() == 'S' && look(1) != 't') {
-    Node *S = getDerived().parseSubstitution();
-    if (S == nullptr)
-      return nullptr;
-    if (look() != 'I')
-      return nullptr;
-    Node *TA = getDerived().parseTemplateArgs(State != nullptr);
-    if (TA == nullptr)
-      return nullptr;
-    if (State) State->EndsWithTemplateArgs = true;
-    return make<NameWithTemplateArgs>(S, TA);
+  Node *Result = nullptr;
+  bool IsSubst = look() == 'S' && look(1) != 't';
+  if (IsSubst) {
+    // A substitution must lead to:
+    //        ::= <unscoped-template-name> <template-args>
+    Result = getDerived().parseSubstitution();
+  } else {
+    // An unscoped name can be one of:
+    //        ::= <unscoped-name>
+    //        ::= <unscoped-template-name> <template-args>
+    Result = getDerived().parseUnscopedName(State);
   }
-
-  Node *N = getDerived().parseUnscopedName(State);
-  if (N == nullptr)
+  if (Result == nullptr)
     return nullptr;
-  //        ::= <unscoped-template-name> <template-args>
+
   if (look() == 'I') {
-    Subs.push_back(N);
+    //        ::= <unscoped-template-name> <template-args>
+    if (!IsSubst)
+      // An unscoped-template-name is substitutable.
+      Subs.push_back(Result);
     Node *TA = getDerived().parseTemplateArgs(State != nullptr);
     if (TA == nullptr)
       return nullptr;
-    if (State) State->EndsWithTemplateArgs = true;
-    return make<NameWithTemplateArgs>(N, TA);
+    if (State)
+      State->EndsWithTemplateArgs = true;
+    Result = make<NameWithTemplateArgs>(Result, TA);
+  } else if (IsSubst) {
+    // The substitution case must be followed by <template-args>.
+    return nullptr;
   }
-  //        ::= <unscoped-name>
-  return N;
+
+  return Result;
 }
 
 // <local-name> := Z <function encoding> E <entity name> [<discriminator>]
@@ -2665,13 +2668,17 @@
 template <typename Derived, typename Alloc>
 Node *
 AbstractManglingParser<Derived, Alloc>::parseUnscopedName(NameState *State) {
-  if (consumeIf("StL") || consumeIf("St")) {
-    Node *R = getDerived().parseUnqualifiedName(State);
-    if (R == nullptr)
-      return nullptr;
-    return make<StdQualifiedName>(R);
-  }
-  return getDerived().parseUnqualifiedName(State);
+  bool IsStd = consumeIf("St");
+  if (IsStd)
+    consumeIf('L');
+
+  Node *Result = getDerived().parseUnqualifiedName(State);
+  if (Result == nullptr)
+    return nullptr;
+  if (IsStd)
+    Result = make<StdQualifiedName>(Result);
+
+  return Result;
 }
 
 // <unqualified-name> ::= <operator-name> [abi-tags]
@@ -4066,9 +4073,9 @@
   }
   //             ::= <substitution>  # See Compression below
   case 'S': {
-    if (look(1) && look(1) != 't') {
-      Node *Sub = getDerived().parseSubstitution();
-      if (Sub == nullptr)
+    if (look(1) != 't') {
+      Result = getDerived().parseSubstitution();
+      if (Result == nullptr)
         return nullptr;
 
       // Sub could be either of:
@@ -4085,13 +4092,13 @@
         Node *TA = getDerived().parseTemplateArgs();
         if (TA == nullptr)
           return nullptr;
-        Result = make<NameWithTemplateArgs>(Sub, TA);
-        break;
+        Result = make<NameWithTemplateArgs>(Result, TA);
+      } else {
+        // If all we parsed was a substitution, don't re-insert into the
+        // substitution table.
+        return Result;
       }
-
-      // If all we parsed was a substitution, don't re-insert into the
-      // substitution table.
-      return Sub;
+      break;
     }
     DEMANGLE_FALLTHROUGH;
   }
Index: llvm/include/llvm/Demangle/StringView.h
===================================================================
--- llvm/include/llvm/Demangle/StringView.h
+++ llvm/include/llvm/Demangle/StringView.h
@@ -14,6 +14,7 @@
 #define DEMANGLE_STRINGVIEW_H
 
 #include "DemangleConfig.h"
+#include <algorithm>
 #include <cassert>
 #include <cstring>
 
@@ -37,16 +38,15 @@
 
   StringView substr(size_t Pos, size_t Len = npos) const {
     assert(Pos <= size());
-    if (Len > size() - Pos)
-      Len = size() - Pos;
-    return StringView(begin() + Pos, Len);
+    return StringView(begin() + Pos, std::min(Len, size() - Pos));
   }
 
   size_t find(char C, size_t From = 0) const {
+    size_t FindBegin = std::min(From, size());
     // Avoid calling memchr with nullptr.
-    if (From < size()) {
+    if (FindBegin < size()) {
       // Just forward to memchr, which is faster than a hand-rolled loop.
-      if (const void *P = ::memchr(First + From, C, size() - From))
+      if (const void *P = ::memchr(First + FindBegin, C, size() - FindBegin))
         return size_t(static_cast<const char *>(P) - First);
     }
     return npos;
@@ -98,7 +98,7 @@
   bool startsWith(StringView Str) const {
     if (Str.size() > size())
       return false;
-    return std::strncmp(Str.begin(), begin(), Str.size()) == 0;
+    return std::equal(Str.begin(), Str.end(), begin());
   }
 
   const char &operator[](size_t Idx) const { return *(begin() + Idx); }
@@ -111,7 +111,7 @@
 
 inline bool operator==(const StringView &LHS, const StringView &RHS) {
   return LHS.size() == RHS.size() &&
-         std::strncmp(LHS.begin(), RHS.begin(), LHS.size()) == 0;
+         std::equal(LHS.begin(), LHS.end(), RHS.begin());
 }
 
 DEMANGLE_NAMESPACE_END
Index: llvm/include/llvm/Demangle/Utility.h
===================================================================
--- llvm/include/llvm/Demangle/Utility.h
+++ llvm/include/llvm/Demangle/Utility.h
@@ -14,11 +14,10 @@
 #define DEMANGLE_UTILITY_H
 
 #include "StringView.h"
-#include <array>
 #include <cstdint>
 #include <cstdlib>
 #include <cstring>
-#include <exception>
+#include <iterator>
 #include <limits>
 
 DEMANGLE_NAMESPACE_BEGIN
@@ -49,8 +48,8 @@
       return;
     }
 
-    std::array<char, 21> Temp;
-    char *TempPtr = Temp.data() + Temp.size();
+    char Temp[21];
+    char *TempPtr = std::end(Temp);
 
     while (N) {
       *--TempPtr = char('0' + N % 10);
@@ -60,7 +59,7 @@
     // Add negative sign...
     if (isNeg)
       *--TempPtr = '-';
-    this->operator<<(StringView(TempPtr, Temp.data() + Temp.size()));
+    this->operator<<(StringView(TempPtr, std::end(Temp)));
   }
 
 public: