diff --git a/libcxxabi/src/demangle/ItaniumDemangle.h b/libcxxabi/src/demangle/ItaniumDemangle.h
index 01f414a7257bf9..45bb741f5629f4 100644
--- a/libcxxabi/src/demangle/ItaniumDemangle.h
+++ b/libcxxabi/src/demangle/ItaniumDemangle.h
@@ -2593,34 +2593,38 @@ Node *AbstractManglingParser<Derived, Alloc>::parseName(NameState *State) {
   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 +2669,17 @@ Node *AbstractManglingParser<Derived, Alloc>::parseLocalName(NameState *State) {
 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 +4074,9 @@ Node *AbstractManglingParser<Derived, Alloc>::parseType() {
   }
   //             ::= <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 +4093,13 @@ Node *AbstractManglingParser<Derived, Alloc>::parseType() {
         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;
   }
diff --git a/libcxxabi/test/test_demangle.pass.cpp b/libcxxabi/test/test_demangle.pass.cpp
index b1bc05661144a0..7bbb17c581cc19 100644
--- a/libcxxabi/test/test_demangle.pass.cpp
+++ b/libcxxabi/test/test_demangle.pass.cpp
@@ -29855,6 +29855,8 @@ const char* cases[][2] =
     // 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]);
diff --git a/llvm/include/llvm/Demangle/ItaniumDemangle.h b/llvm/include/llvm/Demangle/ItaniumDemangle.h
index 98ff73e21cca65..a3a0381edf97d9 100644
--- a/llvm/include/llvm/Demangle/ItaniumDemangle.h
+++ b/llvm/include/llvm/Demangle/ItaniumDemangle.h
@@ -2595,34 +2595,38 @@ Node *AbstractManglingParser<Derived, Alloc>::parseName(NameState *State) {
   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>]
@@ -2667,13 +2671,17 @@ Node *AbstractManglingParser<Derived, Alloc>::parseLocalName(NameState *State) {
 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]
@@ -4068,9 +4076,9 @@ Node *AbstractManglingParser<Derived, Alloc>::parseType() {
   }
   //             ::= <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:
@@ -4087,13 +4095,13 @@ Node *AbstractManglingParser<Derived, Alloc>::parseType() {
         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;
   }