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
@@ -280,17 +280,20 @@
 class VendorExtQualType final : public Node {
   const Node *Ty;
   StringView Ext;
+  const Node *TA;
 
 public:
-  VendorExtQualType(const Node *Ty_, StringView Ext_)
-      : Node(KVendorExtQualType), Ty(Ty_), Ext(Ext_) {}
+  VendorExtQualType(const Node *Ty_, StringView Ext_, const Node *TA_)
+      : Node(KVendorExtQualType), Ty(Ty_), Ext(Ext_), TA(TA_) {}
 
-  template<typename Fn> void match(Fn F) const { F(Ty, Ext); }
+  template <typename Fn> void match(Fn F) const { F(Ty, Ext, TA); }
 
   void printLeft(OutputStream &S) const override {
     Ty->print(S);
     S += " ";
     S += Ext;
+    if (TA != nullptr)
+      TA->print(S);
   }
 };
 
@@ -3680,8 +3683,6 @@
     if (Qual.empty())
       return nullptr;
 
-    // FIXME parse the optional <template-args> here!
-
     // extension            ::= U <objc-name> <objc-type>  # objc-type<identifier>
     if (Qual.startsWith("objcproto")) {
       StringView ProtoSourceName = Qual.dropFront(std::strlen("objcproto"));
@@ -3699,10 +3700,17 @@
       return make<ObjCProtoName>(Child, Proto);
     }
 
+    Node *TA = nullptr;
+    if (look() == 'I') {
+      TA = getDerived().parseTemplateArgs();
+      if (TA == nullptr)
+        return nullptr;
+    }
+
     Node *Child = getDerived().parseQualifiedType();
     if (Child == nullptr)
       return nullptr;
-    return make<VendorExtQualType>(Child, Qual);
+    return make<VendorExtQualType>(Child, Qual, TA);
   }
 
   Qualifiers Quals = parseCVQualifiers();
diff --git a/libcxxabi/test/test_demangle.pass.cpp b/libcxxabi/test/test_demangle.pass.cpp
--- a/libcxxabi/test/test_demangle.pass.cpp
+++ b/libcxxabi/test/test_demangle.pass.cpp
@@ -29843,6 +29843,10 @@
     {"_Z1fIL4Enumn1EEvv", "void f<(Enum)-1>()"},
 
     {"_ZN1A1gIiEEDTcldtptfpT1b1fIT_EEEv", "decltype(this->b.f<int>()) A::g<int>()"},
+
+    // Optional template-args for vendor extended type qualifier.
+    // See https://bugs.llvm.org/show_bug.cgi?id=48009.
+    {"_Z3fooILi79EEbU7_ExtIntIXT_EEi", "bool foo<79>(int _ExtInt<79>)"},
 };
 
 const unsigned N = sizeof(cases) / sizeof(cases[0]);
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
@@ -280,17 +280,20 @@
 class VendorExtQualType final : public Node {
   const Node *Ty;
   StringView Ext;
+  const Node *TA;
 
 public:
-  VendorExtQualType(const Node *Ty_, StringView Ext_)
-      : Node(KVendorExtQualType), Ty(Ty_), Ext(Ext_) {}
+  VendorExtQualType(const Node *Ty_, StringView Ext_, const Node *TA_)
+      : Node(KVendorExtQualType), Ty(Ty_), Ext(Ext_), TA(TA_) {}
 
-  template<typename Fn> void match(Fn F) const { F(Ty, Ext); }
+  template <typename Fn> void match(Fn F) const { F(Ty, Ext, TA); }
 
   void printLeft(OutputStream &S) const override {
     Ty->print(S);
     S += " ";
     S += Ext;
+    if (TA != nullptr)
+      TA->print(S);
   }
 };
 
@@ -3680,8 +3683,6 @@
     if (Qual.empty())
       return nullptr;
 
-    // FIXME parse the optional <template-args> here!
-
     // extension            ::= U <objc-name> <objc-type>  # objc-type<identifier>
     if (Qual.startsWith("objcproto")) {
       StringView ProtoSourceName = Qual.dropFront(std::strlen("objcproto"));
@@ -3699,10 +3700,17 @@
       return make<ObjCProtoName>(Child, Proto);
     }
 
+    Node *TA = nullptr;
+    if (look() == 'I') {
+      TA = getDerived().parseTemplateArgs();
+      if (TA == nullptr)
+        return nullptr;
+    }
+
     Node *Child = getDerived().parseQualifiedType();
     if (Child == nullptr)
       return nullptr;
-    return make<VendorExtQualType>(Child, Qual);
+    return make<VendorExtQualType>(Child, Qual, TA);
   }
 
   Qualifiers Quals = parseCVQualifiers();
diff --git a/llvm/unittests/Demangle/DemangleTest.cpp b/llvm/unittests/Demangle/DemangleTest.cpp
--- a/llvm/unittests/Demangle/DemangleTest.cpp
+++ b/llvm/unittests/Demangle/DemangleTest.cpp
@@ -21,4 +21,9 @@
             "invocation function for block in foo(int)");
   EXPECT_EQ(demangle("?foo@@YAXH@Z"), "void __cdecl foo(int)");
   EXPECT_EQ(demangle("foo"), "foo");
+
+  // Regression test for demangling of optional template-args for vendor
+  // extended type qualifier (https://bugs.llvm.org/show_bug.cgi?id=48009)
+  EXPECT_EQ(demangle("_Z3fooILi79EEbU7_ExtIntIXT_EEi"),
+            "bool foo<79>(int _ExtInt<79>)");
 }