diff --git a/clang/include/clang/AST/VTableBuilder.h b/clang/include/clang/AST/VTableBuilder.h
index 241dd13f903e56..e451f3f861b792 100644
--- a/clang/include/clang/AST/VTableBuilder.h
+++ b/clang/include/clang/AST/VTableBuilder.h
@@ -18,6 +18,7 @@
 #include "clang/AST/GlobalDecl.h"
 #include "clang/AST/RecordLayout.h"
 #include "clang/Basic/ABI.h"
+#include "clang/Basic/Thunk.h"
 #include "llvm/ADT/DenseMap.h"
 #include <memory>
 #include <utility>
diff --git a/clang/include/clang/Basic/ABI.h b/clang/include/clang/Basic/ABI.h
index 2401ffa20494e9..231bad799a42cb 100644
--- a/clang/include/clang/Basic/ABI.h
+++ b/clang/include/clang/Basic/ABI.h
@@ -37,174 +37,6 @@ enum CXXDtorType {
     Dtor_Comdat    ///< The COMDAT used for dtors
 };
 
-/// A return adjustment.
-struct ReturnAdjustment {
-  /// The non-virtual adjustment from the derived object to its
-  /// nearest virtual base.
-  int64_t NonVirtual;
-
-  /// Holds the ABI-specific information about the virtual return
-  /// adjustment, if needed.
-  union VirtualAdjustment {
-    // Itanium ABI
-    struct {
-      /// The offset (in bytes), relative to the address point
-      /// of the virtual base class offset.
-      int64_t VBaseOffsetOffset;
-    } Itanium;
-
-    // Microsoft ABI
-    struct {
-      /// The offset (in bytes) of the vbptr, relative to the beginning
-      /// of the derived class.
-      uint32_t VBPtrOffset;
-
-      /// Index of the virtual base in the vbtable.
-      uint32_t VBIndex;
-    } Microsoft;
-
-    VirtualAdjustment() {
-      memset(this, 0, sizeof(*this));
-    }
-
-    bool Equals(const VirtualAdjustment &Other) const {
-      return memcmp(this, &Other, sizeof(Other)) == 0;
-    }
-
-    bool isEmpty() const {
-      VirtualAdjustment Zero;
-      return Equals(Zero);
-    }
-
-    bool Less(const VirtualAdjustment &RHS) const {
-      return memcmp(this, &RHS, sizeof(RHS)) < 0;
-    }
-  } Virtual;
-
-  ReturnAdjustment() : NonVirtual(0) {}
-
-  bool isEmpty() const { return !NonVirtual && Virtual.isEmpty(); }
-
-  friend bool operator==(const ReturnAdjustment &LHS,
-                         const ReturnAdjustment &RHS) {
-    return LHS.NonVirtual == RHS.NonVirtual && LHS.Virtual.Equals(RHS.Virtual);
-  }
-
-  friend bool operator!=(const ReturnAdjustment &LHS, const ReturnAdjustment &RHS) {
-    return !(LHS == RHS);
-  }
-
-  friend bool operator<(const ReturnAdjustment &LHS,
-                        const ReturnAdjustment &RHS) {
-    if (LHS.NonVirtual < RHS.NonVirtual)
-      return true;
-
-    return LHS.NonVirtual == RHS.NonVirtual && LHS.Virtual.Less(RHS.Virtual);
-  }
-};
-
-/// A \c this pointer adjustment.
-struct ThisAdjustment {
-  /// The non-virtual adjustment from the derived object to its
-  /// nearest virtual base.
-  int64_t NonVirtual;
-
-  /// Holds the ABI-specific information about the virtual this
-  /// adjustment, if needed.
-  union VirtualAdjustment {
-    // Itanium ABI
-    struct {
-      /// The offset (in bytes), relative to the address point,
-      /// of the virtual call offset.
-      int64_t VCallOffsetOffset;
-    } Itanium;
-
-    struct {
-      /// The offset of the vtordisp (in bytes), relative to the ECX.
-      int32_t VtordispOffset;
-
-      /// The offset of the vbptr of the derived class (in bytes),
-      /// relative to the ECX after vtordisp adjustment.
-      int32_t VBPtrOffset;
-
-      /// The offset (in bytes) of the vbase offset in the vbtable.
-      int32_t VBOffsetOffset;
-    } Microsoft;
-
-    VirtualAdjustment() {
-      memset(this, 0, sizeof(*this));
-    }
-
-    bool Equals(const VirtualAdjustment &Other) const {
-      return memcmp(this, &Other, sizeof(Other)) == 0;
-    }
-
-    bool isEmpty() const {
-      VirtualAdjustment Zero;
-      return Equals(Zero);
-    }
-
-    bool Less(const VirtualAdjustment &RHS) const {
-      return memcmp(this, &RHS, sizeof(RHS)) < 0;
-    }
-  } Virtual;
-
-  ThisAdjustment() : NonVirtual(0) { }
-
-  bool isEmpty() const { return !NonVirtual && Virtual.isEmpty(); }
-
-  friend bool operator==(const ThisAdjustment &LHS,
-                         const ThisAdjustment &RHS) {
-    return LHS.NonVirtual == RHS.NonVirtual && LHS.Virtual.Equals(RHS.Virtual);
-  }
-
-  friend bool operator!=(const ThisAdjustment &LHS, const ThisAdjustment &RHS) {
-    return !(LHS == RHS);
-  }
-
-  friend bool operator<(const ThisAdjustment &LHS,
-                        const ThisAdjustment &RHS) {
-    if (LHS.NonVirtual < RHS.NonVirtual)
-      return true;
-
-    return LHS.NonVirtual == RHS.NonVirtual && LHS.Virtual.Less(RHS.Virtual);
-  }
-};
-
-class CXXMethodDecl;
-
-/// The \c this pointer adjustment as well as an optional return
-/// adjustment for a thunk.
-struct ThunkInfo {
-  /// The \c this pointer adjustment.
-  ThisAdjustment This;
-
-  /// The return adjustment.
-  ReturnAdjustment Return;
-
-  /// Holds a pointer to the overridden method this thunk is for,
-  /// if needed by the ABI to distinguish different thunks with equal
-  /// adjustments. Otherwise, null.
-  /// CAUTION: In the unlikely event you need to sort ThunkInfos, consider using
-  /// an ABI-specific comparator.
-  const CXXMethodDecl *Method;
-
-  ThunkInfo() : Method(nullptr) { }
-
-  ThunkInfo(const ThisAdjustment &This, const ReturnAdjustment &Return,
-            const CXXMethodDecl *Method = nullptr)
-      : This(This), Return(Return), Method(Method) {}
-
-  friend bool operator==(const ThunkInfo &LHS, const ThunkInfo &RHS) {
-    return LHS.This == RHS.This && LHS.Return == RHS.Return &&
-           LHS.Method == RHS.Method;
-  }
-
-  bool isEmpty() const {
-    return This.isEmpty() && Return.isEmpty() && Method == nullptr;
-  }
-};
-
 } // end namespace clang
 
 #endif
diff --git a/clang/include/clang/Basic/Thunk.h b/clang/include/clang/Basic/Thunk.h
new file mode 100644
index 00000000000000..7f3c4b24ec6d31
--- /dev/null
+++ b/clang/include/clang/Basic/Thunk.h
@@ -0,0 +1,185 @@
+//===----- Thunk.h - Declarations related to VTable Thunks ------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Enums/classes describing THUNK related information about constructors,
+/// destructors and thunks.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_BASIC_THUNK_H
+#define LLVM_CLANG_BASIC_THUNK_H
+
+#include "clang/AST/GlobalDecl.h"
+
+namespace clang {
+
+/// A return adjustment.
+struct ReturnAdjustment {
+  /// The non-virtual adjustment from the derived object to its
+  /// nearest virtual base.
+  int64_t NonVirtual;
+
+  /// Holds the ABI-specific information about the virtual return
+  /// adjustment, if needed.
+  union VirtualAdjustment {
+    // Itanium ABI
+    struct {
+      /// The offset (in bytes), relative to the address point
+      /// of the virtual base class offset.
+      int64_t VBaseOffsetOffset;
+    } Itanium;
+
+    // Microsoft ABI
+    struct {
+      /// The offset (in bytes) of the vbptr, relative to the beginning
+      /// of the derived class.
+      uint32_t VBPtrOffset;
+
+      /// Index of the virtual base in the vbtable.
+      uint32_t VBIndex;
+    } Microsoft;
+
+    VirtualAdjustment() { memset(this, 0, sizeof(*this)); }
+
+    bool Equals(const VirtualAdjustment &Other) const {
+      return memcmp(this, &Other, sizeof(Other)) == 0;
+    }
+
+    bool isEmpty() const {
+      VirtualAdjustment Zero;
+      return Equals(Zero);
+    }
+
+    bool Less(const VirtualAdjustment &RHS) const {
+      return memcmp(this, &RHS, sizeof(RHS)) < 0;
+    }
+  } Virtual;
+
+  ReturnAdjustment() : NonVirtual(0) {}
+
+  bool isEmpty() const { return !NonVirtual && Virtual.isEmpty(); }
+
+  friend bool operator==(const ReturnAdjustment &LHS,
+                         const ReturnAdjustment &RHS) {
+    return LHS.NonVirtual == RHS.NonVirtual && LHS.Virtual.Equals(RHS.Virtual);
+  }
+
+  friend bool operator!=(const ReturnAdjustment &LHS,
+                         const ReturnAdjustment &RHS) {
+    return !(LHS == RHS);
+  }
+
+  friend bool operator<(const ReturnAdjustment &LHS,
+                        const ReturnAdjustment &RHS) {
+    if (LHS.NonVirtual < RHS.NonVirtual)
+      return true;
+
+    return LHS.NonVirtual == RHS.NonVirtual && LHS.Virtual.Less(RHS.Virtual);
+  }
+};
+
+/// A \c this pointer adjustment.
+struct ThisAdjustment {
+  /// The non-virtual adjustment from the derived object to its
+  /// nearest virtual base.
+  int64_t NonVirtual;
+
+  /// Holds the ABI-specific information about the virtual this
+  /// adjustment, if needed.
+  union VirtualAdjustment {
+    // Itanium ABI
+    struct {
+      /// The offset (in bytes), relative to the address point,
+      /// of the virtual call offset.
+      int64_t VCallOffsetOffset;
+    } Itanium;
+
+    struct {
+      /// The offset of the vtordisp (in bytes), relative to the ECX.
+      int32_t VtordispOffset;
+
+      /// The offset of the vbptr of the derived class (in bytes),
+      /// relative to the ECX after vtordisp adjustment.
+      int32_t VBPtrOffset;
+
+      /// The offset (in bytes) of the vbase offset in the vbtable.
+      int32_t VBOffsetOffset;
+    } Microsoft;
+
+    VirtualAdjustment() { memset(this, 0, sizeof(*this)); }
+
+    bool Equals(const VirtualAdjustment &Other) const {
+      return memcmp(this, &Other, sizeof(Other)) == 0;
+    }
+
+    bool isEmpty() const {
+      VirtualAdjustment Zero;
+      return Equals(Zero);
+    }
+
+    bool Less(const VirtualAdjustment &RHS) const {
+      return memcmp(this, &RHS, sizeof(RHS)) < 0;
+    }
+  } Virtual;
+
+  ThisAdjustment() : NonVirtual(0) {}
+
+  bool isEmpty() const { return !NonVirtual && Virtual.isEmpty(); }
+
+  friend bool operator==(const ThisAdjustment &LHS, const ThisAdjustment &RHS) {
+    return LHS.NonVirtual == RHS.NonVirtual && LHS.Virtual.Equals(RHS.Virtual);
+  }
+
+  friend bool operator!=(const ThisAdjustment &LHS, const ThisAdjustment &RHS) {
+    return !(LHS == RHS);
+  }
+
+  friend bool operator<(const ThisAdjustment &LHS, const ThisAdjustment &RHS) {
+    if (LHS.NonVirtual < RHS.NonVirtual)
+      return true;
+
+    return LHS.NonVirtual == RHS.NonVirtual && LHS.Virtual.Less(RHS.Virtual);
+  }
+};
+
+/// The \c this pointer adjustment as well as an optional return
+/// adjustment for a thunk.
+struct ThunkInfo {
+  /// The \c this pointer adjustment.
+  ThisAdjustment This;
+
+  /// The return adjustment.
+  ReturnAdjustment Return;
+
+  /// Holds a pointer to the overridden method this thunk is for,
+  /// if needed by the ABI to distinguish different thunks with equal
+  /// adjustments. Otherwise, null.
+  /// CAUTION: In the unlikely event you need to sort ThunkInfos, consider using
+  /// an ABI-specific comparator.
+  const CXXMethodDecl *Method;
+
+  ThunkInfo() : Method(nullptr) {}
+
+  ThunkInfo(const ThisAdjustment &This, const ReturnAdjustment &Return,
+            const CXXMethodDecl *Method = nullptr)
+      : This(This), Return(Return), Method(Method) {}
+
+  friend bool operator==(const ThunkInfo &LHS, const ThunkInfo &RHS) {
+    return LHS.This == RHS.This && LHS.Return == RHS.Return &&
+           LHS.Method == RHS.Method;
+  }
+
+  bool isEmpty() const {
+    return This.isEmpty() && Return.isEmpty() && Method == nullptr;
+  }
+};
+
+} // end namespace clang
+
+#endif
diff --git a/clang/lib/AST/ItaniumMangle.cpp b/clang/lib/AST/ItaniumMangle.cpp
index 79fec683b7019d..7420103213db2b 100644
--- a/clang/lib/AST/ItaniumMangle.cpp
+++ b/clang/lib/AST/ItaniumMangle.cpp
@@ -14,7 +14,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "clang/AST/Mangle.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/Attr.h"
 #include "clang/AST/Decl.h"
@@ -23,14 +22,16 @@
 #include "clang/AST/DeclOpenMP.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Expr.h"
-#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprCXX.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprObjC.h"
+#include "clang/AST/Mangle.h"
 #include "clang/AST/TypeLoc.h"
 #include "clang/Basic/ABI.h"
 #include "clang/Basic/Module.h"
 #include "clang/Basic/SourceManager.h"
 #include "clang/Basic/TargetInfo.h"
+#include "clang/Basic/Thunk.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"