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diff --git a/clang-tools-extra/clangd/ExpectedTypes.h b/clang-tools-extra/clangd/ExpectedTypes.h
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+//===--- ExpectedTypes.h - Simplified C++ types -----------------*- C++-*--===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// A simplified model of C++ types that can be used to check whether they are
+// convertible between each other for the purposes of code completion ranking
+// without looking at the ASTs. Note that we don't aim to fully mimic the C++
+// conversion rules, merely try to have a model that gives useful improvements
+// to the code completion ranking.
+//
+// We define an encoding of AST types as opaque strings, which can be stored in
+// the index. Similar types (such as `int` and `long`) are folded together,
+// forming equivalence classes with the same encoding.
+//===----------------------------------------------------------------------===//
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_EXPECTED_TYPES_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_EXPECTED_TYPES_H
+
+#include "clang/AST/Type.h"
+#include "llvm/ADT/StringRef.h"
+
+namespace clang {
+class CodeCompletionResult;
+
+namespace clangd {
+/// A representation of a type that can be computed based on clang AST and
+/// compared for equality. The encoding is stable between different ASTs, this
+/// allows the representation to be stored in the index and compared with types
+/// coming from a different AST later.
+/// OpaqueType is a strongly-typedefed std::string, you can get the underlying
+/// string with raw().
+class OpaqueType {
+public:
+  /// Create a type from a code completion result.
+  static llvm::Optional<OpaqueType>
+  fromCompletionResult(ASTContext &Ctx, const CodeCompletionResult &R);
+  /// Construct an instance from a clang::QualType. This is usually a
+  /// PreferredType from a clang's completion context.
+  static llvm::Optional<OpaqueType> fromType(ASTContext &Ctx, QualType Type);
+
+  /// Get the raw byte representation of the type. You can only rely on the
+  /// types being equal iff their raw representation is the same. The particular
+  /// details of the used encoding might change over time and one should not
+  /// rely on it.
+  llvm::StringRef raw() const { return Data; }
+
+  friend bool operator==(const OpaqueType &L, const OpaqueType &R) {
+    return L.Data == R.Data;
+  }
+  friend bool operator!=(const OpaqueType &L, const OpaqueType &R) {
+    return !(L == R);
+  }
+
+private:
+  static llvm::Optional<OpaqueType> encode(ASTContext &Ctx, QualType Type);
+  explicit OpaqueType(std::string Data);
+
+  std::string Data;
+};
+} // namespace clangd
+} // namespace clang
+#endif