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diff --git a/include/clang/Tooling/Transformer/MatchConsumer.h b/include/clang/Tooling/Transformer/MatchConsumer.h
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+++ b/include/clang/Tooling/Transformer/MatchConsumer.h
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+//===--- MatchConsumer.h - MatchConsumer abstraction ------------*- 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 This file defines the *MatchConsumer* abstraction: a computation over
+/// match results, specifically the `ast_matchers::MatchFinder::MatchResult`
+/// class.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLING_TRANSFORMER_MATCH_CONSUMER_H_
+#define LLVM_CLANG_TOOLING_TRANSFORMER_MATCH_CONSUMER_H_
+
+#include "clang/AST/ASTTypeTraits.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Errc.h"
+#include "llvm/Support/Error.h"
+
+namespace clang {
+namespace transformer {
+/// A failable computation over nodes bound by AST matchers.
+///
+/// The computation should report any errors though its return value (rather
+/// than terminating the program) to enable usage in interactive scenarios like
+/// clang-query.
+///
+/// This is a central abstraction of the Transformer framework.
+template <typename T>
+using MatchConsumer =
+    std::function<Expected<T>(const ast_matchers::MatchFinder::MatchResult &)>;
+
+/// Creates an error that signals that a `MatchConsumer` expected a certain node
+/// to be bound by AST matchers, but it was not actually bound.
+inline llvm::Error notBoundError(llvm::StringRef Id) {
+  return llvm::make_error<llvm::StringError>(llvm::errc::invalid_argument,
+                                             "Id not bound: " + Id);
+}
+
+/// Chooses between the two consumers, based on whether \p ID is bound in the
+/// match.
+template <typename T>
+MatchConsumer<T> ifBound(std::string ID, MatchConsumer<T> TrueC,
+                         MatchConsumer<T> FalseC) {
+  return [=](const ast_matchers::MatchFinder::MatchResult &Result) {
+    auto &Map = Result.Nodes.getMap();
+    return (Map.find(ID) != Map.end() ? TrueC : FalseC)(Result);
+  };
+}
+} // namespace transformer
+
+namespace tooling {
+// DEPRECATED: Temporary alias supporting client migration to the `transformer`
+// namespace.
+using transformer::ifBound;
+} // namespace tooling
+} // namespace clang
+#endif // LLVM_CLANG_TOOLING_TRANSFORMER_MATCH_CONSUMER_H_