diff --git a/clang/include/clang/Tooling/Refactoring/Transformer.h b/clang/include/clang/Tooling/Refactoring/Transformer.h
--- a/clang/include/clang/Tooling/Refactoring/Transformer.h
+++ b/clang/include/clang/Tooling/Refactoring/Transformer.h
@@ -44,12 +44,21 @@
   Name,
 };
 
-using TextGenerator =
-    std::function<std::string(const ast_matchers::MatchFinder::MatchResult &)>;
+// \c TextGenerator may fail, because it processes dynamically-bound match
+// results.  For example, a typo in the name of a bound node or a mismatch in
+// the node's type can lead to a failure in the string generation code.  We
+// allow the generator to return \c Expected, rather than assert on such
+// failures, so that the Transformer client can choose how to handle the error.
+// For example, if used in a UI (for example, clang-query or a web app), in
+// which the user specifies the rewrite rule, the backend might choose to return
+// a diagnostic error, rather than crash.
+using TextGenerator = std::function<Expected<std::string>(
+    const ast_matchers::MatchFinder::MatchResult &)>;
 
-/// Wraps a string as a TextGenerator.
+/// Wraps a string as a (trivially successful) TextGenerator.
 inline TextGenerator text(std::string M) {
-  return [M](const ast_matchers::MatchFinder::MatchResult &) { return M; };
+  return [M](const ast_matchers::MatchFinder::MatchResult &)
+             -> Expected<std::string> { return M; };
 }
 
 // Description of a source-code edit, expressed in terms of an AST node.
diff --git a/clang/lib/Tooling/Refactoring/Transformer.cpp b/clang/lib/Tooling/Refactoring/Transformer.cpp
--- a/clang/lib/Tooling/Refactoring/Transformer.cpp
+++ b/clang/lib/Tooling/Refactoring/Transformer.cpp
@@ -157,12 +157,16 @@
         Edit.Target, It->second, Edit.Kind, Edit.Part, *Result.Context);
     if (auto Err = RangeOrErr.takeError())
       return std::move(Err);
-    Transformation T;
-    T.Range = *RangeOrErr;
-    if (T.Range.isInvalid() ||
-        isOriginMacroBody(*Result.SourceManager, T.Range.getBegin()))
+    auto &Range = *RangeOrErr;
+    if (Range.isInvalid() ||
+        isOriginMacroBody(*Result.SourceManager, Range.getBegin()))
       return SmallVector<Transformation, 0>();
-    T.Replacement = Edit.Replacement(Result);
+    auto ReplacementOrErr = Edit.Replacement(Result);
+    if (auto Err = ReplacementOrErr.takeError())
+      return std::move(Err);
+    Transformation T;
+    T.Range = Range;
+    T.Replacement = std::move(*ReplacementOrErr);
     Transformations.push_back(std::move(T));
   }
   return Transformations;