diff --git a/clang-tools-extra/pseudo/include/clang-pseudo/Token.h b/clang-tools-extra/pseudo/include/clang-pseudo/Token.h
--- a/clang-tools-extra/pseudo/include/clang-pseudo/Token.h
+++ b/clang-tools-extra/pseudo/include/clang-pseudo/Token.h
@@ -199,12 +199,15 @@
     clang::Language = clang::Language::CXX,
     clang::LangStandard::Kind = clang::LangStandard::lang_unspecified);
 
-/// Derives a token stream by decoding escapes, interpreting raw_identifiers and
-/// splitting the greatergreater token.
+/// Decoding raw tokens written in the source code, returning a derived stream.
 ///
-/// Tokens containing UCNs, escaped newlines, trigraphs etc are decoded and
-/// their backing data is owned by the returned stream.
-/// raw_identifier tokens are assigned specific types (identifier, keyword etc).
+/// - escaped newlines within tokens are removed
+/// - trigraphs are replaced with the characters they encode
+/// - UCNs within raw_identifiers are replaced by the characters they encode
+///   (UCNs within strings, comments etc are not translated)
+/// - raw_identifier tokens are assigned their correct keyword type
+/// - the >> token is split into separate > > tokens
+///   (we use a modified grammar where >> is a nonterminal, not a token)
 ///
 /// The StartsPPLine flag is preserved.
 ///
diff --git a/clang-tools-extra/pseudo/lib/Lex.cpp b/clang-tools-extra/pseudo/lib/Lex.cpp
--- a/clang-tools-extra/pseudo/lib/Lex.cpp
+++ b/clang-tools-extra/pseudo/lib/Lex.cpp
@@ -90,12 +90,23 @@
         assert(CharSize != 0 && "no progress!");
         Pos += CharSize;
       }
-      // Remove universal character names (UCN).
+      llvm::StringRef Text = CleanBuffer;
       llvm::SmallString<64> UCNBuffer;
-      clang::expandUCNs(UCNBuffer, CleanBuffer);
+      // A surface reading of the standard suggests UCNs might appear anywhere.
+      // But we need only decode them in raw_identifiers.
+      //  - they cannot appear in punctuation/keyword tokens, because UCNs
+      //    cannot encode basic characters outside of literals [lex.charset]
+      //  - they can appear in literals, but we need not unescape them now.
+      //    We treat them as escape sequences when evaluating the literal.
+      //  - comments are handled similarly to literals
+      // This is good fortune, because expandUCNs requires its input to be a
+      // reasonably valid identifier (e.g. without stray backslashes).
+      if (Tok.Kind == tok::raw_identifier) {
+        clang::expandUCNs(UCNBuffer, CleanBuffer);
+        Text = UCNBuffer;
+      }
 
-      llvm::StringRef Text = llvm::StringRef(UCNBuffer).copy(*CleanedStorage);
-      Tok.Data = Text.data();
+      Tok.Data = Text.copy(*CleanedStorage).data();
       Tok.Length = Text.size();
       Tok.Flags &= ~static_cast<decltype(Tok.Flags)>(LexFlags::NeedsCleaning);
     }
diff --git a/clang-tools-extra/pseudo/tool/ClangPseudo.cpp b/clang-tools-extra/pseudo/tool/ClangPseudo.cpp
--- a/clang-tools-extra/pseudo/tool/ClangPseudo.cpp
+++ b/clang-tools-extra/pseudo/tool/ClangPseudo.cpp
@@ -17,6 +17,7 @@
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/Signals.h"
 
 using clang::pseudo::Grammar;
 using llvm::cl::desc;
@@ -52,6 +53,7 @@
 
 int main(int argc, char *argv[]) {
   llvm::cl::ParseCommandLineOptions(argc, argv, "");
+  llvm::sys::PrintStackTraceOnErrorSignal(argv[0]);
 
   clang::LangOptions LangOpts = clang::pseudo::genericLangOpts();
   std::string SourceText;