diff --git a/clang/lib/Parse/ParseDeclCXX.cpp b/clang/lib/Parse/ParseDeclCXX.cpp
--- a/clang/lib/Parse/ParseDeclCXX.cpp
+++ b/clang/lib/Parse/ParseDeclCXX.cpp
@@ -1045,8 +1045,16 @@
                                                SourceLocation StartLoc,
                                                SourceLocation EndLoc) {
   // make sure we have a token we can turn into an annotation token
-  if (PP.isBacktrackEnabled())
+  if (PP.isBacktrackEnabled()) {
     PP.RevertCachedTokens(1);
+    if (DS.getTypeSpecType() == TST_error) {
+      // We encountered an error in parsing 'decltype(...)' so lets annotate all
+      // the tokens in the backtracking cache - that we likely had to skip over
+      // to get to a token that allows us to resume parsing, such as a
+      // semi-colon.
+      EndLoc = PP.getLastCachedTokenLocation();
+    }
+  }
   else
     PP.EnterToken(Tok, /*IsReinject*/true);
 
diff --git a/clang/test/Parser/PR42049.cpp b/clang/test/Parser/PR42049.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Parser/PR42049.cpp
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+// https://bugs.llvm.org/show_bug.cgi?id=42049
+
+void f() {
+  g<decltype>(); // expected-error 2 {{expected}} expected-note {{to match}}
+  g2<decltype x; // expected-error 2{{expected}} expected-note {{to match}}
+}                   
+
+template<class T> void f2() {
+  g<decltype>(); // expected-error 2 {{expected}} expected-note {{to match}}
+  g2<decltype x; // expected-error 2{{expected}} expected-note {{to match}}
+}
+