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
@@ -1048,11 +1048,15 @@
   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();
+      // We encountered an error in parsing 'decltype(...)' so lets revert
+      // the tokens (that we likely had to skip over to get to a stop token such
+      // as a semi-colon) in the backtracking cache to the EndLoc of where we
+      // think the decltype specifier tokens should have ended. Since these
+      // tokens are in the same token cache, their SourceLocations must have the
+      // same FileID and so should have well defined behavior when comparing.
+      assert(EndLoc <= PP.getLastCachedTokenLocation());
+      while (EndLoc != PP.getLastCachedTokenLocation())
+        PP.RevertCachedTokens(1);
     }
   }
   else
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  {{use of undeclared identifier}} expected-error {{expected}}
+  g2<decltype x; // expected-error 2{{expected}} expected-note {{to match}}
+}                   
+
+template<class T> void f2() {
+  g<decltype>(); // expected-error {{use of undeclared identifier}} expected-error {{expected}}
+  g2<decltype x; // expected-error 2{{expected}} expected-note {{to match}}
+}
+