diff --git a/clang/lib/Parse/ParseExpr.cpp b/clang/lib/Parse/ParseExpr.cpp
index bbc7dadde8f3..9719d857f39e 100644
--- a/clang/lib/Parse/ParseExpr.cpp
+++ b/clang/lib/Parse/ParseExpr.cpp
@@ -1049,6 +1049,11 @@ ExprResult Parser::ParseCastExpression(bool isUnaryExpression,
     //                     simple-type-specifier braced-init-list
     //
     DeclSpec DS(AttrFactory);
+
+    if (!Actions.isSimpleTypeSpecifier(Tok.getKind()))
+      // This can happen if we tried to recover from errors earlier.
+      return ExprError();
+
     ParseCXXSimpleTypeSpecifier(DS);
     if (Tok.isNot(tok::l_paren) &&
         (!getLangOpts().CPlusPlus11 || Tok.isNot(tok::l_brace)))
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 68762e97fa2b..92400df00ce2 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -108,6 +108,7 @@ bool Sema::isSimpleTypeSpecifier(tok::TokenKind Kind) const {
   case tok::kw_char16_t:
   case tok::kw_char32_t:
   case tok::kw_typeof:
+  case tok::annot_decltype:
   case tok::kw_decltype:
     return getLangOpts().CPlusPlus;
 
diff --git a/clang/test/Parser/cxx-decl.cpp b/clang/test/Parser/cxx-decl.cpp
index 281e33b20f8b..c33a8d561caf 100644
--- a/clang/test/Parser/cxx-decl.cpp
+++ b/clang/test/Parser/cxx-decl.cpp
@@ -209,6 +209,14 @@ namespace PR5066 {
   using T = int (*T)(); // expected-error {{type-id cannot have a name}} expected-warning {{C++11}}
 }
 
+namespace PR17255 {
+void foo() {
+  typename A::template B<>; // expected-error {{use of undeclared identifier 'A'}} \
+                            // expected-error {{expected a qualified name after 'typename'}} \
+                            // expected-warning {{'template' keyword outside of a template}}
+}
+}
+
 // PR8380
 extern ""      // expected-error {{unknown linkage language}}
 test6a { ;// expected-error {{C++ requires a type specifier for all declarations}} \