Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -8572,9 +8572,9 @@
   "that uses neither 'co_await' nor 'co_yield'">,
   InGroup<DiagGroup<"coreturn-without-coawait">>;
 def err_implied_std_coroutine_traits_not_found : Error<
-  "you need to include <coroutine> before defining a coroutine">;
+  "you need to include <experimental/coroutine> before defining a coroutine">;
 def err_malformed_std_coroutine_traits : Error<
-  "'std::coroutine_traits' must be a class template">;
+  "'std::experimental::coroutine_traits' must be a class template">;
 def err_implied_std_coroutine_traits_promise_type_not_found : Error<
   "this function cannot be a coroutine: %q0 has no member named 'promise_type'">;
 def err_implied_std_coroutine_traits_promise_type_not_class : Error<
Index: include/clang/Sema/Sema.h
===================================================================
--- include/clang/Sema/Sema.h
+++ include/clang/Sema/Sema.h
@@ -725,6 +725,10 @@
   /// standard library.
   LazyDeclPtr StdAlignValT;
 
+  /// \brief The C++ "std::experimental" namespace, where the experimental parts
+  /// of the standard library resides.
+  NamespaceDecl *StdExperimentalNamespaceCache;
+
   /// \brief The C++ "std::initializer_list" template, which is defined in
   /// \<initializer_list>.
   ClassTemplateDecl *StdInitializerList;
@@ -4255,6 +4259,8 @@
   NamespaceDecl *getStdNamespace() const;
   NamespaceDecl *getOrCreateStdNamespace();
 
+  NamespaceDecl *lookupStdExperimentalNamespace();
+
   CXXRecordDecl *getStdBadAlloc() const;
   EnumDecl *getStdAlignValT() const;
 
Index: lib/Sema/Sema.cpp
===================================================================
--- lib/Sema/Sema.cpp
+++ lib/Sema/Sema.cpp
@@ -88,8 +88,8 @@
     VisContext(nullptr),
     IsBuildingRecoveryCallExpr(false),
     Cleanup{}, LateTemplateParser(nullptr),
-    LateTemplateParserCleanup(nullptr),
-    OpaqueParser(nullptr), IdResolver(pp), StdInitializerList(nullptr),
+    LateTemplateParserCleanup(nullptr), OpaqueParser(nullptr), IdResolver(pp),
+    StdExperimentalNamespaceCache(nullptr), StdInitializerList(nullptr),
     CXXTypeInfoDecl(nullptr), MSVCGuidDecl(nullptr),
     NSNumberDecl(nullptr), NSValueDecl(nullptr),
     NSStringDecl(nullptr), StringWithUTF8StringMethod(nullptr),
Index: lib/Sema/SemaCoroutine.cpp
===================================================================
--- lib/Sema/SemaCoroutine.cpp
+++ lib/Sema/SemaCoroutine.cpp
@@ -26,15 +26,15 @@
 static QualType lookupPromiseType(Sema &S, const FunctionProtoType *FnType,
                                   SourceLocation Loc) {
   // FIXME: Cache std::coroutine_traits once we've found it.
-  NamespaceDecl *Std = S.getStdNamespace();
-  if (!Std) {
+  NamespaceDecl *StdExp = S.lookupStdExperimentalNamespace();
+  if (!StdExp) {
     S.Diag(Loc, diag::err_implied_std_coroutine_traits_not_found);
     return QualType();
   }
 
   LookupResult Result(S, &S.PP.getIdentifierTable().get("coroutine_traits"),
                       Loc, Sema::LookupOrdinaryName);
-  if (!S.LookupQualifiedName(Result, Std)) {
+  if (!S.LookupQualifiedName(Result, StdExp)) {
     S.Diag(Loc, diag::err_implied_std_coroutine_traits_not_found);
     return QualType();
   }
@@ -86,7 +86,7 @@
   QualType PromiseType = S.Context.getTypeDeclType(Promise);
   if (!PromiseType->getAsCXXRecordDecl()) {
     // Use the fully-qualified name of the type.
-    auto *NNS = NestedNameSpecifier::Create(S.Context, nullptr, Std);
+    auto *NNS = NestedNameSpecifier::Create(S.Context, nullptr, StdExp);
     NNS = NestedNameSpecifier::Create(S.Context, NNS, false,
                                       CoroTrait.getTypePtr());
     PromiseType = S.Context.getElaboratedType(ETK_None, NNS, PromiseType);
@@ -345,6 +345,7 @@
   }
   return BuildCoreturnStmt(Loc, E);
 }
+
 StmtResult Sema::BuildCoreturnStmt(SourceLocation Loc, Expr *E) {
   auto *Coroutine = checkCoroutineContext(*this, Loc, "co_return");
   if (!Coroutine)
Index: lib/Sema/SemaDeclCXX.cpp
===================================================================
--- lib/Sema/SemaDeclCXX.cpp
+++ lib/Sema/SemaDeclCXX.cpp
@@ -8275,6 +8275,20 @@
                                  StdNamespace.get(Context.getExternalSource()));
 }
 
+NamespaceDecl *Sema::lookupStdExperimentalNamespace() {
+  if (!StdExperimentalNamespaceCache) {
+    if (auto Std = getStdNamespace()) {
+      LookupResult Result(*this, &PP.getIdentifierTable().get("experimental"),
+                          SourceLocation(), LookupNamespaceName);
+      if (!LookupQualifiedName(Result, Std) ||
+          !(StdExperimentalNamespaceCache =
+                Result.getAsSingle<NamespaceDecl>()))
+        Result.suppressDiagnostics();
+    }
+  }
+  return StdExperimentalNamespaceCache;
+}
+
 /// \brief Retrieve the special "std" namespace, which may require us to 
 /// implicitly define the namespace.
 NamespaceDecl *Sema::getOrCreateStdNamespace() {
Index: test/SemaCXX/coroutines.cpp
===================================================================
--- test/SemaCXX/coroutines.cpp
+++ test/SemaCXX/coroutines.cpp
@@ -1,18 +1,18 @@
 // RUN: %clang_cc1 -std=c++14 -fcoroutines-ts -verify %s
 
 void no_coroutine_traits_bad_arg_await() {
-  co_await a; // expected-error {{include <coroutine>}}
+  co_await a; // expected-error {{include <experimental/coroutine>}}
   // expected-error@-1 {{use of undeclared identifier 'a'}}
 }
 
 void no_coroutine_traits_bad_arg_yield() {
-  co_yield a; // expected-error {{include <coroutine>}}
+  co_yield a; // expected-error {{include <experimental/coroutine>}}
   // expected-error@-1 {{use of undeclared identifier 'a'}}
 }
 
 
 void no_coroutine_traits_bad_arg_return() {
-  co_return a; // expected-error {{include <coroutine>}}
+  co_return a; // expected-error {{include <experimental/coroutine>}}
   // expected-error@-1 {{use of undeclared identifier 'a'}}
 }
 
@@ -36,45 +36,59 @@
 };
 
 void no_coroutine_traits() {
-  co_await a; // expected-error {{need to include <coroutine>}}
+  co_await a; // expected-error {{need to include <experimental/coroutine>}}
 }
 
 namespace std {
-  template<typename ...T> struct coroutine_traits; // expected-note {{declared here}}
-};
+namespace experimental {
+template <typename... T>
+struct coroutine_traits; // expected-note {{declared here}}
+}
+}
 
 template<typename Promise> struct coro {};
-template<typename Promise, typename... Ps>
-struct std::coroutine_traits<coro<Promise>, Ps...> {
+template <typename Promise, typename... Ps>
+struct std::experimental::coroutine_traits<coro<Promise>, Ps...> {
   using promise_type = Promise;
 };
 
 void no_specialization() {
-  co_await a; // expected-error {{implicit instantiation of undefined template 'std::coroutine_traits<void>'}}
+  co_await a; // expected-error {{implicit instantiation of undefined template 'std::experimental::coroutine_traits<void>'}}
 }
 
-template<typename ...T> struct std::coroutine_traits<int, T...> {};
+template <typename... T>
+struct std::experimental::coroutine_traits<int, T...> {};
 
 int no_promise_type() {
-  co_await a; // expected-error {{this function cannot be a coroutine: 'std::coroutine_traits<int>' has no member named 'promise_type'}}
+  co_await a; // expected-error {{this function cannot be a coroutine: 'std::experimental::coroutine_traits<int>' has no member named 'promise_type'}}
 }
 
-template<> struct std::coroutine_traits<double, double> { typedef int promise_type; };
+template <>
+struct std::experimental::coroutine_traits<double, double> { typedef int promise_type; };
 double bad_promise_type(double) {
-  co_await a; // expected-error {{this function cannot be a coroutine: 'std::coroutine_traits<double, double>::promise_type' (aka 'int') is not a class}}
+  co_await a; // expected-error {{this function cannot be a coroutine: 'experimental::coroutine_traits<double, double>::promise_type' (aka 'int') is not a class}}
 }
 
-template<> struct std::coroutine_traits<double, int> {
+template <>
+struct std::experimental::coroutine_traits<double, int> {
   struct promise_type {};
 };
 double bad_promise_type_2(int) {
-  co_yield 0; // expected-error {{no member named 'yield_value' in 'std::coroutine_traits<double, int>::promise_type'}}
+  co_yield 0; // expected-error {{no member named 'yield_value' in 'std::experimental::coroutine_traits<double, int>::promise_type'}}
 }
 
 struct promise; // expected-note 2{{forward declaration}}
-template<typename ...T> struct std::coroutine_traits<void, T...> { using promise_type = promise; };
+template <typename... T>
+struct std::experimental::coroutine_traits<void, T...> { using promise_type = promise; };
+
+namespace std {
+namespace experimental {
+template <typename Promise = void>
+struct coroutine_handle;
+}
+}
 
-  // FIXME: This diagnostic is terrible.
+// FIXME: This diagnostic is terrible.
 void undefined_promise() { // expected-error {{variable has incomplete type 'promise_type'}}
   // FIXME: This diagnostic doesn't make any sense.
   // expected-error@-2 {{incomplete definition of type 'promise'}}
@@ -200,7 +214,8 @@
 }
 
 struct yield_fn_tag {};
-template<> struct std::coroutine_traits<void, yield_fn_tag> {
+template <>
+struct std::experimental::coroutine_traits<void, yield_fn_tag> {
   struct promise_type {
     // FIXME: add an await_transform overload for functions
     awaitable yield_value(int());
@@ -285,7 +300,7 @@
 }
 
 
-template<> struct std::coroutine_traits<int, int, const char**>
+template<> struct std::experimental::coroutine_traits<int, int, const char**>
 { using promise_type = promise; };
 
 int main(int, const char**) { // expected-error {{'main' cannot be a coroutine}}