Index: cfe/trunk/include/clang/Sema/Sema.h
===================================================================
--- cfe/trunk/include/clang/Sema/Sema.h
+++ cfe/trunk/include/clang/Sema/Sema.h
@@ -824,6 +824,10 @@
   /// \<initializer_list>.
   ClassTemplateDecl *StdInitializerList;
 
+  /// The C++ "std::coroutine_traits" template, which is defined in
+  /// \<coroutine_traits>
+  ClassTemplateDecl *StdCoroutineTraitsCache;
+
   /// The C++ "type_info" declaration, which is defined in \<typeinfo>.
   RecordDecl *CXXTypeInfoDecl;
 
@@ -8536,6 +8540,8 @@
   bool buildCoroutineParameterMoves(SourceLocation Loc);
   VarDecl *buildCoroutinePromise(SourceLocation Loc);
   void CheckCompletedCoroutineBody(FunctionDecl *FD, Stmt *&Body);
+  ClassTemplateDecl *lookupCoroutineTraits(SourceLocation KwLoc,
+                                           SourceLocation FuncLoc);
 
   //===--------------------------------------------------------------------===//
   // OpenCL extensions.
Index: cfe/trunk/lib/Sema/Sema.cpp
===================================================================
--- cfe/trunk/lib/Sema/Sema.cpp
+++ cfe/trunk/lib/Sema/Sema.cpp
@@ -131,9 +131,9 @@
       IsBuildingRecoveryCallExpr(false), Cleanup{}, LateTemplateParser(nullptr),
       LateTemplateParserCleanup(nullptr), OpaqueParser(nullptr), IdResolver(pp),
       StdExperimentalNamespaceCache(nullptr), StdInitializerList(nullptr),
-      CXXTypeInfoDecl(nullptr), MSVCGuidDecl(nullptr), NSNumberDecl(nullptr),
-      NSValueDecl(nullptr), NSStringDecl(nullptr),
-      StringWithUTF8StringMethod(nullptr),
+      StdCoroutineTraitsCache(nullptr), CXXTypeInfoDecl(nullptr),
+      MSVCGuidDecl(nullptr), NSNumberDecl(nullptr), NSValueDecl(nullptr),
+      NSStringDecl(nullptr), StringWithUTF8StringMethod(nullptr),
       ValueWithBytesObjCTypeMethod(nullptr), NSArrayDecl(nullptr),
       ArrayWithObjectsMethod(nullptr), NSDictionaryDecl(nullptr),
       DictionaryWithObjectsMethod(nullptr), GlobalNewDeleteDeclared(false),
Index: cfe/trunk/lib/Sema/SemaCoroutine.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaCoroutine.cpp
+++ cfe/trunk/lib/Sema/SemaCoroutine.cpp
@@ -60,20 +60,8 @@
     return QualType();
   }
 
-  LookupResult Result(S, &S.PP.getIdentifierTable().get("coroutine_traits"),
-                      FuncLoc, Sema::LookupOrdinaryName);
-  if (!S.LookupQualifiedName(Result, StdExp)) {
-    S.Diag(KwLoc, diag::err_implied_coroutine_type_not_found)
-        << "std::experimental::coroutine_traits";
-    return QualType();
-  }
-
-  ClassTemplateDecl *CoroTraits = Result.getAsSingle<ClassTemplateDecl>();
+  ClassTemplateDecl *CoroTraits = S.lookupCoroutineTraits(KwLoc, FuncLoc);
   if (!CoroTraits) {
-    Result.suppressDiagnostics();
-    // We found something weird. Complain about the first thing we found.
-    NamedDecl *Found = *Result.begin();
-    S.Diag(Found->getLocation(), diag::err_malformed_std_coroutine_traits);
     return QualType();
   }
 
@@ -1538,3 +1526,27 @@
     return StmtError();
   return Res;
 }
+
+ClassTemplateDecl *Sema::lookupCoroutineTraits(SourceLocation KwLoc,
+                                               SourceLocation FuncLoc) {
+  if (!StdCoroutineTraitsCache) {
+    if (auto StdExp = lookupStdExperimentalNamespace()) {
+      LookupResult Result(*this,
+                          &PP.getIdentifierTable().get("coroutine_traits"),
+                          FuncLoc, LookupOrdinaryName);
+      if (!LookupQualifiedName(Result, StdExp)) {
+        Diag(KwLoc, diag::err_implied_coroutine_type_not_found)
+            << "std::experimental::coroutine_traits";
+        return nullptr;
+      }
+      if (!(StdCoroutineTraitsCache =
+                Result.getAsSingle<ClassTemplateDecl>())) {
+        Result.suppressDiagnostics();
+        NamedDecl *Found = *Result.begin();
+        Diag(Found->getLocation(), diag::err_malformed_std_coroutine_traits);
+        return nullptr;
+      }
+    }
+  }
+  return StdCoroutineTraitsCache;
+}