3 files changed, 21 insertions, 6 deletions
diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index 081d45b61eb..9ea5cba94ee 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -88,8 +88,8 @@ Sema::Sema(Preprocessor &pp, ASTContext &ctxt, ASTConsumer &consumer,
     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),
diff --git a/clang/lib/Sema/SemaCoroutine.cpp b/clang/lib/Sema/SemaCoroutine.cpp
index 461ca8eddcf..f9c3970e021 100644
--- a/clang/lib/Sema/SemaCoroutine.cpp
+++ b/clang/lib/Sema/SemaCoroutine.cpp
@@ -26,15 +26,15 @@ using namespace sema;
 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 @@ static QualType lookupPromiseType(Sema &S, const FunctionProtoType *FnType,
   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 @@ StmtResult Sema::ActOnCoreturnStmt(SourceLocation Loc, Expr *E) {
   }
   return BuildCoreturnStmt(Loc, E);
 }
+
 StmtResult Sema::BuildCoreturnStmt(SourceLocation Loc, Expr *E) {
   auto *Coroutine = checkCoroutineContext(*this, Loc, "co_return");
   if (!Coroutine)
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 133f1f47cc4..2fe520b8238 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -8275,6 +8275,20 @@ NamespaceDecl *Sema::getStdNamespace() const {
                                  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() {