Index: cfe/trunk/lib/Sema/TreeTransform.h
===================================================================
--- cfe/trunk/lib/Sema/TreeTransform.h
+++ cfe/trunk/lib/Sema/TreeTransform.h
@@ -7163,13 +7163,22 @@
   Builder.ReturnValue = Res.get();
 
   if (S->hasDependentPromiseType()) {
-    assert(!Promise->getType()->isDependentType() &&
-           "the promise type must no longer be dependent");
-    assert(!S->getFallthroughHandler() && !S->getExceptionHandler() &&
-           !S->getReturnStmtOnAllocFailure() && !S->getDeallocate() &&
-           "these nodes should not have been built yet");
-    if (!Builder.buildDependentStatements())
-      return StmtError();
+    // PR41909: We may find a generic coroutine lambda definition within a
+    // template function that is being instantiated. In this case, the lambda
+    // will have a dependent promise type, until it is used in an expression
+    // that creates an instantiation with a non-dependent promise type. We
+    // should not assert or build coroutine dependent statements for such a
+    // generic lambda.
+    auto *MD = dyn_cast_or_null<CXXMethodDecl>(FD);
+    if (!MD || !MD->getParent()->isGenericLambda()) {
+      assert(!Promise->getType()->isDependentType() &&
+             "the promise type must no longer be dependent");
+      assert(!S->getFallthroughHandler() && !S->getExceptionHandler() &&
+             !S->getReturnStmtOnAllocFailure() && !S->getDeallocate() &&
+             "these nodes should not have been built yet");
+      if (!Builder.buildDependentStatements())
+        return StmtError();
+    }
   } else {
     if (auto *OnFallthrough = S->getFallthroughHandler()) {
       StmtResult Res = getDerived().TransformStmt(OnFallthrough);
Index: cfe/trunk/test/SemaCXX/coroutines.cpp
===================================================================
--- cfe/trunk/test/SemaCXX/coroutines.cpp
+++ cfe/trunk/test/SemaCXX/coroutines.cpp
@@ -720,6 +720,16 @@
   co_await 42;
 }
 
+template<typename T> void ok_generic_lambda_coawait_PR41909() {
+  [](auto& arg) -> coro<good_promise_1> { // expected-warning {{expression result unused}}
+    co_await 12;
+  };
+  [](auto &arg) -> coro<good_promise_1> {
+    co_await 24;
+  }("argument");
+}
+template void ok_generic_lambda_coawait_PR41909<int>(); // expected-note {{in instantiation of function template specialization 'ok_generic_lambda_coawait_PR41909<int>' requested here}}
+
 template<> struct std::experimental::coroutine_traits<int, int, const char**>
 { using promise_type = promise; };