[PATCH] D62550: [coroutines][PR41909] Don't build dependent coroutine statements for generic lambda

[Brian Gesiak via Phabricator via cfe-commits]

modocache created this revision.
modocache added reviewers: GorNishanov, EricWF, lewissbaker, tks2103.
Herald added a project: clang.

https://bugs.llvm.org/show_bug.cgi?id=41909 describes an issue in which
a generic lambda that takes a dependent argument `auto set` causes the
template instantiation machinery for coroutine body statements to crash
with an ICE. The issue is two-fold:

1. The paths taken by the template instantiator contain several asserts that the coroutine promise must not have a dependent type.
2. The template instantiator unconditionally builds corotuine statements that depend on the promise type, which cannot be dependent.

To work around the issue, prevent the template instantiator from building
dependent coroutine statements if the coroutine promise type is dependent.
Since we only expect this to occur in the case of a generic lambda, limit
the workaround behavior to just that case.


Repository:
  rC Clang

https://reviews.llvm.org/D62550

Files:
  lib/Sema/TreeTransform.h
  test/SemaCXX/coroutines.cpp


Index: test/SemaCXX/coroutines.cpp
===================================================================
--- test/SemaCXX/coroutines.cpp
+++ 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; };
 
Index: lib/Sema/TreeTransform.h
===================================================================
--- lib/Sema/TreeTransform.h
+++ lib/Sema/TreeTransform.h
@@ -7155,13 +7155,22 @@
   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);


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