Diagnose use of VLAs in a coroutine

[AaronBallman]

Fixes #65858

[llvmbot]

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-coroutines

Author: Aaron Ballman (AaronBallman)

Changes

Fixes #65858

---

Full diff: https://github.com/llvm/llvm-project/pull/70341.diff

5 Files Affected:

• (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+2)
• (modified) clang/include/clang/Sema/ScopeInfo.h (+8)
• (modified) clang/lib/Sema/SemaCoroutine.cpp (+5)
• (modified) clang/lib/Sema/SemaType.cpp (+12-6)
• (added) clang/test/SemaCXX/coroutine-vla.cpp (+29)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index a673ce726d6c220..453bd8a9a340425 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -166,6 +166,8 @@ def ext_vla_folded_to_constant : ExtWarn<
   InGroup<GNUFoldingConstant>;
 def err_vla_unsupported : Error<
   "variable length arrays are not supported for %select{the current target|'%1'}0">;
+def err_vla_in_coroutine_unsupported : Error<
+  "variable length arrays in a coroutine are not supported">;
 def note_vla_unsupported : Note<
   "variable length arrays are not supported for the current target">;
 
diff --git a/clang/include/clang/Sema/ScopeInfo.h b/clang/include/clang/Sema/ScopeInfo.h
index 02b22af89ff035d..b2f6e3289f41fce 100644
--- a/clang/include/clang/Sema/ScopeInfo.h
+++ b/clang/include/clang/Sema/ScopeInfo.h
@@ -189,6 +189,9 @@ class FunctionScopeInfo {
   /// First SEH '__try' statement in the current function.
   SourceLocation FirstSEHTryLoc;
 
+  /// First use of a VLA within the current function.
+  SourceLocation FirstVLALoc;
+
 private:
   /// Used to determine if errors occurred in this function or block.
   DiagnosticErrorTrap ErrorTrap;
@@ -473,6 +476,11 @@ class FunctionScopeInfo {
     FirstSEHTryLoc = TryLoc;
   }
 
+  void setHasVLA(SourceLocation VLALoc) {
+    if (FirstVLALoc.isInvalid())
+      FirstVLALoc = VLALoc;
+  }
+
   bool NeedsScopeChecking() const {
     return !HasDroppedStmt && (HasIndirectGoto || HasMustTail ||
                                (HasBranchProtectedScope && HasBranchIntoScope));
diff --git a/clang/lib/Sema/SemaCoroutine.cpp b/clang/lib/Sema/SemaCoroutine.cpp
index d2b0922a4bb9c4c..e1f3b5acb3cadec 100644
--- a/clang/lib/Sema/SemaCoroutine.cpp
+++ b/clang/lib/Sema/SemaCoroutine.cpp
@@ -1198,6 +1198,11 @@ void Sema::CheckCompletedCoroutineBody(FunctionDecl *FD, Stmt *&Body) {
   if (FD->hasAttr<AlwaysInlineAttr>())
     Diag(FD->getLocation(), diag::warn_always_inline_coroutine);
 
+  // We don't allow use of VLAs within a coroutine, so diagnose if we've seen
+  // a VLA in the body of this function.
+  if (Fn->FirstVLALoc.isValid())
+    Diag(Fn->FirstVLALoc, diag::err_vla_in_coroutine_unsupported);
+
   // [stmt.return.coroutine]p1:
   //   A coroutine shall not enclose a return statement ([stmt.return]).
   if (Fn->FirstReturnLoc.isValid()) {
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index 28b81c1768a3004..dea77fae4cadb59 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -2706,12 +2706,18 @@ QualType Sema::BuildArrayType(QualType T, ArrayType::ArraySizeModifier ASM,
     }
   }
 
-  if (T->isVariableArrayType() && !Context.getTargetInfo().isVLASupported()) {
-    // CUDA device code and some other targets don't support VLAs.
-    bool IsCUDADevice = (getLangOpts().CUDA && getLangOpts().CUDAIsDevice);
-    targetDiag(Loc,
-               IsCUDADevice ? diag::err_cuda_vla : diag::err_vla_unsupported)
-        << (IsCUDADevice ? CurrentCUDATarget() : 0);
+  if (T->isVariableArrayType()) {
+    if (!Context.getTargetInfo().isVLASupported()) {
+      // CUDA device code and some other targets don't support VLAs.
+      bool IsCUDADevice = (getLangOpts().CUDA && getLangOpts().CUDAIsDevice);
+      targetDiag(Loc,
+                 IsCUDADevice ? diag::err_cuda_vla : diag::err_vla_unsupported)
+          << (IsCUDADevice ? CurrentCUDATarget() : 0);
+    } else if (sema::FunctionScopeInfo *FSI = getCurFunction()) {
+      // VLAs are supported on this target, but we may need to do delayed
+      // checking that the VLA is not being used within a coroutine.
+      FSI->setHasVLA(Loc);
+    }
   }
 
   // If this is not C99, diagnose array size modifiers on non-VLAs.
diff --git a/clang/test/SemaCXX/coroutine-vla.cpp b/clang/test/SemaCXX/coroutine-vla.cpp
new file mode 100644
index 000000000000000..176e35f346e2b45
--- /dev/null
+++ b/clang/test/SemaCXX/coroutine-vla.cpp
@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 %s -std=c++20 -fsyntax-only -Wno-vla-cxx-extension -verify
+#include "Inputs/std-coroutine.h"
+
+struct promise;
+
+struct coroutine : std::coroutine_handle<promise> {
+  using promise_type = ::promise;
+};
+
+struct promise
+{
+    coroutine get_return_object();
+    std::suspend_always initial_suspend() noexcept;
+    std::suspend_always final_suspend() noexcept;
+    void return_void();
+    void unhandled_exception();
+};
+
+coroutine foo(int n) {
+  int array[n]; // expected-error {{variable length arrays in a coroutine are not supported}}
+  co_return;
+}
+
+void lambda() {
+  [](int n) -> coroutine {
+    int array[n]; // expected-error {{variable length arrays in a coroutine are not supported}}
+    co_return;
+  }(10);
+}

[cor3ntin]

LGTM but it is missing a release note :)

[ChuanqiXu9]

Thanks! LGTM with a nit.

[ChuanqiXu9]

Suggested change

	// We don't allow use of VLAs within a coroutine, so diagnose if we've seen
	// VLAs are not allowed within a coroutine, so diagnose if we've seen

nit: it is not us (clang/LLVM) who decide to not support VLA in coroutines. But it is the C++20 coroutines itself can't support VLA by its design naturally.

[ChuanqiXu9]

LGTM but it is missing a release note :)

it may be fine to not mention it too since VLAs in coroutines never got compiled.. user might meet backend crashes..

[erichkeane]

The comment suggestion makes sense to me, else just needs a release note.