Diagnose use of VLAs in a coroutine (#70341)

Fixes #65858

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -636,6 +636,10 @@ Bug Fixes to C++ Support
   (`#46200 <https://github.com/llvm/llvm-project/issues/46200>`_)
   (`#57812 <https://github.com/llvm/llvm-project/issues/57812>`_)
 
+- Diagnose use of a variable-length array in a coroutine. The design of
+  coroutines is such that it is not possible to support VLA use. Fixes:
+  (`#65858 <https://github.com/llvm/llvm-project/issues/65858>`_)
+
 - Fix bug where we were overriding zero-initialization of class members when
   default initializing a base class in a constant expression context. Fixes:
   (`#69890 <https://github.com/llvm/llvm-project/issues/69890>`_)

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">;
 

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));

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);
 
+  // The design of coroutines means we cannot allow use of VLAs within one, 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()) {

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.

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);
+}