For #64088: mark vtable as used if we might emit a reference to it.

Author: zygoloid

clang/lib/Sema/SemaCast.cpp

@@ -935,6 +935,14 @@ void CastOperation::CheckDynamicCast() {
           << isClangCL;
   }
 
+  // For a dynamic_cast to a final type, IR generation might emit a reference
+  // to the vtable.
+  if (DestRecord) {
+    auto *DestDecl = DestRecord->getAsCXXRecordDecl();
+    if (DestDecl->isEffectivelyFinal())
+      Self.MarkVTableUsed(OpRange.getBegin(), DestDecl);
+  }
+
   // Done. Everything else is run-time checks.
   Kind = CK_Dynamic;
 }

clang/test/CodeGenCXX/dynamic-cast-exact.cpp

@@ -76,3 +76,12 @@ H *exact_multi(A *a) {
   // CHECK: phi ptr [ %[[RESULT]], %[[LABEL_NOTNULL]] ], [ null, %[[LABEL_FAILED]] ]
   return dynamic_cast<H*>(a);
 }
+
+namespace GH64088 {
+  // Ensure we mark the B vtable as used here, because we're going to emit a
+  // reference to it.
+  // CHECK: define {{.*}} @_ZN7GH640881BD0
+  struct A { virtual ~A(); };
+  struct B final : A { virtual ~B() = default; };
+  B *cast(A *p) { return dynamic_cast<B*>(p); }
+}