[clang][bytecode] Fix copy constructors for empty unions (#147050)

Nothing to do in that case.

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -5836,6 +5836,8 @@ bool Compiler<Emitter>::compileConstructor(const CXXConstructorDecl *Ctor) {
     return false;
 
   if (R->isUnion() && Ctor->isCopyOrMoveConstructor()) {
+    if (R->getNumFields() == 0)
+      return this->emitRetVoid(Ctor);
     // union copy and move ctors are special.
     assert(cast<CompoundStmt>(Ctor->getBody())->body_empty());
     if (!this->emitThis(Ctor))

clang/test/AST/ByteCode/unions.cpp

@@ -600,6 +600,18 @@ namespace MoveOrAssignOp {
   }
   static_assert(foo());
 }
+
+namespace CopyEmptyUnion {
+  struct A {
+    union {}; // both-warning {{declaration does not declare anything}}
+  };
+  constexpr int foo() {
+     A a;
+     A a2 = a;
+     return 1;
+  }
+  static_assert(foo() == 1);
+}
 #endif
 
 namespace AddressComparison {