[clang][Interp] Fix empty InitListExprs for unions

We still need to handle Inits.size() == 0, but we can do that earlier.

Author: tbaederr

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -1053,6 +1053,9 @@ bool ByteCodeExprGen<Emitter>::visitInitList(ArrayRef<const Expr *> Inits,
     if (Inits.size() == 1 && E->getType() == Inits[0]->getType())
       return this->visitInitializer(Inits[0]);
 
+    if (Inits.size() == 0)
+      return this->emitFinishInit(E);
+
     auto initPrimitiveField = [=](const Record::Field *FieldToInit,
                                   const Expr *Init, PrimType T) -> bool {
       if (!this->visit(Init))

clang/test/AST/Interp/unions.cpp

@@ -42,4 +42,10 @@ namespace SimpleStore {
     return a.b;
   }
   static_assert(foo() == 10, "");
+
+  constexpr int empty() {
+    A a{}; /// Just test that this works.
+    return 10;
+  }
+  static_assert(empty() == 10, "");
 }