[clang][Interp] Create a new local variable in VisitCXXDefaultArgExpr

[tbaeder]

Summary

If the argument is of composite type, we need a pointer to return.

[tbaeder]

Abandoning in favor of https://reviews.llvm.org/D156027

clang/lib/AST/Interp/ByteCodeExprGen.cpp

 bool ByteCodeExprGen<Emitter>::VisitCXXDefaultInitExpr(
     const CXXDefaultInitExpr *E) {
   return this->visit(E->getExpr());
 }
 
 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::VisitCXXDefaultArgExpr(
     const CXXDefaultArgExpr *E) {
-  return this->visit(E->getExpr());
+  const Expr *SubExpr = E->getExpr();
+
+  if (std::optional<PrimType> T = classify(E->getExpr()))
+    return this->visit(SubExpr);
+
+  if (std::optional<unsigned> LocalIndex =
+          allocateLocal(SubExpr, /*IsExtended=*/true)) {
+    if (!this->emitGetPtrLocal(*LocalIndex, E))
+      return false;
+    return this->visitInitializer(SubExpr);
+  }
+  return false;
 }
 
 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::VisitCXXBoolLiteralExpr(
     const CXXBoolLiteralExpr *E) {
   if (DiscardResult)
     return true;
 

clang/test/AST/Interp/records.cpp

namespace MutableFields {
 
   /// FIXME: This should also be rejected in the new interpreter.
   constexpr Foo GlobalF;
   constexpr int foo2() { // ref-error {{never produces a constant expression}}
     return GlobalF.I; // ref-note {{read of mutable member 'I' is not allowed in a constant expression}}
   }
 }
 
+namespace CompositeDefaultArgs {
+  struct Foo {
+    int a;
+    int b;
+    constexpr Foo() : a(12), b(13) {}
+  };
+
+  class Bar {
+  public:
+    bool B = false;
+
+    constexpr int someFunc(Foo F = Foo()) {
+      this->B = true;
+      return 5;
+    }
+  };
+
+  constexpr bool testMe() {
+    Bar B;
+    B.someFunc();
+    return B.B;
+  }
+  static_assert(testMe(), "");
+}