[clang][bytecode] Avoid copies with elidable CXXConstructExprs

To fix the newly added cwg6.cpp.

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -3235,7 +3235,8 @@ bool Compiler<Emitter>::VisitCXXConstructExpr(const CXXConstructExpr *E) {
       return this->visitInitializer(E->getArg(0));
 
     // Zero initialization.
-    if (E->requiresZeroInitialization()) {
+    bool ZeroInit = E->requiresZeroInitialization();
+    if (ZeroInit) {
       const Record *R = getRecord(E->getType());
 
       if (!this->visitZeroRecordInitializer(R, E))
@@ -3246,6 +3247,16 @@ bool Compiler<Emitter>::VisitCXXConstructExpr(const CXXConstructExpr *E) {
         return true;
     }
 
+    // Avoid materializing a temporary for an elidable copy/move constructor.
+    if (!ZeroInit && E->isElidable()) {
+      const Expr *SrcObj = E->getArg(0);
+      assert(SrcObj->isTemporaryObject(Ctx.getASTContext(), Ctor->getParent()));
+      assert(Ctx.getASTContext().hasSameUnqualifiedType(E->getType(),
+                                                        SrcObj->getType()));
+      if (const auto *ME = dyn_cast<MaterializeTemporaryExpr>(SrcObj))
+        return this->visitInitializer(ME->getSubExpr());
+    }
+
     const Function *Func = getFunction(Ctor);
 
     if (!Func)

clang/test/AST/ByteCode/cxx11.cpp

@@ -379,3 +379,11 @@ namespace DiscardedAddrLabel {
   }
 }
 
+struct Counter {
+  int copies;
+  constexpr Counter(int copies) : copies(copies) {}
+  constexpr Counter(const Counter& other) : copies(other.copies + 1) {}
+};
+// Passing an lvalue by value makes a non-elidable copy.
+constexpr int PassByValue(Counter c) { return c.copies; }
+static_assert(PassByValue(Counter(0)) == 0, "expect no copies");