Shall structured binding be returned from a function as rvalue in C++20?

Question

Consider a C++20 program where in function foo there is a structured binding auto [y]. The function returns y, which is converted in object of type A. A can be constructed either from const reference of from rvalue-reference.

#include <tuple> #include <iostream>  struct A {     A(const int &) { std::cout << "A(const int &) "; }     A(int &&) { std::cout << "A(int &&) "; } };  A foo() {     auto [y] = std::make_tuple(1);     return y; }  int main() { foo(); } 

Which one of the constructors shall be selected according to C++20 language standard?

Clang selects A(const int &) and GCC selects A(int &&), demo: https://gcc.godbolt.org/z/5q779vE6T

Does one of the compilers not support yet the standard in that respect?

Answer 1

I believe that Clang is correct.

TL;DR: some lvalues can be implicitly moved, but a structured binding is not such a lvalue.

1. The name of a structured binding is an lvalue:

[dcl.struct.bind]/1:

A structured binding declaration introduces the identifiers v0, v1, v2,… of the identifier-list as names of structured bindings.

[dcl.struct.bind]/4:

Each vi is the name of an lvalue of type Ti that refers to the object bound to ri; the referenced type is ri.

2. An variable name (which is normally an lvalue) can be moved in a return statement if it names an implicitly movable entity:

An implicitly movable entity is a variable of automatic storage duration that is either a non-volatile object or an rvalue reference to a non-volatile object type. In the following copy-initialization contexts, a move operation is first considered before attempting a copy operation:

• If the expression in a return ([stmt.return]) or co_­return ([stmt.return.coroutine]) statement is a (possibly parenthesized) id-expression that names an implicitly movable entity declared in the body or parameter-declaration-clause of the innermost enclosing function or lambda-expression, or
• [...]

3. As can be seen in the definition of implicitly movable entity, only objects and (rvalue) references can be implicitly moved. But a structured binding is neither.

[basic.pre]/3:

An entity is a value, object, reference, [or] structured binding[...].

So I believe that a structured binding cannot be implicitly moved.

If y were an object or reference, then it would be implicitly movable in return y;.

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Edit: C++17 as written specified that structured bindings to tuple members are references. This was corrected by CWG 2313.