How can I write a function template that returns either a reference or a value?

Question

I'd like to write a function template that returns either a reference or a value depending on some compile time expression. What I've tried so far is something like this:

template<typename T>
auto&& Func()
{
  if constexpr (some_compile_time_expression)
  {
    return GetReferenceFromSomewhere();
  }
  else
  {
    return GetValueFromSomewhere();
  }
}

This works fine for all types of references but doesn't work for values. For example, if GetValueFromSomewhere returns a Foo, then the compiler deduces the return type of Func as Foo&& and warns that I'm returning the address of a temporary.

Is there any way to make this work, or am I forced to tease the two branches apart somehow (through function overloads or some such)?

Answer 1

Use decltype(auto) for the return type placeholder, it will preserve the exact value category of the function you're calling in the return statement

template<typename T>
decltype(auto) Func()
{
  if constexpr (some_compile_time_expression_dependent_on_T)
  {
    return GetReferenceFromSomewhere();
  }
  else
  {
    return GetValueFromSomewhere();
  }
}

Live demo