Question

Enabling NRVO when forwarding a function's result via template function

When forwarding a function's result via template function, I encountered different behaviour regarding utilization of "named return value optimization (NRVO)" for clang and gcc. Here a code snippet:

Foo provideFooAsTemporary() 
{
    return Foo{};
}

template <typename TFn>
auto forwardA(TFn &&fn) 
{
    auto result = fn();
    return result;
}

template <typename TFn>
auto forwardB(TFn &&fn) -> decltype(fn()) 
{
    auto result = fn();
    return result;
}

Foo fooA = forwardA(provideFooAsTemporary);  
Foo fooB = forwardB(provideFooAsTemporary);

Here my observations (also see godbolt example):

gcc 14.1: Avoids any move operations for both forwardA and forwardB.
clang 18.0.1: Avoids moving only for forwardB, but a move operation occurs for forwardA.

I would have expected that forwardA triggers NRVO also for clang.

Here my questions:

What exactly is the difference between forwardA and forwardB? Why does the specified trailing return type make a difference?
Is there a simpler way to enable NRVO for clang than what is shown in forwardB?

3 55 3

1 Jan 1970