[SOLVED] Determining the return type of a class method without warning on gcc

Issue

I would like to alias a return type for a templated class method, on clang I use

template <class R, unsigned int BlockSize>
struct block {
    using KeyType = decltype(((R*)nullptr)->getKey());
}

This works fine on clang, but on gcc 11.3.0 I get a warning:

 warning: ‘this’ pointer is null [-Wnonnull]

My question is what is the proper way to get the KeyType without warning?

Solution

To be able to use member functions in unevaluated contexts such as decltype expressions one should use std::declval:

template <class R, unsigned int BlockSize>
struct block {
    using KeyType = decltype(std::declval<R>().getKey());
}

Answered By - Osyotr

Answer Checked By - Willingham (WPSolving Volunteer)