It wouldn't pollute the file scope namespace. You can define it in eye sight of the sort call, steal variables from the nesting functions stack etc.

... but how's that different of what we have today ?

    std::sort(vec.begin(), vec.end(), 
              [] (const auto& lhs, const auto& rhs) { 
      return lhs.whatever < rhs.whatever;
    });

or

    auto sort_by_whatever = [] (const auto& lhs, const auto& rhs) { 
      return lhs.whatever < rhs.whatever;
    };
    std::sort(vec.begin(), vec.end(), sort_by_whatever);

Sorry was refering to C in that comment with no lambdas, with the GCC extension.