Error while parsing stream redirection inside dollar-parentheses

Snippet which fails:

function ff () {
    local xxxx="$(<aa)"
    case x in
      * ) echo "blabla"; return 1 ;;
    esac &> foo || { echo "ffff"; return 0; }
}

[Yanpas]

According to official docs it is short form of cat command substitution:

https://www.gnu.org/software/bash/manual/bash.html#Command-Substitution

[Yanpas]

it means store the content of a file aa in variable xxxx. Here is example:

function ff () {
    local xxxx="$(<aa)"
    echo "xxxx is $xxxx"
}

echo foo > aa

ff

output is xxxx is foo