solidity/test/libsolidity/syntaxTests/types/mapping/function_type_return_external.sol

contract C {
    function f(function() external returns (mapping(uint=>uint) storage)) public pure {
    }
}
// ----
// TypeError: (57-76): Data location must be "memory" for return parameter in function, but "storage" was given.
// TypeError: (57-76): Internal type cannot be used for external function type.