solidity/test/libsolidity/semanticTests/functionCall/call_unimplemented_base.sol

abstract contract I
{
    function a() internal view virtual returns(uint256);
}
abstract contract V is I
{
    function b() public view returns(uint256) { return a(); }
}
contract C is V
{
    function a() internal view override returns (uint256) { return 42;}
}
// ----
// b() -> 42