Slither crashes when specifying array size as constant expression

[Xenomega]

The following contract will cause slither to throw an error:

contract C {
  uint constant x = 100;
  uint constant y = 10;
  function f(uint[x / y] z) public  {
    // the input parameter in this function is problematic
  }
}

The issue is that array sizes can be specified by a literal or constant expression, and slither seems to fail to initialize arrays via constant expressions (tested with +, *, / )

Stack trace:

ERROR:root:Traceback (most recent call last):
  File "c:\users\vyper\documents\github\slither\slither\__main__.py", line 278, in main_impl
    (results_tmp, number_contracts_tmp) = process(filename, args, detector_classes, printer_classes)
  File "c:\users\vyper\documents\github\slither\slither\__main__.py", line 40, in process
    slither = Slither(filename, args.solc, args.disable_solc_warnings, args.solc_args, ast)
  File "c:\users\vyper\documents\github\slither\slither\slither.py", line 41, in __init__
    self._analyze_contracts()
  File "c:\users\vyper\documents\github\slither\slither\solc_parsing\slitherSolc.py", line 210, in _analyze_contracts
    self._analyze_third_part(contracts_to_be_analyzed, libraries)
  File "c:\users\vyper\documents\github\slither\slither\solc_parsing\slitherSolc.py", line 294, in _analyze_third_part
    self._analyze_variables_modifiers_functions(contract)
  File "c:\users\vyper\documents\github\slither\slither\solc_parsing\slitherSolc.py", line 329, in _analyze_variables_modifiers_functions
    contract.analyze_params_functions()
  File "c:\users\vyper\documents\github\slither\slither\solc_parsing\declarations\contract.py", line 348, in analyze_params_functions
    self._functions[function.full_name] = function
  File "c:\users\vyper\documents\github\slither\slither\core\declarations\function.py", line 358, in full_name
    name, parameters, _ = self.signature
  File "c:\users\vyper\documents\github\slither\slither\core\declarations\function.py", line 341, in signature
    return self.name, [str(x.type) for x in self.parameters], [str(x.type) for x in self.returns]
  File "c:\users\vyper\documents\github\slither\slither\core\declarations\function.py", line 341, in <listcomp>
    return self.name, [str(x.type) for x in self.parameters], [str(x.type) for x in self.returns]
  File "c:\users\vyper\documents\github\slither\slither\core\solidity_types\array_type.py", line 25, in __str__
    if isinstance(self._length.value, Variable) and self._length.value.is_constant:
AttributeError: 'BinaryOperation' object has no attribute 'value'

[montyly]

Note: solving this issue requires building an expression visitor that evaluates simple expressions.

That should be easy using the visitor expression module https://github.com/trailofbits/slither/tree/master/slither/visitors/expression