prompt: We are currently solving the following issue within our repository. Here is the issue text:
  --- BEGIN ISSUE ---
  AssertionError parsing function-type return values
  The following contract causes an assertion error:
  ```
  contract FunctionReturningFunction {
      function f() public { }
  
      function g() internal returns (function() internal) {
          return f;
      }
  }
  ```
  
  Output:
  ```
  ERROR:root:Error in .\function_returning_function.sol
  ERROR:root:Traceback (most recent call last):
    File "c:\users\x\documents\github\slither\slither\__main__.py", line 610, in main_impl
      (slither_instances, results_detectors, results_printers, number_contracts) = process_all(filename, args,
    File "c:\users\x\documents\github\slither\slither\__main__.py", line 67, in process_all
      (slither, current_results_detectors, current_results_printers, current_analyzed_count) = process_single(
    File "c:\users\x\documents\github\slither\slither\__main__.py", line 53, in process_single
      slither = Slither(target,
    File "c:\users\x\documents\github\slither\slither\slither.py", line 86, in __init__
      self._parser.analyze_contracts()
    File "c:\users\x\documents\github\slither\slither\solc_parsing\slitherSolc.py", line 345, in analyze_contracts
      self._convert_to_slithir()
    File "c:\users\x\documents\github\slither\slither\solc_parsing\slitherSolc.py", line 489, in _convert_to_slithir
      func.generate_slithir_and_analyze()
    File "c:\users\x\documents\github\slither\slither\core\declarations\function.py", line 1652, in generate_slithir_and_analyze
      node.slithir_generation()
    File "c:\users\x\documents\github\slither\slither\core\cfg\node.py", line 702, in slithir_generation
      self._irs = convert_expression(expression, self)
    File "c:\users\x\documents\github\slither\slither\slithir\convert.py", line 64, in convert_expression
      visitor = ExpressionToSlithIR(expression, node)
    File "c:\users\x\documents\github\slither\slither\visitors\slithir\expression_to_slithir.py", line 105, in __init__
      r = Return(get(self.expression))
    File "c:\users\x\documents\github\slither\slither\slithir\operations\return_operation.py", line 18, in __init__
      assert is_valid_rvalue(values) or isinstance(values, TupleVariable) or values is None
  AssertionError
  ```
  AssertionError parsing function-type return values
  The following contract causes an assertion error:
  ```
  contract FunctionReturningFunction {
      function f() public { }
  
      function g() internal returns (function() internal) {
          return f;
      }
  }
  ```
  
  Output:
  ```
  ERROR:root:Error in .\function_returning_function.sol
  ERROR:root:Traceback (most recent call last):
    File "c:\users\x\documents\github\slither\slither\__main__.py", line 610, in main_impl
      (slither_instances, results_detectors, results_printers, number_contracts) = process_all(filename, args,
    File "c:\users\x\documents\github\slither\slither\__main__.py", line 67, in process_all
      (slither, current_results_detectors, current_results_printers, current_analyzed_count) = process_single(
    File "c:\users\x\documents\github\slither\slither\__main__.py", line 53, in process_single
      slither = Slither(target,
    File "c:\users\x\documents\github\slither\slither\slither.py", line 86, in __init__
      self._parser.analyze_contracts()
    File "c:\users\x\documents\github\slither\slither\solc_parsing\slitherSolc.py", line 345, in analyze_contracts
      self._convert_to_slithir()
    File "c:\users\x\documents\github\slither\slither\solc_parsing\slitherSolc.py", line 489, in _convert_to_slithir
      func.generate_slithir_and_analyze()
    File "c:\users\x\documents\github\slither\slither\core\declarations\function.py", line 1652, in generate_slithir_and_analyze
      node.slithir_generation()
    File "c:\users\x\documents\github\slither\slither\core\cfg\node.py", line 702, in slithir_generation
      self._irs = convert_expression(expression, self)
    File "c:\users\x\documents\github\slither\slither\slithir\convert.py", line 64, in convert_expression
      visitor = ExpressionToSlithIR(expression, node)
    File "c:\users\x\documents\github\slither\slither\visitors\slithir\expression_to_slithir.py", line 105, in __init__
      r = Return(get(self.expression))
    File "c:\users\x\documents\github\slither\slither\slithir\operations\return_operation.py", line 18, in __init__
      assert is_valid_rvalue(values) or isinstance(values, TupleVariable) or values is None
  AssertionError
  ```
  
  --- END ISSUE ---
  
  Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
  --- BEGIN FILES ---
  Path: `slither/slithir/operations/return_operation.py`
  Content:
  ```
  1 from slither.slithir.operations.operation import Operation
  2 
  3 from slither.slithir.variables.tuple import TupleVariable
  4 from slither.slithir.utils.utils import is_valid_rvalue
  5 
  6 
  7 class Return(Operation):
  8     """
  9        Return
  10        Only present as last operation in RETURN node
  11     """
  12 
  13     def __init__(self, values):
  14         # Note: Can return None 
  15         # ex: return call()
  16         # where call() dont return
  17         if not isinstance(values, list):
  18             assert is_valid_rvalue(values) or isinstance(values, TupleVariable) or values is None
  19             if values is None:
  20                 values = []
  21             else:
  22                 values = [values]
  23         else:
  24             # Remove None
  25             # Prior Solidity 0.5
  26             # return (0,)
  27             # was valid for returns(uint)
  28             values = [v for v in values if not v is None]
  29             self._valid_value(values)
  30         super(Return, self).__init__()
  31         self._values = values
  32 
  33     def _valid_value(self, value):
  34         if isinstance(value, list):
  35             assert all(self._valid_value(v) for v in value)
  36         else:
  37             assert is_valid_rvalue(value) or isinstance(value, TupleVariable)
  38         return True
  39 
  40     @property
  41     def read(self):
  42         return self._unroll(self.values)
  43 
  44     @property
  45     def values(self):
  46         return self._unroll(self._values)
  47 
  48     def __str__(self):
  49         return "RETURN {}".format(','.join(['{}'.format(x) for x in self.values]))
  50
  ```
  --- END FILES ---
  
  Please first localize the bug based on the issue statement, and then generate a patch according to the `git diff` format fenced by three backticks.
  
  Here is an example:
  
  ```diff
  diff --git a/examples/server_async.py b/examples/server_async.py
  --- a/examples/server_async.py
  +++ b/examples/server_async.py
  @@ -313,4 +313,4 @@
   
   
   if __name__ == "__main__":
  -    asyncio.run(run_async_server("."), debug=True)
  +    asyncio.run(run_async_server(), debug=True)
  diff --git a/examples/server_sync.py b/examples/server_sync.py
  --- a/examples/server_sync.py
  +++ b/examples/server_sync.py
  @@ -313,5 +313,5 @@
   
   
   if __name__ == "__main__":
  -    server = run_sync_server(".")
  +    server = run_sync_server()
       server.shutdown()
  
  ```
  
golden_diff: diff --git a/slither/slithir/operations/return_operation.py b/slither/slithir/operations/return_operation.py
  --- a/slither/slithir/operations/return_operation.py
  +++ b/slither/slithir/operations/return_operation.py
  @@ -1,3 +1,4 @@
  +from slither.core.declarations import Function
   from slither.slithir.operations.operation import Operation
   
   from slither.slithir.variables.tuple import TupleVariable
  @@ -15,7 +16,7 @@
           # ex: return call()
           # where call() dont return
           if not isinstance(values, list):
  -            assert is_valid_rvalue(values) or isinstance(values, TupleVariable) or values is None
  +            assert is_valid_rvalue(values) or isinstance(values, (TupleVariable, Function)) or values is None
               if values is None:
                   values = []
               else:
  @@ -34,7 +35,7 @@
           if isinstance(value, list):
               assert all(self._valid_value(v) for v in value)
           else:
  -            assert is_valid_rvalue(value) or isinstance(value, TupleVariable)
  +            assert is_valid_rvalue(value) or isinstance(value, (TupleVariable, Function))
           return True
   
       @property
  
verification_info: {"golden_diff": "diff --git a/slither/slithir/operations/return_operation.py b/slither/slithir/operations/return_operation.py\n--- a/slither/slithir/operations/return_operation.py\n+++ b/slither/slithir/operations/return_operation.py\n@@ -1,3 +1,4 @@\n+from slither.core.declarations import Function\n from slither.slithir.operations.operation import Operation\n \n from slither.slithir.variables.tuple import TupleVariable\n@@ -15,7 +16,7 @@\n         # ex: return call()\n         # where call() dont return\n         if not isinstance(values, list):\n-            assert is_valid_rvalue(values) or isinstance(values, TupleVariable) or values is None\n+            assert is_valid_rvalue(values) or isinstance(values, (TupleVariable, Function)) or values is None\n             if values is None:\n                 values = []\n             else:\n@@ -34,7 +35,7 @@\n         if isinstance(value, list):\n             assert all(self._valid_value(v) for v in value)\n         else:\n-            assert is_valid_rvalue(value) or isinstance(value, TupleVariable)\n+            assert is_valid_rvalue(value) or isinstance(value, (TupleVariable, Function))\n         return True\n \n     @property\n", "issue": "AssertionError parsing function-type return values\nThe following contract causes an assertion error:\r\n```\r\ncontract FunctionReturningFunction {\r\n    function f() public { }\r\n\r\n    function g() internal returns (function() internal) {\r\n        return f;\r\n    }\r\n}\r\n```\r\n\r\nOutput:\r\n```\r\nERROR:root:Error in .\\function_returning_function.sol\r\nERROR:root:Traceback (most recent call last):\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\__main__.py\", line 610, in main_impl\r\n    (slither_instances, results_detectors, results_printers, number_contracts) = process_all(filename, args,\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\__main__.py\", line 67, in process_all\r\n    (slither, current_results_detectors, current_results_printers, current_analyzed_count) = process_single(\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\__main__.py\", line 53, in process_single\r\n    slither = Slither(target,\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\slither.py\", line 86, in __init__\r\n    self._parser.analyze_contracts()\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\solc_parsing\\slitherSolc.py\", line 345, in analyze_contracts\r\n    self._convert_to_slithir()\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\solc_parsing\\slitherSolc.py\", line 489, in _convert_to_slithir\r\n    func.generate_slithir_and_analyze()\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\core\\declarations\\function.py\", line 1652, in generate_slithir_and_analyze\r\n    node.slithir_generation()\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\core\\cfg\\node.py\", line 702, in slithir_generation\r\n    self._irs = convert_expression(expression, self)\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\slithir\\convert.py\", line 64, in convert_expression\r\n    visitor = ExpressionToSlithIR(expression, node)\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\visitors\\slithir\\expression_to_slithir.py\", line 105, in __init__\r\n    r = Return(get(self.expression))\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\slithir\\operations\\return_operation.py\", line 18, in __init__\r\n    assert is_valid_rvalue(values) or isinstance(values, TupleVariable) or values is None\r\nAssertionError\r\n```\nAssertionError parsing function-type return values\nThe following contract causes an assertion error:\r\n```\r\ncontract FunctionReturningFunction {\r\n    function f() public { }\r\n\r\n    function g() internal returns (function() internal) {\r\n        return f;\r\n    }\r\n}\r\n```\r\n\r\nOutput:\r\n```\r\nERROR:root:Error in .\\function_returning_function.sol\r\nERROR:root:Traceback (most recent call last):\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\__main__.py\", line 610, in main_impl\r\n    (slither_instances, results_detectors, results_printers, number_contracts) = process_all(filename, args,\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\__main__.py\", line 67, in process_all\r\n    (slither, current_results_detectors, current_results_printers, current_analyzed_count) = process_single(\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\__main__.py\", line 53, in process_single\r\n    slither = Slither(target,\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\slither.py\", line 86, in __init__\r\n    self._parser.analyze_contracts()\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\solc_parsing\\slitherSolc.py\", line 345, in analyze_contracts\r\n    self._convert_to_slithir()\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\solc_parsing\\slitherSolc.py\", line 489, in _convert_to_slithir\r\n    func.generate_slithir_and_analyze()\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\core\\declarations\\function.py\", line 1652, in generate_slithir_and_analyze\r\n    node.slithir_generation()\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\core\\cfg\\node.py\", line 702, in slithir_generation\r\n    self._irs = convert_expression(expression, self)\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\slithir\\convert.py\", line 64, in convert_expression\r\n    visitor = ExpressionToSlithIR(expression, node)\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\visitors\\slithir\\expression_to_slithir.py\", line 105, in __init__\r\n    r = Return(get(self.expression))\r\n  File \"c:\\users\\x\\documents\\github\\slither\\slither\\slithir\\operations\\return_operation.py\", line 18, in __init__\r\n    assert is_valid_rvalue(values) or isinstance(values, TupleVariable) or values is None\r\nAssertionError\r\n```\n", "before_files": [{"content": "from slither.slithir.operations.operation import Operation\n\nfrom slither.slithir.variables.tuple import TupleVariable\nfrom slither.slithir.utils.utils import is_valid_rvalue\n\n\nclass Return(Operation):\n    \"\"\"\n       Return\n       Only present as last operation in RETURN node\n    \"\"\"\n\n    def __init__(self, values):\n        # Note: Can return None \n        # ex: return call()\n        # where call() dont return\n        if not isinstance(values, list):\n            assert is_valid_rvalue(values) or isinstance(values, TupleVariable) or values is None\n            if values is None:\n                values = []\n            else:\n                values = [values]\n        else:\n            # Remove None\n            # Prior Solidity 0.5\n            # return (0,)\n            # was valid for returns(uint)\n            values = [v for v in values if not v is None]\n            self._valid_value(values)\n        super(Return, self).__init__()\n        self._values = values\n\n    def _valid_value(self, value):\n        if isinstance(value, list):\n            assert all(self._valid_value(v) for v in value)\n        else:\n            assert is_valid_rvalue(value) or isinstance(value, TupleVariable)\n        return True\n\n    @property\n    def read(self):\n        return self._unroll(self.values)\n\n    @property\n    def values(self):\n        return self._unroll(self._values)\n\n    def __str__(self):\n        return \"RETURN {}\".format(','.join(['{}'.format(x) for x in self.values]))\n", "path": "slither/slithir/operations/return_operation.py"}], "after_files": [{"content": "from slither.core.declarations import Function\nfrom slither.slithir.operations.operation import Operation\n\nfrom slither.slithir.variables.tuple import TupleVariable\nfrom slither.slithir.utils.utils import is_valid_rvalue\n\n\nclass Return(Operation):\n    \"\"\"\n       Return\n       Only present as last operation in RETURN node\n    \"\"\"\n\n    def __init__(self, values):\n        # Note: Can return None \n        # ex: return call()\n        # where call() dont return\n        if not isinstance(values, list):\n            assert is_valid_rvalue(values) or isinstance(values, (TupleVariable, Function)) or values is None\n            if values is None:\n                values = []\n            else:\n                values = [values]\n        else:\n            # Remove None\n            # Prior Solidity 0.5\n            # return (0,)\n            # was valid for returns(uint)\n            values = [v for v in values if not v is None]\n            self._valid_value(values)\n        super(Return, self).__init__()\n        self._values = values\n\n    def _valid_value(self, value):\n        if isinstance(value, list):\n            assert all(self._valid_value(v) for v in value)\n        else:\n            assert is_valid_rvalue(value) or isinstance(value, (TupleVariable, Function))\n        return True\n\n    @property\n    def read(self):\n        return self._unroll(self.values)\n\n    @property\n    def values(self):\n        return self._unroll(self._values)\n\n    def __str__(self):\n        return \"RETURN {}\".format(','.join(['{}'.format(x) for x in self.values]))\n", "path": "slither/slithir/operations/return_operation.py"}]}