Adds check for function parameter write to prevent reporting public -> external visibility change.

slither/detectors/functions/external_function.py

@@ -96,6 +96,13 @@ class ExternalFunction(AbstractDetector):
                                        for function in derived_contract.functions
                                        if function.full_name == base_most_function.full_name]
 
+    @staticmethod
+    def function_parameters_written(function):
+        for node in function.nodes:
+            if any (var.name == parameter.name for var in node.local_variables_written for parameter in function.parameters):
+                return True
+        return False
+
     def _detect(self):
         results = []
 
@@ -130,6 +137,11 @@ class ExternalFunction(AbstractDetector):
                 if function in completed_functions:
                     continue
 
+                # If the function has parameters which are written-to in function body, we skip
+                # because parameters of external functions will be allocated in calldata region which is immutable
+                if self.function_parameters_written(function):
+                    continue
+                
                 # Get the base-most function to know our origin of this function.
                 base_most_function = self.get_base_most_function(function)
 

tests/external_function.sol

@@ -68,3 +68,15 @@ contract InternalCall {
     }
     
 }
+
+contract FunctionParameterWrite {
+  
+  function parameter_read_ok_for_external (uint i) public {
+    uint local = i;
+  }
+
+  function parameter_read_not_ok_for_external (uint i) public returns (uint) {
+    i += 1;
+    return (i);
+  }
+}