Add support for "selector" access on custom error (#984)

* Add support for "selector" access on custom error

slither/slithir/convert.py

@@ -623,7 +623,19 @@ def propagate_types(ir, node: "Node"):  # pylint: disable=too-many-locals
                     b.set_expression(ir.expression)
                     b.set_node(ir.node)
                     return b
-                if ir.variable_right == "selector" and isinstance(ir.variable_left.type, Function):
+                if ir.variable_right == "selector" and isinstance(ir.variable_left, (CustomError)):
+                    assignment = Assignment(
+                        ir.lvalue,
+                        Constant(str(get_function_id(ir.variable_left.solidity_signature))),
+                        ElementaryType("bytes4"),
+                    )
+                    assignment.set_expression(ir.expression)
+                    assignment.set_node(ir.node)
+                    assignment.lvalue.set_type(ElementaryType("bytes4"))
+                    return assignment
+                if ir.variable_right == "selector" and isinstance(
+                    ir.variable_left.type, (Function)
+                ):
                     assignment = Assignment(
                         ir.lvalue,
                         Constant(str(get_function_id(ir.variable_left.type.full_name))),

slither/slithir/operations/member.py

@@ -1,4 +1,5 @@
 from slither.core.declarations import Contract, Function
+from slither.core.declarations.custom_error import CustomError
 from slither.core.declarations.enum import Enum
 from slither.core.declarations.solidity_import_placeholder import SolidityImportPlaceHolder
 from slither.slithir.operations.lvalue import OperationWithLValue
@@ -21,7 +22,7 @@ class Member(OperationWithLValue):
         #     }
         # }
         assert is_valid_rvalue(variable_left) or isinstance(
-            variable_left, (Contract, Enum, Function, SolidityImportPlaceHolder)
+            variable_left, (Contract, Enum, Function, CustomError, SolidityImportPlaceHolder)
         )
 
         assert isinstance(variable_right, Constant)

tests/ast-parsing/custom_error-0.8.4.sol

@@ -37,5 +37,9 @@ contract B is A{
     function g() public{
         revert MyError(2);
     }
+
+    function h() public returns(bytes4){
+        return MyError.selector;
+    }
 }