Allow redefinition of underscore functions

Allow functions called '_' to be redefined, instead of showing an error
as we would if a function with any other name was redefined.

Author: pranavrajpal

mypy/checkexpr.py

@@ -333,6 +333,11 @@ def visit_call_expr_inner(self, e: CallExpr, allow_none_return: bool = False) ->
                 isinstance(callee_type, CallableType)
                 and callee_type.implicit):
             self.msg.untyped_function_call(callee_type, e)
+
+        if isinstance(callee_type, CallableType) and callee_type.name == "_":
+            self.msg.underscore_function_call(e)
+            return AnyType(TypeOfAny.from_error)
+
         # Figure out the full name of the callee for plugin lookup.
         object_type = None
         member = None

mypy/fastparse.py

@@ -455,7 +455,7 @@ def fix_function_overloads(self, stmts: List[Statement]) -> List[Statement]:
                 elif len(current_overload) > 1:
                     ret.append(OverloadedFuncDef(current_overload))
 
-                if isinstance(stmt, Decorator):
+                if isinstance(stmt, Decorator) and stmt.name != "_":
                     current_overload = [stmt]
                     current_overload_name = stmt.name
                 else:

mypy/messages.py

@@ -701,6 +701,9 @@ def does_not_return_value(self, callee_type: Optional[Type], context: Context) -
         else:
             self.fail('Function does not return a value', context, code=codes.FUNC_RETURNS_VALUE)
 
+    def underscore_function_call(self, context: Context) -> None:
+        self.fail('Calling function named "_" is not allowed', context)
+
     def deleted_as_rvalue(self, typ: DeletedType, context: Context) -> None:
         """Report an error about using an deleted type as an rvalue."""
         if typ.source is None:

mypy/semanal.py

@@ -666,6 +666,8 @@ def f(): ...  # Error: 'f' redefined
         """
         if isinstance(new, Decorator):
             new = new.func
+        if isinstance(previous, (FuncDef, Decorator)) and new.name == previous.name == "_":
+            return True
         if isinstance(previous, (FuncDef, Var, Decorator)) and new.is_conditional:
             new.original_def = previous
             return True