Mutable default argument in generic function

Detects when a generic function or method accepts a mutable object as a default parameter value, which can lead to runtime type errors.

When a generic function uses a mutable default value (like a list, dict, or set), that default is shared across all invocations of the function. This creates a scenario where the mutable object can accumulate values of different types from different calls.

What gets flagged

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def append_and_return[T](x: T, y: list[T] = []) -> list[T]:
    y.append(x)
    return y

# Each call with a different type shares the same list
int_list = append_and_return(1)
str_list = append_and_return("hello")

# This is a int at runtime but str at type check time.
value = str_list[0]

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error[mutable-generic-default]: Using a mutable default argument for a generic parameter in a function can lead to runtime type errors.
 --> main.py:1:45
  |
1 | def append_and_return[T](x: T, y: list[T] = []) -> list[T]:
  |                                             ^^
2 |     y.append(x)
3 |     return y
  |
info: rule `mutable-generic-default` was selected in the configuration file

In this example, both calls to append_and_return share the same default list. If code appends to this list in different calls with different type parameters, the list ends up containing mixed types, breaking the type soundness that list[T] promises.