Fix check unused arguments false positive bug (#8542) (#8545)

Problem: the special method `__new__` must match the arguments of the
`__init__` method even if `__new__` method does not use them. This
generate `unused-argument` for the `__new__` method.

Fix: the unused arguments check should not be done on the `__new__`
method if the `__init__` method is defined in the same class.

Update `unused-argument` test to include a check for the case of
`__init__` and `__new__` being defined in a class but `__new__` does not
use all of the argument. This is fine because `__new__` must have the
same argument of `__init__`.

Update with a second check in case of `__init__` being not defined in a
class. Then the unused arguments check must be done on `__new__`.

Fixes https://github.com/pylint-dev/pylint/issues/3670

(cherry picked from commit 156da64d0fb4c06e15c5b619b91ce550d594a770)

Co-authored-by: Théo Battrel <theo.util@protonmail.ch>

1 files changed, 10 insertions, 0 deletions

diff --git a/pylint/checkers/variables.py b/pylint/checkers/variables.py
index f55b71264..79d9ded08 100644
--- a/pylint/checkers/variables.py
+++ b/pylint/checkers/variables.py
@@ -2602,6 +2602,16 @@ class VariablesChecker(BaseChecker):
         argnames = node.argnames()
         # Care about functions with unknown argument (builtins)
         if name in argnames:
+            if node.name == "__new__":
+                is_init_def = False
+                # Look for the `__init__` method in all the methods of the same class.
+                for n in node.parent.get_children():
+                    is_init_def = hasattr(n, "name") and (n.name == "__init__")
+                    if is_init_def:
+                        break
+                # Ignore unused arguments check for `__new__` if `__init__` is defined.
+                if is_init_def:
+                    return
             self._check_unused_arguments(name, node, stmt, argnames, nonlocal_names)
         else:
             if stmt.parent and isinstance(