[Backport maintenance/2.17.x] Fix check unused arguments false positive bug

doc/whatsnew/fragments/3670.false_positive

@@ -0,0 +1,3 @@
+Fix `unused-argument` false positive when `__new__` does not use all the arguments of `__init__`.
+
+Closes #3670

pylint/checkers/variables.py

@@ -2602,6 +2602,16 @@ def _check_is_unused(
         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(

tests/functional/u/unused/unused_argument.py

@@ -107,3 +107,24 @@ class Descendant(Ancestor):
     def set_thing(self, thing, *, other=None):
         """Subclass does not raise unused-argument"""
         self.thing = thing
+
+
+# Test that Class with both `__init__` and `__new__` don't check
+# on `__new__` for unused arguments
+
+# pylint: disable=invalid-name
+
+class TestClassWithInitAndNew:
+    def __init__(self, argA, argB):
+        self.argA = argA
+        self.argB = argB
+
+    def __new__(cls, argA, argB):
+        return object.__new__(cls)
+
+# Test that `__new__` method is checked for unused arguments
+# when `__init__` is not in the Class
+
+class TestClassWithOnlyNew:
+    def __new__(cls, argA, argB): # [unused-argument, unused-argument]
+        return object.__new__(cls)

tests/functional/u/unused/unused_argument.txt

@@ -7,3 +7,5 @@ unused-argument:73:0:None:None:AAAA.selected:Unused argument 'args':INFERENCE
 unused-argument:73:0:None:None:AAAA.selected:Unused argument 'kwargs':INFERENCE
 unused-argument:92:23:92:26:BBBB.__init__:Unused argument 'arg':INFERENCE
 unused-argument:103:34:103:39:Ancestor.set_thing:Unused argument 'other':INFERENCE
+unused-argument:129:21:129:25:TestClassWithOnlyNew.__new__:Unused argument 'argA':INFERENCE
+unused-argument:129:27:129:31:TestClassWithOnlyNew.__new__:Unused argument 'argB':INFERENCE