Fix false positive undefined variable in decorator with list comprehe…

…nsion (#4737)

Closes #3791

CONTRIBUTORS.txt

@@ -519,3 +519,5 @@ contributors:
   - Documented Jupyter integration
 
 * Yilei Yang: contributor
+
+* Marcin Kurczewski (rr-): contributor

pylint/checkers/variables.py

@@ -44,6 +44,7 @@
 # Copyright (c) 2021 Lorena B <46202743+lorena-b@users.noreply.github.com>
 # Copyright (c) 2021 haasea <44787650+haasea@users.noreply.github.com>
 # Copyright (c) 2021 Alexander Kapshuna <kapsh@kap.sh>
+# Copyright (c) 2021 Marcin Kurczewski <rr-@sakuya.pl>
 
 # Licensed under the GPL: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
 # For details: https://github.com/PyCQA/pylint/blob/main/LICENSE
@@ -1006,9 +1007,12 @@ def visit_name(self, node):
             # variable used outside the loop
             # avoid the case where there are homonyms inside function scope and
             # comprehension current scope (avoid bug #1731)
-            if name in current_consumer.consumed and not (
-                current_consumer.scope_type == "comprehension"
-                and self._has_homonym_in_upper_function_scope(node, i)
+            if name in current_consumer.consumed and (
+                utils.is_func_decorator(current_consumer.node)
+                or not (
+                    current_consumer.scope_type == "comprehension"
+                    and self._has_homonym_in_upper_function_scope(node, i)
+                )
             ):
                 defnode = utils.assign_parent(current_consumer.consumed[name][0])
                 self._check_late_binding_closure(node, defnode)

tests/functional/u/undefined/undefined_variable.py

@@ -309,3 +309,22 @@ def undefined_annotation(a:x): # [undefined-variable]
     ]
     for item in lst
 ]
+
+
+# 3791
+@decorator(x for x in range(3))
+def decorated1(x):
+    print(x)
+
+@decorator(x * x for x in range(3))
+def decorated2(x):
+    print(x)
+
+@decorator(x)  # [undefined-variable]
+@decorator(x * x for x in range(3))
+def decorated3(x):
+    print(x)
+
+@decorator(x * x * y for x in range(3))  # [undefined-variable]
+def decorated4(x):
+    print(x)

tests/functional/u/undefined/undefined_variable.txt

@@ -28,3 +28,5 @@ used-before-assignment:254:26:func_should_fail:Using variable 'datetime' before
 undefined-variable:281:18:not_using_loop_variable_accordingly:Undefined variable 'iteree'
 undefined-variable:292:27:undefined_annotation:Undefined variable 'x'
 used-before-assignment:293:7:undefined_annotation:Using variable 'x' before assignment
+undefined-variable:323:11:decorated3:Undefined variable 'x'
+undefined-variable:328:19:decorated4:Undefined variable 'y'