resolved false positive on unused variables in decorator functions (#4373)

* resolved false positive on unused variables in decorator functions and added corresponding test cases

Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>

5 files changed, 22 insertions, 0 deletions

diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index 0252731b5..62bcc2004 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -469,6 +469,8 @@ contributors:
 
 * Andreas Finkler: contributor
 
+* Aidan Haase, Elizabeth Bott: contributor
+
 * Sebastian Müller: contributor
 
 * Ramiro Leal-Cavazos (ramiro050): Fixed bug preventing pylint from working with emacs tramp
diff --git a/ChangeLog b/ChangeLog
index a1dd0606a..ffbaec574 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -9,6 +9,10 @@ Release date: Undefined
 ..
   Put new features here and also in 'doc/whatsnew/2.8.rst'
 
+* Resolve false positives on unused variables in decorator functions
+
+  Closes #4252
+
 * Add new extension ``ConfusingConsecutiveElifChecker``. This optional checker emits a refactoring message (R5601 ``confusing-consecutive-elif``)
   if if/elif statements with different indentation levels follow directly one after the other.
 
diff --git a/pylint/checkers/variables.py b/pylint/checkers/variables.py
index 2a1587eeb..c611ae2f0 100644
--- a/pylint/checkers/variables.py
+++ b/pylint/checkers/variables.py
@@ -1695,6 +1695,9 @@ class VariablesChecker(BaseChecker):
                     return
                 message_name = "unused-variable"
 
+            if isinstance(stmt, astroid.FunctionDef) and stmt.decorators:
+                return
+
             # Don't check function stubs created only for type information
             if utils.is_overload_stub(node):
                 return
diff --git a/tests/functional/d/decorator_unused.py b/tests/functional/d/decorator_unused.py
new file mode 100644
index 000000000..add5b3b40
--- /dev/null
+++ b/tests/functional/d/decorator_unused.py
@@ -0,0 +1,13 @@
+# pylint: disable=C
+def foo():
+    _dispatch = {}
+
+    def save(fn):
+        _dispatch[fn.__name__] = fn
+        return fn
+
+    @save
+    def bar():
+        ...
+    # do whatever
+foo()
diff --git a/tests/functional/d/decorator_unused.txt b/tests/functional/d/decorator_unused.txt
new file mode 100644
index 000000000..e69de29bb
--- /dev/null
+++ b/tests/functional/d/decorator_unused.txt