Warn about self-referencing named expressions lacking prior assignments (#6915)

4 files changed, 37 insertions, 12 deletions

diff --git a/doc/whatsnew/2/2.15/index.rst b/doc/whatsnew/2/2.15/index.rst
index cac82dd23..0f284c81c 100644
--- a/doc/whatsnew/2/2.15/index.rst
+++ b/doc/whatsnew/2/2.15/index.rst
@@ -36,6 +36,11 @@ False negatives fixed
 
   Closes #6648
 
+* Emit ``used-before-assignment`` for self-referencing named expressions (``:=``) lacking
+  prior assignments.
+
+  Closes #5653
+
 * Emit ``using-constant-test`` when testing the truth value of a call that only returns generators.
 
   Closes #6909
diff --git a/pylint/checkers/variables.py b/pylint/checkers/variables.py
index d03a74ca2..9389a6982 100644
--- a/pylint/checkers/variables.py
+++ b/pylint/checkers/variables.py
@@ -1973,10 +1973,14 @@ class VariablesChecker(BaseChecker):
                         )
                     )
                 ):
-                    # Expressions, with assignment expressions
-                    # Use only after assignment
-                    # b = (c := 2) and c
-                    maybe_before_assign = False
+                    # Relation of a name to the same name in a named expression
+                    # Could be used before assignment if self-referencing:
+                    # (b := b)
+                    # Otherwise, safe if used after assignment:
+                    # (b := 2) and b
+                    maybe_before_assign = any(
+                        anc is defnode.value for anc in node.node_ancestors()
+                    )
 
             # Look for type checking definitions inside a type checking guard.
             if isinstance(defstmt, (nodes.Import, nodes.ImportFrom)):
@@ -2076,6 +2080,14 @@ class VariablesChecker(BaseChecker):
                 if (
                     not isinstance(ref_node.parent, nodes.AnnAssign)
                     or ref_node.parent.value
+                ) and not (
+                    # EXCEPTION: will not have a value if a self-referencing named expression
+                    # var: int
+                    # if (var := var * var)  <-- "var" still undefined
+                    isinstance(ref_node.parent, nodes.NamedExpr)
+                    and any(
+                        anc is ref_node.parent.value for anc in node.node_ancestors()
+                    )
                 ):
                     return False
             parent = parent_scope.parent
diff --git a/tests/functional/u/undefined/undefined_variable_py38.py b/tests/functional/u/undefined/undefined_variable_py38.py
index 93ba1eb2b..7a6aed295 100644
--- a/tests/functional/u/undefined/undefined_variable_py38.py
+++ b/tests/functional/u/undefined/undefined_variable_py38.py
@@ -14,7 +14,13 @@ def typing_and_assignment_expression():
 def typing_and_self_referencing_assignment_expression():
     """The variable gets assigned in an assignment expression that references itself"""
     var: int
-    if (var := var ** 2):  # false negative: https://github.com/PyCQA/pylint/issues/5653
+    if (var := var ** 2):  # [used-before-assignment]
+        print(var)
+
+
+def self_referencing_assignment_expression():
+    """An invalid self-referencing assignment expression"""
+    if (var := var()):  # [used-before-assignment]
         print(var)
 
 
diff --git a/tests/functional/u/undefined/undefined_variable_py38.txt b/tests/functional/u/undefined/undefined_variable_py38.txt
index e912cbbf9..cc438c0d7 100644
--- a/tests/functional/u/undefined/undefined_variable_py38.txt
+++ b/tests/functional/u/undefined/undefined_variable_py38.txt
@@ -1,7 +1,9 @@
-undefined-variable:42:6:42:16::Undefined variable 'no_default':UNDEFINED
-undefined-variable:50:6:50:22::Undefined variable 'again_no_default':UNDEFINED
-undefined-variable:76:6:76:19::Undefined variable 'else_assign_1':INFERENCE
-undefined-variable:99:6:99:19::Undefined variable 'else_assign_2':INFERENCE
-used-before-assignment:134:10:134:16:type_annotation_used_improperly_after_comprehension:Using variable 'my_int' before assignment:HIGH
-used-before-assignment:141:10:141:16:type_annotation_used_improperly_after_comprehension_2:Using variable 'my_int' before assignment:HIGH
-used-before-assignment:171:12:171:16:expression_in_ternary_operator_inside_container_wrong_position:Using variable 'val3' before assignment:HIGH
+used-before-assignment:17:15:17:18:typing_and_self_referencing_assignment_expression:Using variable 'var' before assignment:HIGH
+used-before-assignment:23:15:23:18:self_referencing_assignment_expression:Using variable 'var' before assignment:HIGH
+undefined-variable:48:6:48:16::Undefined variable 'no_default':UNDEFINED
+undefined-variable:56:6:56:22::Undefined variable 'again_no_default':UNDEFINED
+undefined-variable:82:6:82:19::Undefined variable 'else_assign_1':INFERENCE
+undefined-variable:105:6:105:19::Undefined variable 'else_assign_2':INFERENCE
+used-before-assignment:140:10:140:16:type_annotation_used_improperly_after_comprehension:Using variable 'my_int' before assignment:HIGH
+used-before-assignment:147:10:147:16:type_annotation_used_improperly_after_comprehension_2:Using variable 'my_int' before assignment:HIGH
+used-before-assignment:177:12:177:16:expression_in_ternary_operator_inside_container_wrong_position:Using variable 'val3' before assignment:HIGH