Fix used-before-assignment false negative with bare type annotations

doc/whatsnew/fragments/10847.false_negative

@@ -0,0 +1,4 @@
+Fix ``used-before-assignment`` false negative when a variable has a bare type
+annotation (without a value) and is only assigned inside ``except`` blocks.
+
+Closes #10847

pylint/checkers/variables.py

@@ -651,6 +651,22 @@ def get_next_to_consume(self, node: nodes.Name) -> list[nodes.NodeNG] | None:
             uncertain_nodes_set = set(uncertain_nodes)
             found_nodes = [n for n in found_nodes if n not in uncertain_nodes_set]
 
+        # Treat bare type annotations (AnnAssign without a value) as uncertain
+        # when there are other uncertain definitions. A bare annotation like
+        # `x: int` does not actually assign a value, so it should not suppress
+        # possibly-used-before-assignment when the real assignments are uncertain
+        # (e.g., in except blocks).
+        if found_nodes and self.consumed_uncertain.get(name):
+            bare_annotations = [
+                n
+                for n in found_nodes
+                if isinstance(n.parent, nodes.AnnAssign) and n.parent.value is None
+            ]
+            if bare_annotations:
+                self.consumed_uncertain[name] += bare_annotations
+                bare_set = set(bare_annotations)
+                found_nodes = [n for n in found_nodes if n not in bare_set]
+
         return found_nodes
 
     def _inferred_to_define_name_raise_or_return(

tests/functional/u/used/used_before_assignment_type_annotations.py

@@ -107,3 +107,31 @@ def loop_conditional_annotated_assignment():
             data={"cat": "harf"}
     token: str = data.get("cat")  # [possibly-used-before-assignment]
     print(token)
+
+
+def bare_annotation_with_except_assignment():
+    """A bare type annotation should not suppress used-before-assignment
+    when the only real assignments are in except blocks.
+
+    https://github.com/pylint-dev/pylint/issues/10847
+    """
+    result = None
+    err: int  # pylint: disable=unused-variable
+    try:
+        result = 1
+    except Exception:  # pylint: disable=broad-exception-caught
+        err = 1
+    if not result:
+        print(err)  # [used-before-assignment]
+
+
+def bare_annotation_with_value_and_except():
+    """A type annotation with a value should suppress the warning."""
+    result = None
+    err: int = 0
+    try:
+        result = 1
+    except Exception:  # pylint: disable=broad-exception-caught
+        err = 1
+    if not result:
+        print(err)

tests/functional/u/used/used_before_assignment_type_annotations.txt

@@ -3,3 +3,4 @@ used-before-assignment:28:10:28:18:value_assignment_after_access:Using variable
 undefined-variable:62:14:62:17:decorator_returning_incorrect_function.wrapper_with_type_and_no_value:Undefined variable 'var':HIGH
 possibly-used-before-assignment:97:17:97:21:conditional_annotated_assignment:Possibly using variable 'data' before assignment:CONTROL_FLOW
 possibly-used-before-assignment:108:17:108:21:loop_conditional_annotated_assignment:Possibly using variable 'data' before assignment:CONTROL_FLOW
+used-before-assignment:125:14:125:17:bare_annotation_with_except_assignment:Using variable 'err' before assignment:CONTROL_FLOW