Resolve possibly-used-before-assignment false positives from match block assignments

doc/whatsnew/fragments/9668.false_positive

@@ -0,0 +1,4 @@
+Fix false positives for `possibly-used-before-assignment` when variables are exhaustively
+assigned within a `match` block.
+
+Closes #9668

pylint/checkers/variables.py

@@ -674,6 +674,12 @@ def _inferred_to_define_name_raise_or_return(
         if isinstance(node, (nodes.With, nodes.For, nodes.While)):
             return NamesConsumer._defines_name_raises_or_returns_recursive(name, node)
 
+        if isinstance(node, nodes.Match):
+            return all(
+                NamesConsumer._defines_name_raises_or_returns_recursive(name, case)
+                for case in node.cases
+            )
+
         if not isinstance(node, nodes.If):
             return False
 
@@ -723,6 +729,7 @@ def _branch_handles_name(self, name: str, body: Iterable[nodes.NodeNG]) -> bool:
                         nodes.With,
                         nodes.For,
                         nodes.While,
+                        nodes.Match,
                     ),
                 )
                 and self._inferred_to_define_name_raise_or_return(name, if_body_stmt)
@@ -975,6 +982,11 @@ def _defines_name_raises_or_returns_recursive(
                 and NamesConsumer._defines_name_raises_or_returns_recursive(name, stmt)
             ):
                 return True
+            if isinstance(stmt, nodes.Match):
+                return all(
+                    NamesConsumer._defines_name_raises_or_returns_recursive(name, case)
+                    for case in stmt.cases
+                )
         return False
 
     @staticmethod

tests/functional/u/used/used_before_assignment_py310.py

@@ -33,7 +33,7 @@ def check_value_if_then_match_return(example: Example, should_check: bool) -> st
             case _:
                 return None
 
-    return result  # [possibly-used-before-assignment] FALSE POSITIVE
+    return result
 
 def check_value_if_then_match_raise(example: Example, should_check: bool) -> str | None:
     if should_check:
@@ -47,7 +47,7 @@ def check_value_if_then_match_raise(example: Example, should_check: bool) -> str
             case _:
                 raise ValueError("Not a valid enum")
 
-    return result  # [possibly-used-before-assignment] FALSE POSITIVE
+    return result
 
 def check_value_if_then_match_assert_never(example: Example, should_check: bool) -> str | None:
     if should_check:
@@ -61,7 +61,7 @@ def check_value_if_then_match_assert_never(example: Example, should_check: bool)
             case _:
                 assert_never(example)
 
-    return result  # [possibly-used-before-assignment] FALSE POSITIVE
+    return result
 
 def g(x):
     if x is None:
@@ -73,4 +73,36 @@ def g(x):
             case _:
                 raise TypeError(type(x))
 
-    return y  # [possibly-used-before-assignment] FALSE POSITIVE
+    return y
+
+def check_value_if_then_match_nested(
+    example: Example, example_inner: Example, should_check: bool
+) -> str | None:
+    if should_check:
+        result = None
+    else:
+        match example:
+            case Example.FOO:
+                match example_inner:
+                    case Example.BAR:
+                        result = "bar"
+                    case _:
+                        return None
+            case _:
+                return None
+
+    return result
+
+def check_value_if_then_match_non_exhaustive(example: Example, should_check: bool) -> str | None:
+    if should_check:
+        result = None
+    else:
+        match example:
+            case Example.FOO:
+                result = "foo"
+            case Example.BAR:
+                pass
+            case _:
+                return None
+
+    return result  # [possibly-used-before-assignment]

tests/functional/u/used/used_before_assignment_py310.txt

@@ -1,4 +1 @@
-possibly-used-before-assignment:36:11:36:17:check_value_if_then_match_return:Possibly using variable 'result' before assignment:CONTROL_FLOW
-possibly-used-before-assignment:50:11:50:17:check_value_if_then_match_raise:Possibly using variable 'result' before assignment:CONTROL_FLOW
-possibly-used-before-assignment:64:11:64:17:check_value_if_then_match_assert_never:Possibly using variable 'result' before assignment:CONTROL_FLOW
-possibly-used-before-assignment:76:11:76:12:g:Possibly using variable 'y' before assignment:CONTROL_FLOW
+possibly-used-before-assignment:108:11:108:17:check_value_if_then_match_non_exhaustive:Possibly using variable 'result' before assignment:CONTROL_FLOW