Fix possibly-undefined false-positive with nesting (python#20276)

Closes python#14309, python#17055

commit 3f1c0f8 adds a test that demonstrates the false-positive

commit acb345a fixes the false-positive based on the suggestion in
python#14309 (comment)

Author: grayjk

mypy/partially_defined.py

@@ -206,11 +206,13 @@ def __init__(self) -> None:
         # disable_branch_skip is used to disable skipping a branch due to a return/raise/etc. This is useful
         # in things like try/except/finally statements.
         self.disable_branch_skip = False
+        self.in_finally = False
 
     def copy(self) -> DefinedVariableTracker:
         result = DefinedVariableTracker()
         result.scopes = [s.copy() for s in self.scopes]
         result.disable_branch_skip = self.disable_branch_skip
+        result.in_finally = self.in_finally
         return result
 
     def _scope(self) -> Scope:
@@ -579,7 +581,9 @@ def process_try_stmt(self, o: TryStmt) -> None:
         self.tracker.end_branch_statement()
 
         if o.finally_body is not None:
+            self.tracker.in_finally = True
             o.finally_body.accept(self)
+            self.tracker.in_finally = False
 
     def visit_while_stmt(self, o: WhileStmt) -> None:
         o.expr.accept(self)
@@ -620,7 +624,10 @@ def visit_starred_pattern(self, o: StarredPattern) -> None:
     def visit_name_expr(self, o: NameExpr) -> None:
         if o.name in self.builtins and self.tracker.in_scope(ScopeType.Global):
             return
-        if self.tracker.is_possibly_undefined(o.name):
+        if (
+            self.tracker.is_possibly_undefined(o.name)
+            and self.tracker.in_finally == self.tracker.disable_branch_skip
+        ):
             # A variable is only defined in some branches.
             self.variable_may_be_undefined(o.name, o)
             # We don't want to report the error on the same variable multiple times.

test-data/unit/check-possibly-undefined.test

@@ -628,7 +628,7 @@ def f7() -> int:
             x = 1
         assert False
     except:
-        pass
+        y = x  # E: Name "x" may be undefined
     return x  # E: Name "x" may be undefined
 [builtins fixtures/exception.pyi]
 
@@ -678,7 +678,7 @@ def f3() -> int:
         pass
     finally:
         y = x  # E: Name "x" may be undefined
-    return x
+    return x  # E: Name "x" may be undefined
 
 def f4() -> int:
     try:
@@ -708,6 +708,17 @@ def f6() -> int:
     finally:
         a = x  # E: Name "x" may be undefined
     return a
+
+def f7() -> int:
+    try:
+        if int():
+            x = 1
+        else:
+            raise BaseException
+        return x  # No error.
+    finally:
+        y = x  # E: Name "x" may be undefined
+    return 0
 [builtins fixtures/exception.pyi]
 
 [case testTryElse]