Fix bug involving calling set on a template parameter within all branches of an if block

CHANGES.rst

@@ -45,6 +45,9 @@ Unreleased
     filter. :issue:`1624`
 -   Using ``set`` for multiple assignment (``a, b = 1, 2``) does not fail when the
     target is a namespace attribute. :issue:`1413`
+-   Using ``set`` in all branches of ``{% if %}{% elif %}{% else %}`` blocks
+    does not cause the variable to be considered initially undefined.
+    :issue:`1253`
 
 
 Version 3.1.4

src/jinja2/idtracking.py

@@ -121,23 +121,20 @@ def load(self, name: str) -> None:
             self._define_ref(name, load=(VAR_LOAD_RESOLVE, name))
 
     def branch_update(self, branch_symbols: t.Sequence["Symbols"]) -> None:
-        stores: t.Dict[str, int] = {}
+        stores: t.Set[str] = set()
+
         for branch in branch_symbols:
-            for target in branch.stores:
-                if target in self.stores:
-                    continue
-                stores[target] = stores.get(target, 0) + 1
+            stores.update(branch.stores)
+
+        stores.difference_update(self.stores)
 
         for sym in branch_symbols:
             self.refs.update(sym.refs)
             self.loads.update(sym.loads)
             self.stores.update(sym.stores)
 
-        for name, branch_count in stores.items():
-            if branch_count == len(branch_symbols):
-                continue
-
-            target = self.find_ref(name)  # type: ignore
+        for name in stores:
+            target = self.find_ref(name)
             assert target is not None, "should not happen"
 
             if self.parent is not None:

tests/test_regression.py

@@ -750,6 +750,16 @@ def is_foo(ctx, s):
         assert tmpl.render() == "foo"
 
 
+def test_load_parameter_when_set_in_all_if_branches(env):
+    tmpl = env.from_string(
+        "{% if True %}{{ a.b }}{% set a = 1 %}"
+        "{% elif False %}{% set a = 2 %}"
+        "{% else %}{% set a = 3 %}{% endif %}"
+        "{{ a }}"
+    )
+    assert tmpl.render(a={"b": 0}) == "01"
+
+
 @pytest.mark.parametrize("unicode_char", ["\N{FORM FEED}", "\x85"])
 def test_unicode_whitespace(env, unicode_char):
     content = "Lorem ipsum\n" + unicode_char + "\nMore text"