Fix bug where set would sometimes fail within if

There was a bug that came as the result of an early optimization done
within ID tracking that caused loading parameters to fail in a very
specific and rare edge case. That edge case only occurred when the
parameter was being set within all 3 standard branches of an if block,
since the optimization would assume that the parameter was never being
referenced and was only ever being set. This would cause the variable to
be set to undefined.

The fix for this was to remove the optimization and still continue to
load in the parameter even if it is set in all 3 branches.

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"