Do not report names assigned by every if branch as undeclared

[rulfox]

meta.find_undeclared_variables reports a name that is {% set %} in every branch of an {% if %} chain ending in {% else %}, even though such a name is always defined afterwards and is never read from the context:

env.parse("{% if a %}{% set x = 1 %}{% else %}{% set x = 2 %}{% endif %}{{ x }}")
# reports {"a", "x"}; "x" cannot come from the context

Why it regressed

Symbols.branch_update used to skip the context load for a name stored by every branch. #1665 removed that optimisation to fix #1253, because it also skipped the load when a branch read the name before assigning it.

This change

Restores the optimisation behind two guards:

1. Every branch must introduce the name as a fresh local, that is, its load is undefined rather than resolve/alias. This is the case What's wrong with this Jinja template when using {% elif %} #1253 needed.
2. The name must not be read inside a scope FrameSymbolVisitor does not enter. Those reads are invisible to the frame's symbols but still read the frame's variables, so FrameSymbolVisitor now records them and visit_If consults them. Every field the compiler visits with a child frame is tracked: For.body/else_/test, Macro.body/defaults, CallBlock.body/defaults, FilterBlock.body/filter, With.body, AssignBlock.body/filter, Scope.body, Block.body, OverlayScope.context/body.

visit_If also tracks each {% elif %} body as its own branch, so plain {% if %}/{% else %} is recognised too. That case was reported in #2069 as failing on 3.1.4 as well.

The scan of nested scopes is lazy, so a frame with no exhaustive {% if %} chain does no extra work.

Verification

• The three snippets in meta.find_undeclared_variables returning variables set in if blocks as of 3.1.5 #2069 (with elif, without elif, and the no-else control case) all behave as the reporter expected.
• Differential run over ~1260 generated templates x 8 contexts covering for, for recursive, macro, call, filter, with, set/endset, block, extends, include, import, namespaces, tuple targets and nested chains: no rendered output changes, only the intended reduction in find_undeclared_variables results.
• Compile time measured at roughly +2% on a 200-unit page template.

Tests

• tests/test_api.py — parametrized find_undeclared_variables cases, including the negative ones: no else, a branch that does not assign, read-before-assign, read in an elif test, read in a nested scope.
• tests/test_regression.py — rendering tests beside the existing What's wrong with this Jinja template when using {% elif %} #1253 test, covering a read in each child-frame field.
• tests/test_idtracking.py — symbol-level assertions for an exhaustive chain and for OverlayScope.context.

Full suite: 936 passed. ruff clean; mypy reports the same pre-existing errors as main.

fixes #2069

[davidism]

https://palletsprojects.com/contributing/llm-ai