Fix #13981: Assignment inside match statement no longer recognized

[phpstan-bot]

Summary

Assignment inside match arm conditions (e.g. is_dir($baseDir = dirname(__DIR__).'/lang') => $baseDir) stopped being recognized in 2.1.34, causing false positive "Variable $baseDir might not be defined" errors. This was a regression from commit 3beb8c6 ("Fix match analysis").

Changes

• src/Analyser/NodeScopeResolver.php: After computing the match arm body scope via filterByTruthyValue, transfer newly-defined variables from the condition processing scope ($armCondResultScope) to the body scope. This ensures variables assigned within match arm conditions are visible in the arm body.
• tests/PHPStan/Rules/Variables/data/bug-13981.php: New test data file reproducing the issue with match(true) and an assignment inside an is_dir() condition.
• tests/PHPStan/Rules/Variables/DefinedVariableRuleTest.php: New testBug13981() test method.

Root cause

Commit 3beb8c6 changed the match arm body scope computation from:

$bodyScope = $this->processExprNode($stmt, $filteringExpr, $matchScope, ...)->getTruthyScope();

to:

$bodyScope = $matchScope->filterByTruthyValue($filteringExpr);

The old code called processExprNode which walked the filtering expression's AST tree, discovering any assignments within the condition sub-expressions (like $baseDir = dirname(__DIR__).'/lang'). The new code only applies type narrowing via filterByTruthyValue without walking the AST, so assignments in conditions were lost.

The fix iterates over variables defined during condition processing (tracked in $armCondResultScope) and transfers any that are new (not already in $matchScope) to the body scope. This preserves correct type narrowing from filterByTruthyValue while restoring variable visibility.

Test

Added tests/PHPStan/Rules/Variables/data/bug-13981.php with a match(true) expression that assigns a variable inside the condition of a match arm. The test expects no errors (the variable should be recognized as defined). Verified the test fails without the fix and passes with it.

Fixes phpstan/phpstan#13981