Fix assignment inside match arm condition not recognized

- Variable assignments in match arm conditions (e.g. `is_dir($baseDir = ...)`)
  were lost after commit 3beb8c6 replaced processExprNode with filterByTruthyValue
- filterByTruthyValue only narrows types without walking the AST, so assignments
  within condition expressions were not discovered for the arm body scope
- Fix transfers newly-defined variables from the condition processing scope to
  the body scope after applying truthiness filtering
- New regression test in tests/PHPStan/Rules/Variables/data/bug-13981.php

Closes phpstan/phpstan#13981

Author: ondrejmirtes

src/Analyser/NodeScopeResolver.php

@@ -208,6 +208,7 @@
 use UnhandledMatchError;
 use function array_fill_keys;
 use function array_filter;
+use function array_flip;
 use function array_key_exists;
 use function array_key_last;
 use function array_keys;
@@ -4355,6 +4356,7 @@ function (MutatingScope $scope) use ($stmt, $expr, $nodeCallback, $context, $sto
 				$filteringExprs = [];
 				$armCondScope = $matchScope;
 				$condNodes = [];
+				$armCondResultScope = $matchScope;
 				foreach ($arm->conds as $j => $armCond) {
 					if (isset($armCondsToSkip[$i][$j])) {
 						continue;
@@ -4376,6 +4378,30 @@ function (MutatingScope $scope) use ($stmt, $expr, $nodeCallback, $context, $sto
 
 				$filteringExpr = $this->getFilteringExprForMatchArm($expr, $filteringExprs);
 				$bodyScope = $matchScope->filterByTruthyValue($filteringExpr);
+				$condResultScope = $armCondResultScope;
+				$matchScopeKnownVars = array_flip(array_merge($matchScope->getDefinedVariables(), $matchScope->getMaybeDefinedVariables()));
+				foreach ($condResultScope->getDefinedVariables() as $varName) {
+					if (isset($matchScopeKnownVars[$varName])) {
+						continue;
+					}
+					$bodyScope = $bodyScope->assignVariable(
+						$varName,
+						$condResultScope->getVariableType($varName),
+						$condResultScope->getNativeType(new Variable($varName)),
+						$condResultScope->hasVariableType($varName),
+					);
+				}
+				foreach ($condResultScope->getMaybeDefinedVariables() as $varName) {
+					if (isset($matchScopeKnownVars[$varName])) {
+						continue;
+					}
+					$bodyScope = $bodyScope->assignVariable(
+						$varName,
+						$condResultScope->getVariableType($varName),
+						$condResultScope->getNativeType(new Variable($varName)),
+						$condResultScope->hasVariableType($varName),
+					);
+				}
 				$matchArmBody = new MatchExpressionArmBody($bodyScope, $arm->body);
 				$armNodes[$i] = new MatchExpressionArm($matchArmBody, $condNodes, $arm->getStartLine());
 

tests/PHPStan/Rules/Variables/DefinedVariableRuleTest.php

@@ -1257,4 +1257,15 @@ public function testBug12944(): void
 		$this->analyse([__DIR__ . '/data/bug-12944.php'], []);
 	}
 
+	#[RequiresPhp('>= 8.0')]
+	public function testBug13981(): void
+	{
+		$this->cliArgumentsVariablesRegistered = true;
+		$this->polluteScopeWithLoopInitialAssignments = true;
+		$this->checkMaybeUndefinedVariables = true;
+		$this->polluteScopeWithAlwaysIterableForeach = true;
+
+		$this->analyse([__DIR__ . '/data/bug-13981.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Variables/data/bug-13981.php

@@ -0,0 +1,10 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug13981;
+
+$path = match (true) {
+	is_dir($baseDir = dirname(__DIR__).'/lang') => $baseDir,
+	default => '/translations',
+};