diff --git a/src/Analyser/MutatingScope.php b/src/Analyser/MutatingScope.php
index c202834e4e..beb3b85d86 100644
--- a/src/Analyser/MutatingScope.php
+++ b/src/Analyser/MutatingScope.php
@@ -119,6 +119,7 @@
 use function array_map;
 use function array_merge;
 use function array_pop;
+use function array_reverse;
 use function array_slice;
 use function count;
 use function explode;
@@ -3720,7 +3721,6 @@ public function filterBySpecifiedTypes(SpecifiedTypes $specifiedTypes): self
 		$specifiedExpressions = [];
 		foreach ($typeSpecifications as $typeSpecification) {
 			$expr = $typeSpecification['expr'];
-			$specifiedExpressions[$this->getNodeKey($expr)] = true;
 			$type = $typeSpecification['type'];
 			if ($typeSpecification['sure']) {
 				if ($specifiedTypes->shouldOverwrite()) {
@@ -3731,27 +3731,24 @@ public function filterBySpecifiedTypes(SpecifiedTypes $specifiedTypes): self
 			} else {
 				$scope = $scope->removeTypeFromExpression($expr, $type);
 			}
+			$specifiedExpressions[$this->getNodeKey($expr)] = $scope->getType($expr);
 		}
 
-		$newConditionalExpressions = $specifiedTypes->getNewConditionalExpressionHolders();
-		foreach ($scope->conditionalExpressions as $variableExprString => $conditionalExpressions) {
-			if (array_key_exists($variableExprString, $specifiedExpressions)) {
-				continue;
-			}
-			$newConditionalExpressions[$variableExprString] = $conditionalExpressions;
-			foreach ($conditionalExpressions as $conditionalExpression) {
-				$targetTypeHolder = $conditionalExpression->getTypeHolder();
-				foreach ($conditionalExpression->getConditionExpressionTypeHolders() as $conditionalTypeHolder) {
-					if (!$scope->invalidateExpression($targetTypeHolder->getExpr())->getType($conditionalTypeHolder->getExpr())->equals($conditionalTypeHolder->getType())) {
+		foreach ($scope->conditionalExpressions as $conditionalExprString => $conditionalExpressions) {
+			foreach (array_reverse($conditionalExpressions) as $conditionalExpression) {
+				foreach ($conditionalExpression->getConditionExpressionTypeHolders() as $holderExprString => $conditionalTypeHolder) {
+					if (!array_key_exists($holderExprString, $specifiedExpressions) || !$specifiedExpressions[$holderExprString]->equals($conditionalTypeHolder->getType())) {
 						continue 2;
 					}
 				}
 
-				if ($targetTypeHolder->getCertainty()->no()) {
-					unset($scope->expressionTypes[$variableExprString]);
+				if ($conditionalExpression->getTypeHolder()->getCertainty()->no()) {
+					unset($scope->expressionTypes[$conditionalExprString]);
 				} else {
-					$scope->expressionTypes[$variableExprString] = $targetTypeHolder;
+					$scope->expressionTypes[$conditionalExprString] = $conditionalExpression->getTypeHolder();
+					$specifiedExpressions[$conditionalExprString] = $conditionalExpression->getTypeHolder()->getType();
 				}
+				continue 2;
 			}
 		}
 
@@ -3762,7 +3759,7 @@ public function filterBySpecifiedTypes(SpecifiedTypes $specifiedTypes): self
 			$scope->getNamespace(),
 			$scope->expressionTypes,
 			$scope->nativeExpressionTypes,
-			$newConditionalExpressions,
+			array_merge($specifiedTypes->getNewConditionalExpressionHolders(), $scope->conditionalExpressions),
 			$scope->inClosureBindScopeClass,
 			$scope->anonymousFunctionReflection,
 			$scope->inFirstLevelStatement,
diff --git a/tests/PHPStan/Rules/SlowdownRuleTest.php b/tests/PHPStan/Rules/SlowdownRuleTest.php
new file mode 100644
index 0000000000..892468c048
--- /dev/null
+++ b/tests/PHPStan/Rules/SlowdownRuleTest.php
@@ -0,0 +1,43 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules;
+
+use PhpParser\Node;
+use PHPStan\Analyser\Scope;
+use PHPStan\Testing\RuleTestCase;
+
+/**
+ * @extends RuleTestCase<Rule>
+ */
+class SlowdownRuleTest extends RuleTestCase
+{
+
+	/**
+	 * @return Rule<Node>
+	 */
+	protected function getRule(): Rule
+	{
+		return new class implements Rule {
+
+			public function getNodeType(): string
+			{
+				return Node::class;
+			}
+
+			/**
+			 * @return string[]
+			 */
+			public function processNode(Node $node, Scope $scope): array
+			{
+				return [];
+			}
+
+		};
+	}
+
+	public function testRule(): void
+	{
+		$this->analyse([__DIR__ . '/data/1.9.x-slowdown.php'], []);
+	}
+
+}
diff --git a/tests/PHPStan/Rules/Variables/DefinedVariableRuleTest.php b/tests/PHPStan/Rules/Variables/DefinedVariableRuleTest.php
index 106320e217..d0687165fb 100644
--- a/tests/PHPStan/Rules/Variables/DefinedVariableRuleTest.php
+++ b/tests/PHPStan/Rules/Variables/DefinedVariableRuleTest.php
@@ -918,4 +918,13 @@ public function testBug4173(): void
 		]);
 	}
 
+	public function testBug5803(): void
+	{
+		$this->cliArgumentsVariablesRegistered = true;
+		$this->polluteScopeWithLoopInitialAssignments = false;
+		$this->checkMaybeUndefinedVariables = true;
+		$this->polluteScopeWithAlwaysIterableForeach = true;
+		$this->analyse([__DIR__ . '/data/bug-5803.php'], []);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Variables/data/bug-5803.php b/tests/PHPStan/Rules/Variables/data/bug-5803.php
new file mode 100644
index 0000000000..60d74283f8
--- /dev/null
+++ b/tests/PHPStan/Rules/Variables/data/bug-5803.php
@@ -0,0 +1,15 @@
+<?php declare(strict_types = 1);
+
+namespace Bug5803;
+
+function () {
+	if (!($_GET['foo'])) { // if 'foo' is falsy, SET $var
+		$var = "set";
+	}
+
+	if ($_GET['foo']) {
+
+	} else {
+		echo $var;
+	}
+};
diff --git a/tests/PHPStan/Rules/data/1.9.x-slowdown.php b/tests/PHPStan/Rules/data/1.9.x-slowdown.php
new file mode 100644
index 0000000000..8d3186719e
--- /dev/null
+++ b/tests/PHPStan/Rules/data/1.9.x-slowdown.php
@@ -0,0 +1,39 @@
+<?php declare(strict_types = 1);
+
+namespace Foo;
+
+use function trim;
+
+class Foo
+{
+
+	public const FIELD_TITLE = 'title';
+	public const FIELD_SOURCE = 'source';
+	public const FIELD_BODY = 'body';
+	public const EMPTY_NOTE_BODY = '-';
+
+	public const FIELD_NOTES = 'notes';
+	public const SUBFIELD_NOTE = 'note';
+
+	/**
+	 * @param array<mixed> $data
+	 * @return array<mixed>
+	 */
+	private function someMethod(array $data): array
+	{
+		foreach ($data[self::FIELD_NOTES][self::SUBFIELD_NOTE] ?? [] as $index => $noteData) {
+			$noteTitle = $noteData[self::FIELD_TITLE] ?? null;
+			$noteSource = $noteData[self::FIELD_SOURCE] ?? null;
+			$noteBody = $noteData[self::FIELD_BODY] ?? null;
+
+			if ($noteBody === null || trim($noteBody) === '') {
+				$data[self::FIELD_NOTES] = self::EMPTY_NOTE_BODY;
+			}
+		}
+
+		if (isset($data[self::FIELD_NOTES][self::SUBFIELD_NOTE])) {}
+
+		return $data;
+	}
+
+}