diff --git a/src/Analyser/MutatingScope.php b/src/Analyser/MutatingScope.php
index 5659a107bc..3993085894 100644
--- a/src/Analyser/MutatingScope.php
+++ b/src/Analyser/MutatingScope.php
@@ -4900,6 +4900,68 @@ public function processAlwaysIterableForeachScopeWithoutPollute(self $finalScope
 		);
 	}
 
+	public function processAlwaysIterableForScopeWithoutPollute(self $finalScope, self $initScope): self
+	{
+		$expressionTypes = $this->expressionTypes;
+		$initScopeExpressionTypes = $initScope->expressionTypes;
+		foreach ($finalScope->expressionTypes as $variableExprString => $variableTypeHolder) {
+			if (!isset($expressionTypes[$variableExprString])) {
+				if (isset($initScopeExpressionTypes[$variableExprString])) {
+					$expressionTypes[$variableExprString] = ExpressionTypeHolder::createMaybe($variableTypeHolder->getExpr(), $variableTypeHolder->getType());
+					continue;
+				}
+
+				$expressionTypes[$variableExprString] = $variableTypeHolder;
+				continue;
+			}
+
+			$expressionTypes[$variableExprString] = new ExpressionTypeHolder(
+				$variableTypeHolder->getExpr(),
+				$variableTypeHolder->getType(),
+				$variableTypeHolder->getCertainty()->and($expressionTypes[$variableExprString]->getCertainty()),
+			);
+		}
+
+		$nativeTypes = $this->nativeExpressionTypes;
+		$initScopeNativeExpressionTypes = $initScope->nativeExpressionTypes;
+		foreach ($finalScope->nativeExpressionTypes as $variableExprString => $variableTypeHolder) {
+			if (!isset($nativeTypes[$variableExprString])) {
+				if (isset($initScopeNativeExpressionTypes[$variableExprString])) {
+					$nativeTypes[$variableExprString] = ExpressionTypeHolder::createMaybe($variableTypeHolder->getExpr(), $variableTypeHolder->getType());
+					continue;
+				}
+
+				$nativeTypes[$variableExprString] = $variableTypeHolder;
+				continue;
+			}
+
+			$nativeTypes[$variableExprString] = new ExpressionTypeHolder(
+				$variableTypeHolder->getExpr(),
+				$variableTypeHolder->getType(),
+				$variableTypeHolder->getCertainty()->and($nativeTypes[$variableExprString]->getCertainty()),
+			);
+		}
+
+		return $this->scopeFactory->create(
+			$this->context,
+			$this->isDeclareStrictTypes(),
+			$this->getFunction(),
+			$this->getNamespace(),
+			$expressionTypes,
+			$nativeTypes,
+			$this->conditionalExpressions,
+			$this->inClosureBindScopeClasses,
+			$this->anonymousFunctionReflection,
+			$this->inFirstLevelStatement,
+			[],
+			[],
+			[],
+			$this->afterExtractCall,
+			$this->parentScope,
+			$this->nativeTypesPromoted,
+		);
+	}
+
 	public function generalizeWith(self $otherScope): self
 	{
 		$variableTypeHolders = $this->generalizeVariableTypeHolders(
diff --git a/src/Analyser/NodeScopeResolver.php b/src/Analyser/NodeScopeResolver.php
index 1a6a069d28..31dde8718b 100644
--- a/src/Analyser/NodeScopeResolver.php
+++ b/src/Analyser/NodeScopeResolver.php
@@ -1410,7 +1410,7 @@ private function processStmtNode(
 				}
 			} else {
 				if (!$this->polluteScopeWithLoopInitialAssignments) {
-					$finalScope = $finalScope->mergeWith($scope);
+					$finalScope = $scope->processAlwaysIterableForScopeWithoutPollute($finalScope, $initScope);
 				}
 			}
 
diff --git a/tests/PHPStan/Analyser/ForLoopNoScopePollutionTest.php b/tests/PHPStan/Analyser/ForLoopNoScopePollutionTest.php
new file mode 100644
index 0000000000..9b07f9ab2b
--- /dev/null
+++ b/tests/PHPStan/Analyser/ForLoopNoScopePollutionTest.php
@@ -0,0 +1,36 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Analyser;
+
+use PHPStan\Testing\TypeInferenceTestCase;
+
+class ForLoopNoScopePollutionTest extends TypeInferenceTestCase
+{
+
+	/** @return iterable<array<string, mixed[]>> */
+	public function dataFileAsserts(): iterable
+	{
+		yield from $this->gatherAssertTypes(__DIR__ . '/data/for-loop-no-scope-pollution.php');
+	}
+
+	/**
+	 * @dataProvider dataFileAsserts
+	 * @param mixed ...$args
+	 */
+	public function testFileAsserts(
+		string $assertType,
+		string $file,
+		...$args,
+	): void
+	{
+		$this->assertFileAsserts($assertType, $file, ...$args);
+	}
+
+	public static function getAdditionalConfigFiles(): array
+	{
+		return [
+			__DIR__ . '/forLoopNoScopePollution.neon',
+		];
+	}
+
+}
diff --git a/tests/PHPStan/Analyser/data/for-loop-no-scope-pollution.php b/tests/PHPStan/Analyser/data/for-loop-no-scope-pollution.php
new file mode 100644
index 0000000000..e7c0bef9ff
--- /dev/null
+++ b/tests/PHPStan/Analyser/data/for-loop-no-scope-pollution.php
@@ -0,0 +1,107 @@
+<?php declare(strict_types=1);
+
+namespace ForLoopNoScopePollution;
+
+
+use PHPStan\TrinaryLogic;
+use function PHPStan\Testing\assertNativeType;
+use function PHPStan\Testing\assertType;
+use function PHPStan\Testing\assertVariableCertainty;
+
+class ForLoop
+{
+
+	/** @param int $b */
+	public function loopThatIteratesAtLeastOnce(int $a, $b): void
+	{
+		$j = 0;
+		for ($i = 0, $j = 10; $i < 10; $i++, $j--) {
+			$a = rand(0, 1);
+			$b = rand(0, 1);
+			$c = rand(0, 1);
+		}
+
+		assertType('int<10, max>', $i);
+		assertNativeType('int<10, max>', $i);
+		assertVariableCertainty(TrinaryLogic::createMaybe(), $i);
+
+		assertType('int<min, 10>', $j);
+		assertNativeType('int<min, 10>', $j);
+		assertVariableCertainty(TrinaryLogic::createYes(), $j);
+
+		assertType('int<0, 1>', $a);
+		assertNativeType('int', $a);
+		assertVariableCertainty(TrinaryLogic::createYes(), $a);
+
+		assertType('int<0, 1>', $b);
+		assertNativeType('int', $b);
+		assertVariableCertainty(TrinaryLogic::createYes(), $b);
+
+		assertType('int<0, 1>', $c);
+		assertNativeType('int', $c);
+		assertVariableCertainty(TrinaryLogic::createYes(), $c);
+	}
+
+	/** @param int $b */
+	public function loopThatMightIterateAtLeastOnce(int $a, $b): void
+	{
+		$j = 0;
+		for ($i = 0, $j = 10; $i < rand(0, 1); $i++, $j--) {
+			$a = rand(0, 1);
+			$b = rand(0, 1);
+			$c = rand(0, 1);
+		}
+
+		assertType('int<0, max>', $i);
+		assertNativeType('int<0, max>', $i);
+		assertVariableCertainty(TrinaryLogic::createMaybe(), $i);
+
+		assertType('int<min, 10>', $j);
+		assertNativeType('int<min, 10>', $j);
+		assertVariableCertainty(TrinaryLogic::createYes(), $j);
+
+		assertType('int', $a);
+		assertNativeType('int', $a);
+		assertVariableCertainty(TrinaryLogic::createYes(), $a);
+
+		assertType('int', $b);
+		assertNativeType('mixed', $b);
+		assertVariableCertainty(TrinaryLogic::createYes(), $b);
+
+		assertType('int<0, 1>', $c);
+		assertNativeType('int', $c);
+		assertVariableCertainty(TrinaryLogic::createMaybe(), $c);
+	}
+
+	/** @param int $b */
+	public function loopThatNeverIterates(int $a, $b): void
+	{
+		$j = 0;
+		for ($i = 0, $j = 10; $i > 10; $i++, $j--) {
+			$a = rand(0, 1);
+			$b = rand(0, 1);
+			$c = rand(0, 1);
+		}
+
+		assertType('*ERROR*', $i);
+		assertNativeType('*ERROR*', $i);
+		assertVariableCertainty(TrinaryLogic::createNo(), $i);
+
+		assertType('0', $j);
+		assertNativeType('0', $j);
+		assertVariableCertainty(TrinaryLogic::createYes(), $j);
+
+		assertType('int', $a);
+		assertNativeType('int', $a);
+		assertVariableCertainty(TrinaryLogic::createYes(), $a);
+
+		assertType('int', $b);
+		assertNativeType('mixed', $b);
+		assertVariableCertainty(TrinaryLogic::createYes(), $b);
+
+		assertType('*ERROR*', $c);
+		assertNativeType('*ERROR*', $c);
+		assertVariableCertainty(TrinaryLogic::createNo(), $c);
+	}
+
+}
diff --git a/tests/PHPStan/Analyser/forLoopNoScopePollution.neon b/tests/PHPStan/Analyser/forLoopNoScopePollution.neon
new file mode 100644
index 0000000000..47811f500e
--- /dev/null
+++ b/tests/PHPStan/Analyser/forLoopNoScopePollution.neon
@@ -0,0 +1,2 @@
+parameters:
+	polluteScopeWithLoopInitialAssignments: false
diff --git a/tests/PHPStan/Analyser/nsrt/for-loop.php b/tests/PHPStan/Analyser/nsrt/for-loop.php
new file mode 100644
index 0000000000..f15cdeb811
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/for-loop.php
@@ -0,0 +1,107 @@
+<?php declare(strict_types=1);
+
+namespace ForLoop;
+
+
+use PHPStan\TrinaryLogic;
+use function PHPStan\Testing\assertNativeType;
+use function PHPStan\Testing\assertType;
+use function PHPStan\Testing\assertVariableCertainty;
+
+class ForLoop
+{
+
+	/** @param int $b */
+	public function loopThatIteratesAtLeastOnce(int $a, $b): void
+	{
+		$j = 0;
+		for ($i = 0, $j = 10; $i < 10; $i++, $j--) {
+			$a = rand(0, 1);
+			$b = rand(0, 1);
+			$c = rand(0, 1);
+		}
+
+		assertType('int<10, max>', $i);
+		assertNativeType('int<10, max>', $i);
+		assertVariableCertainty(TrinaryLogic::createYes(), $i);
+
+		assertType('int<min, 10>', $j);
+		assertNativeType('int<min, 10>', $j);
+		assertVariableCertainty(TrinaryLogic::createYes(), $j);
+
+		assertType('int<0, 1>', $a);
+		assertNativeType('int', $a);
+		assertVariableCertainty(TrinaryLogic::createYes(), $a);
+
+		assertType('int<0, 1>', $b);
+		assertNativeType('int', $b);
+		assertVariableCertainty(TrinaryLogic::createYes(), $b);
+
+		assertType('int<0, 1>', $c);
+		assertNativeType('int', $c);
+		assertVariableCertainty(TrinaryLogic::createYes(), $c);
+	}
+
+	/** @param int $b */
+	public function loopThatMightIterateAtLeastOnce(int $a, $b): void
+	{
+		$j = 0;
+		for ($i = 0, $j = 10; $i < rand(0, 1); $i++, $j--) {
+			$a = rand(0, 1);
+			$b = rand(0, 1);
+			$c = rand(0, 1);
+		}
+
+		assertType('int<0, max>', $i);
+		assertNativeType('int<0, max>', $i);
+		assertVariableCertainty(TrinaryLogic::createYes(), $i);
+
+		assertType('int<min, 10>', $j);
+		assertNativeType('int<min, 10>', $j);
+		assertVariableCertainty(TrinaryLogic::createYes(), $j);
+
+		assertType('int', $a);
+		assertNativeType('int', $a);
+		assertVariableCertainty(TrinaryLogic::createYes(), $a);
+
+		assertType('int', $b);
+		assertNativeType('mixed', $b);
+		assertVariableCertainty(TrinaryLogic::createYes(), $b);
+
+		assertType('int<0, 1>', $c);
+		assertNativeType('int', $c);
+		assertVariableCertainty(TrinaryLogic::createMaybe(), $c);
+	}
+
+	/** @param int $b */
+	public function loopThatNeverIterates(int $a, $b): void
+	{
+		$j = 0;
+		for ($i = 0, $j = 10; $i > 10; $i++, $j--) {
+			$a = rand(0, 1);
+			$b = rand(0, 1);
+			$c = rand(0, 1);
+		}
+
+		assertType('0', $i);
+		assertNativeType('0', $i);
+		assertVariableCertainty(TrinaryLogic::createYes(), $i);
+
+		assertType('10', $j);
+		assertNativeType('10', $j);
+		assertVariableCertainty(TrinaryLogic::createYes(), $j);
+
+		assertType('int', $a);
+		assertNativeType('int', $a);
+		assertVariableCertainty(TrinaryLogic::createYes(), $a);
+
+		assertType('int', $b);
+		assertNativeType('mixed', $b);
+		assertVariableCertainty(TrinaryLogic::createYes(), $b);
+
+		assertType('*ERROR*', $c);
+		assertNativeType('*ERROR*', $c);
+		assertVariableCertainty(TrinaryLogic::createNo(), $c);
+	}
+
+}
diff --git a/tests/PHPStan/Rules/Variables/DefinedVariableRuleTest.php b/tests/PHPStan/Rules/Variables/DefinedVariableRuleTest.php
index 94f0b0ffeb..9a2346eb77 100644
--- a/tests/PHPStan/Rules/Variables/DefinedVariableRuleTest.php
+++ b/tests/PHPStan/Rules/Variables/DefinedVariableRuleTest.php
@@ -1068,4 +1068,13 @@ public function testBug10228(): void
 		$this->analyse([__DIR__ . '/data/bug-10228.php'], []);
 	}
 
+	public function testBug9550(): void
+	{
+		$this->cliArgumentsVariablesRegistered = true;
+		$this->polluteScopeWithLoopInitialAssignments = false;
+		$this->checkMaybeUndefinedVariables = true;
+		$this->polluteScopeWithAlwaysIterableForeach = true;
+		$this->analyse([__DIR__ . '/data/bug-9550.php'], []);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Variables/data/bug-9550.php b/tests/PHPStan/Rules/Variables/data/bug-9550.php
new file mode 100644
index 0000000000..ab260ad44c
--- /dev/null
+++ b/tests/PHPStan/Rules/Variables/data/bug-9550.php
@@ -0,0 +1,32 @@
+<?php declare(strict_types = 1);
+
+namespace Bug9550;
+
+class Foo
+{
+	protected static function makeShortTextFromScalar(string $value, int $maxUtf8Length = 50): string
+	{
+		$maxUtf8Length = max(20, min($maxUtf8Length, 100));
+
+		$vStrLonger = mb_substr($value, 0, $maxUtf8Length + 1000);
+
+		$withThreeDots = false;
+		\PHPStan\dumpType($maxUtf8Length);
+		for ($l = $maxUtf8Length; $l > 0; --$l) {
+			$vStr = mb_substr($vStrLonger, 0, $l);
+			if ($vStr !== $vStrLonger) {
+				$vStrLonger = $vStr;
+				$vStr = mb_substr($vStr, 0, $l - 3);
+				$withThreeDots = true;
+			} else {
+				$vStrLonger = $vStr;
+			}
+			$vStr = str_replace(["\0", "\t", "\n", "\r"], ['\0', '\t', '\n', '\r'], $vStr);
+			if (mb_strlen($vStr) <= $maxUtf8Length - ($withThreeDots ? 3 : 0)) {
+				break;
+			}
+		}
+
+		return '\'' . $vStr . '\'' . ($withThreeDots ? '...' : '');
+	}
+}