Improve for loop initial statement scope pollution

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(

src/Analyser/NodeScopeResolver.php

@@ -1410,7 +1410,7 @@ private function processStmtNode(
 				}
 			} else {
 				if (!$this->polluteScopeWithLoopInitialAssignments) {
-					$finalScope = $finalScope->mergeWith($scope);
+					$finalScope = $scope->processAlwaysIterableForScopeWithoutPollute($finalScope, $initScope);
 				}
 			}
 

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'], []);
+	}
+
 }

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 ? '...' : '');
+	}
+}