diff --git a/src/Rules/Comparison/ImpossibleCheckTypeHelper.php b/src/Rules/Comparison/ImpossibleCheckTypeHelper.php
index 15e69dc664..08d4d8ec77 100644
--- a/src/Rules/Comparison/ImpossibleCheckTypeHelper.php
+++ b/src/Rules/Comparison/ImpossibleCheckTypeHelper.php
@@ -158,6 +158,12 @@ public function findSpecifiedType(
 		}
 
 		$specifiedTypes = $this->typeSpecifier->specifyTypesInCondition($scope, $node, TypeSpecifierContext::createTruthy());
+
+		// don't validate types on overwrite
+		if ($specifiedTypes->shouldOverwrite()) {
+			return null;
+		}
+
 		$sureTypes = $specifiedTypes->getSureTypes();
 		$sureNotTypes = $specifiedTypes->getSureNotTypes();
 
diff --git a/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeGenericOverwriteRuleTest.php b/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeGenericOverwriteRuleTest.php
new file mode 100644
index 0000000000..7d12e3f769
--- /dev/null
+++ b/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeGenericOverwriteRuleTest.php
@@ -0,0 +1,40 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Comparison;
+
+use PHPStan\Rules\Rule;
+use PHPStan\Testing\RuleTestCase;
+
+/**
+ * @extends RuleTestCase<ImpossibleCheckTypeMethodCallRule>
+ */
+class ImpossibleCheckTypeGenericOverwriteRuleTest extends RuleTestCase
+{
+
+	public function getRule(): Rule
+	{
+		return new ImpossibleCheckTypeMethodCallRule(
+			new ImpossibleCheckTypeHelper(
+				$this->createReflectionProvider(),
+				$this->getTypeSpecifier(),
+				[],
+				true,
+			),
+			true,
+			true,
+		);
+	}
+
+	public function testNoReportedErrorOnOverwrite(): void
+	{
+		$this->analyse([__DIR__ . '/data/generic-type-override.php'], []);
+	}
+
+	public static function getAdditionalConfigFiles(): array
+	{
+		return [
+			__DIR__ . '/impossible-check-type-generic-overwrite.neon',
+		];
+	}
+
+}
diff --git a/tests/PHPStan/Rules/Comparison/data/TestTypeOverwriteSpecifyingExtensions.php b/tests/PHPStan/Rules/Comparison/data/TestTypeOverwriteSpecifyingExtensions.php
new file mode 100644
index 0000000000..4e2acbd198
--- /dev/null
+++ b/tests/PHPStan/Rules/Comparison/data/TestTypeOverwriteSpecifyingExtensions.php
@@ -0,0 +1,58 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Comparison;
+
+use PhpParser\Node\Expr\MethodCall;
+use PHPStan\Analyser\Scope;
+use PHPStan\Analyser\SpecifiedTypes;
+use PHPStan\Analyser\TypeSpecifier;
+use PHPStan\Analyser\TypeSpecifierAwareExtension;
+use PHPStan\Analyser\TypeSpecifierContext;
+use PHPStan\Reflection\MethodReflection;
+use PHPStan\Type\MethodTypeSpecifyingExtension;
+use PHPStan\Type\ObjectType;
+use PHPStan\Type\Generic\GenericObjectType;
+
+class GenericTypeOverride implements MethodTypeSpecifyingExtension, TypeSpecifierAwareExtension
+{
+
+	/** @var TypeSpecifier */
+	private $typeSpecifier;
+
+	public function setTypeSpecifier(TypeSpecifier $typeSpecifier): void
+	{
+		$this->typeSpecifier = $typeSpecifier;
+	}
+
+	public function getClass(): string
+	{
+		return \GenericTypeOverride\Foo::class;
+	}
+
+	public function isMethodSupported(
+		MethodReflection $methodReflection,
+		MethodCall $node,
+		TypeSpecifierContext $context
+	): bool
+	{
+		return $methodReflection->getName() === 'setFetchMode';
+	}
+
+	public function specifyTypes(
+		MethodReflection $methodReflection,
+		MethodCall $node,
+		Scope $scope,
+		TypeSpecifierContext $context
+	): SpecifiedTypes
+	{
+		$newType = new GenericObjectType(\GenericTypeOverride\Foo::class, [new ObjectType(\GenericTypeOverride\Bar::class)]);
+
+		return $this->typeSpecifier->create(
+			$node->var,
+			$newType,
+			TypeSpecifierContext::createTruthy(),
+			true
+		);
+	}
+
+}
diff --git a/tests/PHPStan/Rules/Comparison/data/generic-type-override.php b/tests/PHPStan/Rules/Comparison/data/generic-type-override.php
new file mode 100644
index 0000000000..e5dad0ece2
--- /dev/null
+++ b/tests/PHPStan/Rules/Comparison/data/generic-type-override.php
@@ -0,0 +1,39 @@
+<?php
+
+namespace GenericTypeOverride;
+
+use function PHPStan\Testing\assertType;
+
+class Test {
+	public function doFoo() {
+		$foo = $this->createGenericFoo();
+		assertType('Foo<int>', $foo);
+
+		// $foo generic will be overridden via MethodTypeSpecifyingExtension
+		$foo->setFetchMode();
+		assertType('Foo<Bar>', $foo);
+	}
+
+	/**
+	 * @return Foo<int>
+	 */
+	public function createGenericFoo() {
+
+	}
+}
+
+
+/**
+ * @template T
+ */
+class Foo
+{
+	public function setFetchMode() {
+
+	}
+}
+
+
+class Bar
+{
+}
diff --git a/tests/PHPStan/Rules/Comparison/impossible-check-type-generic-overwrite.neon b/tests/PHPStan/Rules/Comparison/impossible-check-type-generic-overwrite.neon
new file mode 100644
index 0000000000..beb99fbc14
--- /dev/null
+++ b/tests/PHPStan/Rules/Comparison/impossible-check-type-generic-overwrite.neon
@@ -0,0 +1,5 @@
+services:
+	-
+		class: PHPStan\Rules\Comparison\GenericTypeOverride
+		tags:
+			- phpstan.typeSpecifier.methodTypeSpecifyingExtension