diff --git a/src/Rules/Comparison/ImpossibleCheckTypeFunctionCallRule.php b/src/Rules/Comparison/ImpossibleCheckTypeFunctionCallRule.php
index 29b0801ece..9033aa3865 100644
--- a/src/Rules/Comparison/ImpossibleCheckTypeFunctionCallRule.php
+++ b/src/Rules/Comparison/ImpossibleCheckTypeFunctionCallRule.php
@@ -8,6 +8,7 @@
 use PHPStan\Rules\Rule;
 use PHPStan\Rules\RuleErrorBuilder;
 use function sprintf;
+use function strtolower;
 
 /**
  * @implements Rule<Node\Expr\FuncCall>
@@ -37,6 +38,9 @@ public function processNode(Node $node, Scope $scope): array
 		}
 
 		$functionName = (string) $node->name;
+		if (strtolower($functionName) === 'is_a') {
+			return [];
+		}
 		$isAlways = $this->impossibleCheckTypeHelper->findSpecifiedType($scope, $node);
 		if ($isAlways === null) {
 			return [];
diff --git a/src/Type/Php/IsAFunctionTypeSpecifyingExtension.php b/src/Type/Php/IsAFunctionTypeSpecifyingExtension.php
index 4d062efd56..c4000b9aff 100644
--- a/src/Type/Php/IsAFunctionTypeSpecifyingExtension.php
+++ b/src/Type/Php/IsAFunctionTypeSpecifyingExtension.php
@@ -9,12 +9,9 @@
 use PHPStan\Analyser\TypeSpecifierAwareExtension;
 use PHPStan\Analyser\TypeSpecifierContext;
 use PHPStan\Reflection\FunctionReflection;
-use PHPStan\Type\ClassStringType;
 use PHPStan\Type\Constant\ConstantBooleanType;
 use PHPStan\Type\Constant\ConstantStringType;
 use PHPStan\Type\FunctionTypeSpecifyingExtension;
-use PHPStan\Type\ObjectWithoutClassType;
-use PHPStan\Type\TypeCombinator;
 use function count;
 use function strtolower;
 
@@ -50,20 +47,9 @@ public function specifyTypes(FunctionReflection $functionReflection, FuncCall $n
 		$allowStringType = isset($node->getArgs()[2]) ? $scope->getType($node->getArgs()[2]->value) : new ConstantBooleanType(false);
 		$allowString = !$allowStringType->equals(new ConstantBooleanType(false));
 
-		$superType = $allowString
-			? TypeCombinator::union(new ObjectWithoutClassType(), new ClassStringType())
-			: new ObjectWithoutClassType();
-
-		$resultType = $this->isAFunctionTypeSpecifyingHelper->determineType($objectOrClassType, $classType, $allowString, true);
-
-		// prevent false-positives in IsAFunctionTypeSpecifyingHelper
-		if ($resultType->equals($superType) && $resultType->isSuperTypeOf($objectOrClassType)->yes()) {
-			return new SpecifiedTypes([], []);
-		}
-
 		return $this->typeSpecifier->create(
 			$node->getArgs()[0]->value,
-			$resultType,
+			$this->isAFunctionTypeSpecifyingHelper->determineType($objectOrClassType, $classType, $allowString, true),
 			$context,
 			false,
 			$scope,
diff --git a/src/Type/Php/IsSubclassOfFunctionTypeSpecifyingExtension.php b/src/Type/Php/IsSubclassOfFunctionTypeSpecifyingExtension.php
index 0ca6cf4e76..2d52ee99e1 100644
--- a/src/Type/Php/IsSubclassOfFunctionTypeSpecifyingExtension.php
+++ b/src/Type/Php/IsSubclassOfFunctionTypeSpecifyingExtension.php
@@ -9,12 +9,9 @@
 use PHPStan\Analyser\TypeSpecifierAwareExtension;
 use PHPStan\Analyser\TypeSpecifierContext;
 use PHPStan\Reflection\FunctionReflection;
-use PHPStan\Type\ClassStringType;
 use PHPStan\Type\Constant\ConstantBooleanType;
 use PHPStan\Type\FunctionTypeSpecifyingExtension;
 use PHPStan\Type\Generic\GenericClassStringType;
-use PHPStan\Type\ObjectWithoutClassType;
-use PHPStan\Type\TypeCombinator;
 use function count;
 use function strtolower;
 
@@ -51,20 +48,9 @@ public function specifyTypes(FunctionReflection $functionReflection, FuncCall $n
 			return new SpecifiedTypes([], []);
 		}
 
-		$superType = $allowString
-			? TypeCombinator::union(new ObjectWithoutClassType(), new ClassStringType())
-			: new ObjectWithoutClassType();
-
-		$resultType = $this->isAFunctionTypeSpecifyingHelper->determineType($objectOrClassType, $classType, $allowString, false);
-
-		// prevent false-positives in IsAFunctionTypeSpecifyingHelper
-		if ($resultType->equals($superType) && $resultType->isSuperTypeOf($objectOrClassType)->yes()) {
-			return new SpecifiedTypes([], []);
-		}
-
 		return $this->typeSpecifier->create(
 			$node->getArgs()[0]->value,
-			$resultType,
+			$this->isAFunctionTypeSpecifyingHelper->determineType($objectOrClassType, $classType, $allowString, false),
 			$context,
 			false,
 			$scope,
diff --git a/tests/PHPStan/Analyser/TypeSpecifierTest.php b/tests/PHPStan/Analyser/TypeSpecifierTest.php
index 4ad6a7cec4..999c7169a3 100644
--- a/tests/PHPStan/Analyser/TypeSpecifierTest.php
+++ b/tests/PHPStan/Analyser/TypeSpecifierTest.php
@@ -1133,7 +1133,9 @@ public function dataCondition(): iterable
 					new Arg(new Variable('stringOrNull')),
 					new Arg(new Expr\ConstFetch(new Name('false'))),
 				]),
-				[],
+				[
+					'$object' => 'object',
+				],
 				[],
 			],
 			[
diff --git a/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php b/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php
index b15cf528b0..16529f3a74 100644
--- a/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php
+++ b/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php
@@ -1102,11 +1102,4 @@ public function testAlwaysTruePregMatch(): void
 		$this->analyse([__DIR__ . '/data/always-true-preg-match.php'], []);
 	}
 
-	public function testBug3979(): void
-	{
-		$this->checkAlwaysTrueCheckTypeFunctionCall = true;
-		$this->treatPhpDocTypesAsCertain = true;
-		$this->analyse([__DIR__ . '/data/bug-3979.php'], []);
-	}
-
 }
diff --git a/tests/PHPStan/Rules/Comparison/data/bug-3979.php b/tests/PHPStan/Rules/Comparison/data/bug-3979.php
deleted file mode 100644
index f0f21220d1..0000000000
--- a/tests/PHPStan/Rules/Comparison/data/bug-3979.php
+++ /dev/null
@@ -1,130 +0,0 @@
-<?php
-
-namespace Bug3979;
-
-class A { }
-class B extends A { }
-class C { }
-
-/**
- * @param mixed $value
- * @param string $class_type
- */
-function check_class($value, $class_type): bool
-{
-	if (!is_string($value) || !class_exists($value) ||
-		($class_type && !is_subclass_of($value, $class_type)))
-		return false;
-	return true;
-}
-
-var_dump(check_class("B", "A")); // true
-var_dump(check_class("C", "A")); // false
-
-/**
- * @param class-string $value
- * @param string $class_type
- */
-function check_class2($value, $class_type): bool
-{
-	if (is_a($value, $class_type, true)) {
-		return true;
-	}
-	return false;
-}
-
-/**
- * @param class-string|object $value
- * @param string $class_type
- */
-function check_class3($value, $class_type): bool
-{
-	if (is_a($value, $class_type, true)) {
-		return true;
-	}
-	return false;
-}
-
-/**
- * @param class-string|object $value
- * @param string $class_type
- */
-function check_class4($value, $class_type): bool
-{
-	if (is_subclass_of($value, $class_type, true)) {
-		return true;
-	}
-	return false;
-}
-
-/**
- * @param object $value
- * @param string $class_type
- */
-function check_class5($value, $class_type): bool
-{
-	if (is_a($value, $class_type, true)) {
-		return true;
-	}
-	return false;
-}
-
-/**
- * @param object $value
- * @param string $class_type
- */
-function check_class6($value, $class_type): bool
-{
-	if (is_subclass_of($value, $class_type, true)) {
-		return true;
-	}
-	return false;
-}
-
-/**
- * @param object $value
- * @param class-string $class_type
- */
-function check_class7($value, $class_type): bool
-{
-	if (is_a($value, $class_type, true)) {
-		return true;
-	}
-	return false;
-}
-
-/**
- * @param object $value
- * @param class-string $class_type
- */
-function check_class8($value, $class_type): bool
-{
-	if (is_subclass_of($value, $class_type, true)) {
-		return true;
-	}
-	return false;
-}
-
-/**
- * @param class-string $value
- * @param class-string $class_type
- */
-function check_class9($value, $class_type): bool
-{
-	if (is_a($value, $class_type, true)) {
-		return true;
-	}
-	return false;
-}
-
-/**
- * @param class-string $value
- * @param class-string $class_type
- */
-function check_class10($value, $class_type): bool
-{
-	if (is_subclass_of($value, $class_type, true)) {
-		return true;
-	}
-	return false;
-}