diff --git a/phpstan-baseline.neon b/phpstan-baseline.neon
index d7174f5db4..7325f424b4 100644
--- a/phpstan-baseline.neon
+++ b/phpstan-baseline.neon
@@ -1587,12 +1587,6 @@ parameters:
 			count: 1
 			path: src/Type/Php/FilterFunctionReturnTypeHelper.php
 
-		-
-			rawMessage: 'Doing instanceof PHPStan\Type\Constant\ConstantStringType is error-prone and deprecated. Use Type::getConstantStrings() instead.'
-			identifier: phpstanApi.instanceofType
-			count: 1
-			path: src/Type/Php/FunctionExistsFunctionTypeSpecifyingExtension.php
-
 		-
 			rawMessage: 'Doing instanceof PHPStan\Type\Constant\ConstantArrayType is error-prone and deprecated. Use Type::getConstantArrays() instead.'
 			identifier: phpstanApi.instanceofType
diff --git a/src/Rules/Comparison/ImpossibleCheckTypeHelper.php b/src/Rules/Comparison/ImpossibleCheckTypeHelper.php
index 69d934aba8..0a1621c842 100644
--- a/src/Rules/Comparison/ImpossibleCheckTypeHelper.php
+++ b/src/Rules/Comparison/ImpossibleCheckTypeHelper.php
@@ -81,6 +81,7 @@ public function findSpecifiedType(
 					'interface_exists',
 					'trait_exists',
 					'enum_exists',
+					'function_exists',
 				], true)) {
 					return null;
 				}
diff --git a/src/Type/Php/FunctionExistsFunctionTypeSpecifyingExtension.php b/src/Type/Php/FunctionExistsFunctionTypeSpecifyingExtension.php
index 330735d9e4..ccab9e96ae 100644
--- a/src/Type/Php/FunctionExistsFunctionTypeSpecifyingExtension.php
+++ b/src/Type/Php/FunctionExistsFunctionTypeSpecifyingExtension.php
@@ -15,8 +15,8 @@
 use PHPStan\Reflection\FunctionReflection;
 use PHPStan\Type\CallableType;
 use PHPStan\Type\Constant\ConstantBooleanType;
-use PHPStan\Type\Constant\ConstantStringType;
 use PHPStan\Type\FunctionTypeSpecifyingExtension;
+use function count;
 use function ltrim;
 
 #[AutowiredService]
@@ -37,15 +37,23 @@ public function isFunctionSupported(
 	public function specifyTypes(FunctionReflection $functionReflection, FuncCall $node, Scope $scope, TypeSpecifierContext $context): SpecifiedTypes
 	{
 		$argType = $scope->getType($node->getArgs()[0]->value);
-		if ($argType instanceof ConstantStringType) {
-			return $this->typeSpecifier->create(
-				new FuncCall(new FullyQualified('function_exists'), [
-					new Arg(new String_(ltrim($argType->getValue(), '\\'))),
-				]),
-				new ConstantBooleanType(true),
-				$context,
-				$scope,
-			);
+
+		$constantStrings = $argType->getConstantStrings();
+		if (count($constantStrings) === 1) {
+			$specifiedTypes = new SpecifiedTypes();
+
+			foreach ($constantStrings as $constantString) {
+				$specifiedTypes = $specifiedTypes->unionWith($this->typeSpecifier->create(
+					new FuncCall(new FullyQualified('function_exists'), [
+						new Arg(new String_(ltrim($constantString->getValue(), '\\'))),
+					]),
+					new ConstantBooleanType(true),
+					$context,
+					$scope,
+				));
+			}
+
+			return $specifiedTypes;
 		}
 
 		return $this->typeSpecifier->create(
diff --git a/tests/PHPStan/Rules/Comparison/IfConstantConditionRuleTest.php b/tests/PHPStan/Rules/Comparison/IfConstantConditionRuleTest.php
index 5a847ad029..197afcbd3f 100644
--- a/tests/PHPStan/Rules/Comparison/IfConstantConditionRuleTest.php
+++ b/tests/PHPStan/Rules/Comparison/IfConstantConditionRuleTest.php
@@ -261,4 +261,15 @@ public function testBug5020(): void
 		$this->analyse([__DIR__ . '/data/bug-5020.php'], []);
 	}
 
+	public function testBug8980(): void
+	{
+		$this->treatPhpDocTypesAsCertain = true;
+		$this->analyse([__DIR__ . '/data/bug-8980.php'], [
+			[
+				'If condition is always true.',
+				23,
+			],
+		]);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php b/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php
index ce1094a5b2..00306e738b 100644
--- a/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php
+++ b/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php
@@ -1236,6 +1236,12 @@ public function testBug8217(): void
 		$this->analyse([__DIR__ . '/data/bug-8217.php'], []);
 	}
 
+	public function testBug8980(): void
+	{
+		$this->treatPhpDocTypesAsCertain = true;
+		$this->analyse([__DIR__ . '/data/bug-8980.php'], []);
+	}
+
 	public function testBug6211(): void
 	{
 		$this->treatPhpDocTypesAsCertain = true;
diff --git a/tests/PHPStan/Rules/Comparison/data/bug-8980.php b/tests/PHPStan/Rules/Comparison/data/bug-8980.php
new file mode 100644
index 0000000000..fb5709ab11
--- /dev/null
+++ b/tests/PHPStan/Rules/Comparison/data/bug-8980.php
@@ -0,0 +1,26 @@
+<?php declare(strict_types = 1);
+
+namespace Bug8980;
+
+use function function_exists;
+
+// actual bug report snippet: function_exists inside array_filter callback
+$undefined_curl_functions = array_filter(
+	[
+		'curl_multi_add_handle',
+		'curl_multi_exec',
+		'curl_multi_init',
+	],
+	static function( $function_name ) {
+		return ! function_exists( $function_name );
+	}
+);
+
+function testFunctionExistsGuardReturn(): void {
+	if (!function_exists('curl_init')) {
+		return;
+	}
+	if (function_exists('curl_init')) {
+		echo 'exists';
+	}
+}
diff --git a/tests/PHPStan/Rules/Functions/CallToNonExistentFunctionRuleTest.php b/tests/PHPStan/Rules/Functions/CallToNonExistentFunctionRuleTest.php
index 703eaba247..54081d235c 100644
--- a/tests/PHPStan/Rules/Functions/CallToNonExistentFunctionRuleTest.php
+++ b/tests/PHPStan/Rules/Functions/CallToNonExistentFunctionRuleTest.php
@@ -264,4 +264,20 @@ public function testBug14384(): void
 		$this->analyse([__DIR__ . '/data/bug-14384.php'], []);
 	}
 
+	public function testBug8980(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-8980.php'], [
+			[
+				'Function funcA not found.',
+				13,
+				'Learn more at https://phpstan.org/user-guide/discovering-symbols',
+			],
+			[
+				'Function funcB not found.',
+				14,
+				'Learn more at https://phpstan.org/user-guide/discovering-symbols',
+			],
+		]);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Functions/data/bug-8980.php b/tests/PHPStan/Rules/Functions/data/bug-8980.php
new file mode 100644
index 0000000000..5cdcfbb85c
--- /dev/null
+++ b/tests/PHPStan/Rules/Functions/data/bug-8980.php
@@ -0,0 +1,16 @@
+<?php declare(strict_types = 1);
+
+namespace Bug8980;
+
+function doFoo():void {
+	$func = rand(0,1) ? 'funcA' : 'funcB';
+
+	if (!function_exists($func)) {
+		throw new \Exception();
+	}
+
+	// the function_exists() will only assure one of the functions to exist.
+	funcA();
+	funcB();
+	$func();
+}