diff --git a/src/Analyser/NodeScopeResolver.php b/src/Analyser/NodeScopeResolver.php
index 696104b2014..6e1ef776098 100644
--- a/src/Analyser/NodeScopeResolver.php
+++ b/src/Analyser/NodeScopeResolver.php
@@ -186,6 +186,7 @@
 use function is_array;
 use function is_int;
 use function is_string;
+use function max;
 use function sprintf;
 use function strtolower;
 use function trim;
@@ -3301,33 +3302,34 @@ private function doCreateCallableParameters(Scope $scope, Expr $closureExpr, ?ar
 
 			$acceptors = $passedToType->getCallableParametersAcceptors($scope);
 			foreach ($acceptors as $acceptor) {
+				$acceptorParameters = array_map(static fn (ParameterReflection $callableParameter) => new NativeParameterReflection(
+					$callableParameter->getName(),
+					$callableParameter->isOptional(),
+					$callableParameter->getType(),
+					$callableParameter->passedByReference(),
+					$callableParameter->isVariadic(),
+					$callableParameter->getDefaultValue(),
+				), $acceptor->getParameters());
+
 				if ($callableParameters === null) {
-					$callableParameters = array_map(static fn (ParameterReflection $callableParameter) => new NativeParameterReflection(
-						$callableParameter->getName(),
-						$callableParameter->isOptional(),
-						$callableParameter->getType(),
-						$callableParameter->passedByReference(),
-						$callableParameter->isVariadic(),
-						$callableParameter->getDefaultValue(),
-					), $acceptor->getParameters());
+					$callableParameters = $acceptorParameters;
 					continue;
 				}
 
 				$newParameters = [];
-				foreach ($acceptor->getParameters() as $i => $callableParameter) {
+				$parameterCount = max(count($callableParameters), count($acceptorParameters));
+				for ($i = 0; $i < $parameterCount; $i++) {
+					if (!array_key_exists($i, $acceptorParameters)) {
+						$newParameters[] = $callableParameters[$i]->toOptional();
+						continue;
+					}
+
 					if (!array_key_exists($i, $callableParameters)) {
-						$newParameters[] = $callableParameter;
+						$newParameters[] = $acceptorParameters[$i]->toOptional();
 						continue;
 					}
 
-					$newParameters[] = $callableParameters[$i]->union(new NativeParameterReflection(
-						$callableParameter->getName(),
-						$callableParameter->isOptional(),
-						$callableParameter->getType(),
-						$callableParameter->passedByReference(),
-						$callableParameter->isVariadic(),
-						$callableParameter->getDefaultValue(),
-					));
+					$newParameters[] = $callableParameters[$i]->union($acceptorParameters[$i]);
 				}
 
 				$callableParameters = $newParameters;
diff --git a/src/Reflection/Native/NativeParameterReflection.php b/src/Reflection/Native/NativeParameterReflection.php
index e8120868302..19da0a741a0 100644
--- a/src/Reflection/Native/NativeParameterReflection.php
+++ b/src/Reflection/Native/NativeParameterReflection.php
@@ -51,6 +51,23 @@ public function getDefaultValue(): ?Type
 		return $this->defaultValue;
 	}
 
+	/** Used when merging signatures where only some of them declare this parameter. */
+	public function toOptional(): self
+	{
+		if ($this->optional) {
+			return $this;
+		}
+
+		return new self(
+			$this->name,
+			true,
+			$this->type,
+			$this->passedByReference,
+			$this->variadic,
+			$this->defaultValue,
+		);
+	}
+
 	public function union(self $other): self
 	{
 		return new self(
diff --git a/tests/PHPStan/Analyser/AnalyserIntegrationTest.php b/tests/PHPStan/Analyser/AnalyserIntegrationTest.php
index 06788c89415..18218040b39 100644
--- a/tests/PHPStan/Analyser/AnalyserIntegrationTest.php
+++ b/tests/PHPStan/Analyser/AnalyserIntegrationTest.php
@@ -1608,6 +1608,14 @@ public function testBug14707(): void
 		$this->assertNoErrors($errors);
 	}
 
+	#[RequiresPhp('>= 8.0.0')]
+	public function testBug15003(): void
+	{
+		// crash
+		$errors = $this->runAnalyse(__DIR__ . '/data/bug-15003.php');
+		$this->assertNoErrors($errors);
+	}
+
 	/**
 	 * @param string[]|null $allAnalysedFiles
 	 * @return list<Error>
diff --git a/tests/PHPStan/Analyser/data/bug-15003.php b/tests/PHPStan/Analyser/data/bug-15003.php
new file mode 100644
index 00000000000..fcb93192b21
--- /dev/null
+++ b/tests/PHPStan/Analyser/data/bug-15003.php
@@ -0,0 +1,82 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug15003;
+
+use Closure;
+
+/**
+ * @phpstan-type Foo InvokableClass|callable(string, mixed): int
+ */
+class TypeImportShortcut {}
+
+interface InvokableClass
+{
+	/**
+	 * @param  Closure(string, ?string=): string  $fail
+	 */
+	public function __invoke(string $foo, Closure $fail): int;
+}
+
+/** @phpstan-import-type Foo from TypeImportShortcut */
+class A
+{
+
+	/** @param callable(string):Foo|Foo $param */
+	public function foo($param): void {}
+
+}
+
+(new A)->foo(function(string $foo) {
+	return 5;
+});
+
+interface InvokableRule
+{
+
+	/**
+	 * @param Closure(string): string $fail
+	 */
+	public function __invoke(string $attribute, mixed $value, Closure $fail): void;
+
+}
+
+/**
+ * @phpstan-type FieldValidationRule InvokableRule|(callable(string, mixed, Closure): void)
+ * @phpstan-type ValidationRules array<int, FieldValidationRule>|FieldValidationRule
+ */
+final class Field
+{
+
+	/**
+	 * @param (callable(string): ValidationRules)|ValidationRules $rules
+	 */
+	public function rules($rules): self
+	{
+		return $this;
+	}
+
+	/**
+	 * @param (callable(string): ValidationRules)|ValidationRules ...$rules
+	 */
+	public function creationRules($rules): self
+	{
+		return $this;
+	}
+
+}
+
+function rules(): Field
+{
+	return (new Field())->rules(function ($attribute, $value, $fail) {
+	});
+}
+
+function creationRules(): Field
+{
+	return (new Field())->creationRules([
+		function ($attribute, $value, $fail) {
+		},
+	]);
+}
diff --git a/tests/PHPStan/Analyser/nsrt/closure-passed-to-union-of-callables.php b/tests/PHPStan/Analyser/nsrt/closure-passed-to-union-of-callables.php
new file mode 100644
index 00000000000..1cd77896227
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/closure-passed-to-union-of-callables.php
@@ -0,0 +1,89 @@
+<?php declare(strict_types = 1);
+
+namespace ClosurePassedToUnionOfCallables;
+
+use function PHPStan\Testing\assertType;
+
+/** @template T */
+final class Invokable
+{
+
+	/** @param T $a */
+	public function __invoke($a, string $b): void
+	{
+	}
+
+}
+
+final class Foo
+{
+
+	/**
+	 * @param callable(int, string): void|callable(int): void $cb
+	 */
+	public function longestFirst($cb): void
+	{
+	}
+
+	/**
+	 * @param callable(int): void|callable(int, string): void $cb
+	 */
+	public function shortestFirst($cb): void
+	{
+	}
+
+	/**
+	 * @param callable(int): void|Invokable<string>|callable(bool, float): void $cb
+	 */
+	public function withInvokable($cb): void
+	{
+	}
+
+	/**
+	 * @param callable(int...): void|callable(string, string): void $cb
+	 */
+	public function variadicFirst($cb): void
+	{
+	}
+
+	/**
+	 * @param callable(string, string): void|callable(int...): void $cb
+	 */
+	public function variadicLast($cb): void
+	{
+	}
+
+	public function run(): void
+	{
+		$this->longestFirst(function ($a, $b): void {
+			assertType('int', $a);
+			assertType('string', $b);
+		});
+
+		$this->shortestFirst(function ($a, $b): void {
+			assertType('int', $a);
+			assertType('string', $b);
+		});
+
+		$this->longestFirst(fn ($a, $b) => assertType('int', $a));
+		$this->longestFirst(fn ($a, $b) => assertType('string', $b));
+		$this->shortestFirst(fn ($a, $b) => assertType('int', $a));
+		$this->shortestFirst(fn ($a, $b) => assertType('string', $b));
+
+		$this->withInvokable(function ($a, $b): void {
+			assertType('bool|int|string', $a);
+			assertType('float|string', $b);
+		});
+
+		$this->variadicFirst(function ($a, $b): void {
+			assertType('int|string', $a);
+			assertType('string', $b);
+		});
+
+		$this->variadicLast(function ($a, $b): void {
+			assertType('int|string', $a);
+			assertType('string', $b);
+		});
+	}
+
+}