Do not add null to a template type for a default parameter value null

src/Reflection/Php/PhpParameterReflection.php

@@ -6,6 +6,7 @@
 use PHPStan\Reflection\PassedByReference;
 use PHPStan\Type\ConstantTypeHelper;
 use PHPStan\Type\MixedType;
+use PHPStan\Type\NullType;
 use PHPStan\Type\Type;
 use PHPStan\Type\TypeCombinator;
 use PHPStan\Type\TypehintHelper;
@@ -43,7 +44,11 @@ public function getType(): Type
 			$phpDocType = $this->phpDocType;
 			if ($phpDocType !== null) {
 				try {
-					if ($this->reflection->isDefaultValueAvailable() && $this->reflection->getDefaultValue() === null) {
+					if (
+						$this->reflection->isDefaultValueAvailable()
+						&& $this->reflection->getDefaultValue() === null
+						&& (new NullType())->isSuperTypeOf($phpDocType)->no()
+					) {
 						$phpDocType = TypeCombinator::addNull($phpDocType);
 					}
 				} catch (Throwable) {

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -767,6 +767,7 @@ public function dataFileAsserts(): iterable
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-5668.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/generics-empty-array.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/../Rules/Methods/data/bug-5757.php');
+		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-6584.php');
 	}
 
 	/**

tests/PHPStan/Analyser/data/bug-6584.php

@@ -0,0 +1,44 @@
+<?php
+
+namespace Bug6584;
+
+use function PHPStan\Testing\assertType;
+
+class Foo
+{
+
+	public function doFoo(int $int, ?int $intOrNull)
+	{
+		assertType('int', $this->same($int));
+		assertType('int', $this->sameWithDefault($int));
+
+		assertType('int|null', $this->same($intOrNull));
+		assertType('int|null', $this->sameWithDefault($intOrNull));
+
+		assertType('null', $this->same(null));
+		assertType('null', $this->sameWithDefault(null));
+		assertType('null', $this->sameWithDefault());
+	}
+
+
+	/**
+	 * @template T
+	 * @param T $t
+	 * @return T
+	 */
+	function same($t) {
+		assertType('T (method Bug6584\Foo::same(), argument)', $t);
+		return $t;
+	}
+
+	/**
+	 * @template T
+	 * @param T $t
+	 * @return T
+	 */
+	function sameWithDefault($t = null) {
+		assertType('T (method Bug6584\Foo::sameWithDefault(), argument)', $t);
+		return $t;
+	}
+
+}