Generics - fix handling optional parameter

Closes phpstan/phpstan#2573
Closes phpstan/phpstan#4603

src/Reflection/Php/PhpParameterFromParserNodeReflection.php

@@ -62,7 +62,10 @@ public function getType(): Type
 			$phpDocType = $this->phpDocType;
 			if ($phpDocType !== null && $this->defaultValue !== null) {
 				if ($this->defaultValue instanceof NullType) {
-					$phpDocType = \PHPStan\Type\TypeCombinator::addNull($phpDocType);
+					$inferred = $phpDocType->inferTemplateTypes($this->defaultValue);
+					if ($inferred->isEmpty()) {
+						$phpDocType = \PHPStan\Type\TypeCombinator::addNull($phpDocType);
+					}
 				}
 			}
 			$this->type = TypehintHelper::decideType($this->realType, $phpDocType);

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -403,6 +403,7 @@ public function dataFileAsserts(): iterable
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-2413.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-3446.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/getopt.php');
+		yield from $this->gatherAssertTypes(__DIR__ . '/data/generics-default.php');
 	}
 
 	/**

tests/PHPStan/Analyser/data/generics-default.php

@@ -0,0 +1,44 @@
+<?php
+
+namespace GenericsDefault;
+
+use function PHPStan\Testing\assertType;
+
+class Foo
+{
+
+	/**
+	 * @template T of int
+	 * @param T $test
+	 * @return T
+	 */
+	public function doFoo(int $test = 0): int
+	{
+		return $test;
+	}
+
+	public function doBar(): void
+	{
+		assertType('0', $this->doFoo());
+		assertType('1', $this->doFoo(1));
+	}
+
+	/**
+	 * @template T
+	 * @param T $default
+	 */
+	public function doBaz($default = null): void
+	{
+		assertType('T (method GenericsDefault\Foo::doBaz(), argument)', $default);
+	}
+
+	/**
+	 * @template T
+	 * @param T|null $default
+	 */
+	public function doLorem($default = null): void
+	{
+		assertType('T (method GenericsDefault\Foo::doLorem(), argument)|null', $default);
+	}
+
+}

tests/PHPStan/Rules/Methods/IncompatibleDefaultParameterTypeRuleTest.php

@@ -40,4 +40,9 @@ public function testBug4011(): void
 		$this->analyse([__DIR__ . '/data/bug-4011.php'], []);
 	}
 
+	public function testBug2573(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-2573.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Methods/ReturnTypeRuleTest.php

@@ -490,4 +490,17 @@ public function testBug4803(): void
 		$this->analyse([__DIR__ . '/../../Analyser/data/bug-4803.php'], []);
 	}
 
+	public function testBug2573(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-2573-return.php'], []);
+	}
+
+	public function testBug4603(): void
+	{
+		if (PHP_VERSION_ID < 80000 && !self::$useStaticReflectionProvider) {
+			$this->markTestSkipped('Test requires PHP 8.0');
+		}
+		$this->analyse([__DIR__ . '/data/bug-4603.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Methods/data/bug-2573-return.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace Bug2573;
+
+class Bar
+{
+
+	/**
+	 * @template V
+	 * @template D
+	 *
+	 * @param array<V> $array
+	 * @param array-key $key
+	 * @param D $default
+	 * @return V|D
+	 */
+	function idx($array, $key, $default = null) {
+		if (array_key_exists($key, $array)) {
+			return $array[$key];
+		}
+		return $default;
+	}
+
+	/**
+	 * @param array<int, int> $arr
+	 * @return int
+	 */
+	function example($arr) {
+		return $this->idx($arr, 5, 42);
+	}
+
+}

tests/PHPStan/Rules/Methods/data/bug-2573.php

@@ -0,0 +1,19 @@
+<?php
+
+namespace Bug2573;
+
+class Foo
+{
+
+	/**
+	 * @template T1
+	 * @template T2
+	 * @param T1 $a
+	 * @param T2 $b
+	 * @return T1|T2
+	 */
+	function chooseOne($a, $b = []) {
+		return rand(0, 1) ? $a : $b;
+	}
+
+}

tests/PHPStan/Rules/Methods/data/bug-4603.php

@@ -0,0 +1,20 @@
+<?php // lint >= 8.0
+
+namespace Bug4603;
+
+class Foo
+{
+
+	/**
+	 * @param T $val
+	 *
+	 * @return T
+	 *
+	 * @template T
+	 */
+	function fcn(mixed $val = null)
+	{
+		return $val;
+	}
+
+}