Fix nullable closure parameter type

src/Analyser/MutatingScope.php

@@ -1588,7 +1588,7 @@ private function resolveType(Expr $node): Type
 				$parameters[] = new NativeParameterReflection(
 					$param->var->name,
 					$firstOptionalParameterIndex !== null && $i >= $firstOptionalParameterIndex,
-					$this->getFunctionType($param->type, $param->type === null, false),
+					$this->getFunctionType($param->type, $this->isParameterValueNullable($param), false),
 					$param->byRef
 						? PassedByReference::createCreatesNewVariable()
 						: PassedByReference::createNo(),

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -767,6 +767,10 @@ 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');
+
+		if (PHP_VERSION_ID >= 70400) {
+			yield from $this->gatherAssertTypes(__DIR__ . '/data/nullable-closure-parameter.php');
+		}
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-6584.php');
 	}
 

tests/PHPStan/Analyser/data/nullable-closure-parameter.php

@@ -0,0 +1,24 @@
+<?php // lint >= 7.4
+
+namespace NullableClosureParameter;
+
+use function PHPStan\Testing\assertType;
+
+class Foo
+{
+
+	public function doFoo()
+	{
+		$a = function (string $test = null) {
+			assertType('string|null', $test);
+			return $test;
+		};
+		assertType('string|null', $a());
+
+		$b = fn (string $test = null) => $test;
+		assertType('string|null', $b());
+
+		fn (string $test = null): string => assertType('string|null', $test);
+	}
+
+}

tests/PHPStan/Rules/Functions/CallCallablesRuleTest.php

@@ -241,4 +241,25 @@ public function testFirstClassCallables(): void
 		]);
 	}
 
+	public function testBug6701(): void
+	{
+		if (PHP_VERSION_ID < 70400 && !self::$useStaticReflectionProvider) {
+			$this->markTestSkipped('Test requires PHP 7.4.');
+		}
+		$this->analyse([__DIR__ . '/data/bug-6701.php'], [
+			[
+				'Parameter #1 $test of closure expects string|null, int given.',
+				14,
+			],
+			[
+				'Parameter #1 $test of closure expects string|null, int given.',
+				18,
+			],
+			[
+				'Parameter #1 $test of closure expects string|null, int given.',
+				24,
+			],
+		]);
+	}
+
 }

tests/PHPStan/Rules/Functions/data/bug-6701.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace Bug6701;
+
+class Foo
+{
+
+	public function doFoo(int $i)
+	{
+		$a = function ( string $test = null ): string {
+			return $test ?? '';
+		};
+		$a(null);
+		$a($i);
+
+		$b = fn ( string $test = null ): string => $test ?? '';
+		$b(null);
+		$b($i);
+
+		$c = function ( ?string $test = null ): string {
+			return $test ?? '';
+		};
+		$c(null);
+		$c($i);
+	}
+
+}