Enter right-side-assign context for ??= so closures capturing the assigned variable by reference see its assigned type

src/Analyser/ExprHandler/AssignOpHandler.php

@@ -29,6 +29,7 @@
 use PHPStan\Type\Type;
 use function array_merge;
 use function get_class;
+use function is_string;
 use function sprintf;
 
 /**
@@ -68,6 +69,13 @@ static function (MutatingScope $scope) use ($stmt, $expr, $nodeCallback, $contex
 					$scope = $scope->filterByFalseyValue(
 						new BinaryOp\NotIdentical($expr->var, new ConstFetch(new Name('null'))),
 					);
+
+					if ($expr->var instanceof Expr\Variable && is_string($expr->var->name)) {
+						$context = $context->enterRightSideAssign(
+							$expr->var->name,
+							$expr->expr,
+						);
+					}
 				}
 
 				$exprResult = $nodeScopeResolver->processExprNode($stmt, $expr->expr, $scope, $storage, $nodeCallback, $context->enterDeep());

tests/PHPStan/Analyser/nsrt/bug-13810.php

@@ -0,0 +1,68 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug13810;
+
+use function PHPStan\Testing\assertType;
+
+function doFoo(): void
+{
+	static $isSupported;
+	assertType('mixed', $isSupported);
+	$isSupported ??= function (mixed $arg) use (&$isSupported): bool {
+		assertType('Closure(mixed): bool', $isSupported);
+		return $isSupported($arg);
+	};
+
+	assertType('mixed~null', $isSupported);
+	$isSupported('foo');
+}
+
+function doBar($isSupported): void
+{
+	assertType('mixed', $isSupported);
+	$isSupported ??= function (mixed $arg) use (&$isSupported): bool {
+		assertType('Closure(mixed): bool', $isSupported);
+		return $isSupported($arg);
+	};
+
+	assertType('mixed~null', $isSupported);
+	$isSupported('foo');
+}
+
+function doFooBar(): void
+{
+	$isSupported = null;
+	assertType('null', $isSupported);
+	$isSupported ??= function (mixed $arg) use (&$isSupported): bool {
+		assertType('Closure(mixed): bool', $isSupported);
+		return $isSupported($arg);
+	};
+
+	assertType('Closure(mixed): bool', $isSupported);
+	$isSupported('foo');
+}
+
+class HelloWorld
+{
+	public function setValue(mixed $value): void
+	{
+		/** @var ?callable $isSupported */
+		static $isSupported = null;
+		$isSupported ??= function(mixed $arg) use (&$isSupported): bool {
+			if (is_array($arg)) {
+				foreach($arg as $value) {
+					if (!$isSupported($value)) {
+						return false;
+					}
+				}
+				return true;
+			}
+			return is_string($arg);
+		};
+		if (!$isSupported($value)) {
+			throw new InvalidArgumentException('only strings/string arrays are supported');
+		}
+	}
+}

tests/PHPStan/Rules/Functions/CallCallablesRuleTest.php

@@ -429,4 +429,9 @@ public function testMaybeNotCallable(): void
 		$this->analyse([__DIR__ . '/data/maybe-not-callable.php'], $errors);
 	}
 
+	public function testBug13810(): void
+	{
+		$this->analyse([__DIR__ . '/../../Analyser/nsrt/bug-13810.php'], []);
+	}
+
 }