Do not infer anything when context is falsy

src/Type/Php/IsSubclassOfFunctionTypeSpecifyingExtension.php

@@ -29,23 +29,20 @@ public function __construct(
 	public function isFunctionSupported(FunctionReflection $functionReflection, FuncCall $node, TypeSpecifierContext $context): bool
 	{
 		return strtolower($functionReflection->getName()) === 'is_subclass_of'
-			&& !$context->null();
+			&& $context->truthy();
 	}
 
 	public function specifyTypes(FunctionReflection $functionReflection, FuncCall $node, Scope $scope, TypeSpecifierContext $context): SpecifiedTypes
 	{
 		if (count($node->getArgs()) < 2) {
 			return new SpecifiedTypes();
 		}
+
 		$objectOrClassType = $scope->getType($node->getArgs()[0]->value);
 		$classType = $scope->getType($node->getArgs()[1]->value);
 		$allowStringType = isset($node->getArgs()[2]) ? $scope->getType($node->getArgs()[2]->value) : new ConstantBooleanType(true);
 		$allowString = !$allowStringType->equals(new ConstantBooleanType(false));
 
-		if (!$classType instanceof ConstantStringType && !$context->truthy()) {
-			return new SpecifiedTypes([], []);
-		}
-
 		return $this->typeSpecifier->create(
 			$node->getArgs()[0]->value,
 			$this->isAFunctionTypeSpecifyingHelper->determineType($objectOrClassType, $classType, $allowString, false),

tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php

@@ -471,6 +471,13 @@ public function testBug6698(): void
 		$this->analyse([__DIR__ . '/data/bug-6698.php'], []);
 	}
 
+	public function testBug6891(): void
+	{
+		$this->checkAlwaysTrueCheckTypeFunctionCall = true;
+		$this->treatPhpDocTypesAsCertain = true;
+		$this->analyse([__DIR__ . '/data/bug-6891.php'], []);
+	}
+
 	public function testBug5369(): void
 	{
 		$this->checkAlwaysTrueCheckTypeFunctionCall = true;

tests/PHPStan/Rules/Comparison/data/bug-6891.php

@@ -0,0 +1,49 @@
+<?php declare(strict_types = 1);
+
+namespace Comparison\Bug6891;
+
+class HelloWorld
+{
+
+	/**
+	 * @param mixed $context
+	 */
+	public function sayHello($context): bool
+	{
+		if (is_subclass_of($context, HelloWorld::class)) {
+			return true;
+		}
+
+		return HelloWorld::class === $context;
+	}
+
+	/**
+	 * @param mixed $context
+	 */
+	public function sayHello2($context): bool
+	{
+		if (is_subclass_of($context, HelloWorld::class)) {
+			return true;
+		}
+
+		return HelloWorld::class === get_class($context);
+	}
+
+	public function sayHello3(object $context): bool
+	{
+		if (is_subclass_of($context, HelloWorld::class)) {
+			return true;
+		}
+
+		return HelloWorld::class === get_class($context);
+	}
+
+	public function sayHello4(string $context): bool
+	{
+		if (is_subclass_of($context, HelloWorld::class)) {
+			return true;
+		}
+
+		return HelloWorld::class === $context;
+	}
+}