Report always false usages of is_a

src/Rules/Comparison/ImpossibleCheckTypeFunctionCallRule.php

@@ -41,9 +41,6 @@ public function processNode(Node $node, Scope $scope): array
 		}
 
 		$functionName = (string) $node->name;
-		if (strtolower($functionName) === 'is_a') {
-			return [];
-		}
 		$isAlways = $this->impossibleCheckTypeHelper->findSpecifiedType($scope, $node);
 		if ($isAlways === null) {
 			return [];

src/Rules/Comparison/ImpossibleCheckTypeHelper.php

@@ -10,12 +10,14 @@
 use PHPStan\Analyser\TypeSpecifier;
 use PHPStan\Analyser\TypeSpecifierContext;
 use PHPStan\Reflection\ReflectionProvider;
+use PHPStan\TrinaryLogic;
 use PHPStan\Type\Constant\ConstantArrayType;
 use PHPStan\Type\Constant\ConstantBooleanType;
 use PHPStan\Type\Constant\ConstantStringType;
 use PHPStan\Type\MixedType;
 use PHPStan\Type\NeverType;
 use PHPStan\Type\ObjectType;
+use PHPStan\Type\StringType;
 use PHPStan\Type\TypeCombinator;
 use PHPStan\Type\TypeUtils;
 use PHPStan\Type\TypeWithClassName;
@@ -157,6 +159,20 @@ public function findSpecifiedType(
 							}
 						}
 					}
+				} elseif ($functionName === 'is_a') {
+					$allowString = TrinaryLogic::createNo();
+					if (count($node->args) > 2) {
+						$allowStringParameterType = $scope->getType($node->args[2]->value);
+						$allowString = $allowStringParameterType instanceof ConstantBooleanType
+							? TrinaryLogic::createFromBoolean($allowStringParameterType->getValue())
+							: TrinaryLogic::createMaybe();
+					}
+					$objectType = $scope->getType($node->args[0]->value);
+					if ($allowString->no() && (new StringType())->isSuperTypeOf($objectType)->yes()) {
+						return false;
+					}
+
+					return null;
 				}
 			}
 		}

tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php

@@ -385,4 +385,16 @@ public function testBug3994(): void
 		$this->analyse([__DIR__ . '/data/bug-3994.php'], []);
 	}
 
+	public function testBug4371(): void
+	{
+		$this->checkAlwaysTrueCheckTypeFunctionCall = true;
+		$this->treatPhpDocTypesAsCertain = true;
+		$this->analyse([__DIR__ . '/data/bug-4371.php'], [
+			[
+				"Call to function is_a() with 'Bug4371\\\\Bar' and 'Bug4371\\\\Foo' will always evaluate to false.",
+				12,
+			],
+		]);
+	}
+
 }

tests/PHPStan/Rules/Comparison/data/bug-4371.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace Bug4371;
+
+class Foo {
+}
+
+class Bar extends Foo {
+
+}
+
+$a = is_a(Bar::class, Foo::class); // should be reported
+
+$a = is_a(Bar::class, Foo::class, true); // should be fine