Adds type specifier for settype.

conf/config.neon

@@ -1528,6 +1528,11 @@ services:
 		tags:
 			- phpstan.broker.dynamicFunctionReturnTypeExtension
 
+	-
+		class: PHPStan\Type\Php\SetTypeFunctionTypeSpecifyingExtension
+		tags:
+			- phpstan.typeSpecifier.functionTypeSpecifyingExtension
+
 	-
 		class: PHPStan\Type\Php\StrCaseFunctionsReturnTypeExtension
 		tags:

src/Type/Php/SetTypeFunctionTypeSpecifyingExtension.php

@@ -0,0 +1,53 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Type\Php;
+
+use PhpParser\Node\Expr\FuncCall;
+use PHPStan\Analyser\Scope;
+use PHPStan\Analyser\SpecifiedTypes;
+use PHPStan\Analyser\TypeSpecifierContext;
+use PHPStan\Reflection\FunctionReflection;
+use PHPStan\Type\ErrorType;
+use PHPStan\Type\FunctionTypeSpecifyingExtension;
+use PHPStan\Type\NullType;
+use PHPStan\Type\ObjectType;
+use PHPStan\Type\TypeCombinator;
+use stdClass;
+use function count;
+use function strtolower;
+
+class SetTypeFunctionTypeSpecifyingExtension implements FunctionTypeSpecifyingExtension
+{
+
+	public function isFunctionSupported(FunctionReflection $functionReflection, FuncCall $node, TypeSpecifierContext $context): bool
+	{
+		return strtolower($functionReflection->getName()) === 'settype'
+			&& count($node->getArgs()) > 1;
+	}
+
+	public function specifyTypes(FunctionReflection $functionReflection, FuncCall $node, Scope $scope, TypeSpecifierContext $context): SpecifiedTypes
+	{
+		$value = $node->getArgs()[0]->value;
+		$valueType = $scope->getType($value);
+		$castType = $scope->getType($node->getArgs()[1]->value);
+
+		$constantStrings = $castType->getConstantStrings();
+		$types = [];
+
+		foreach ($constantStrings as $constantString) {
+			$types[] = match ($constantString->getValue()) {
+				'bool', 'boolean' => $valueType->toBoolean(),
+				'int', 'integer' => $valueType->toInteger(),
+				'float', 'double' => $valueType->toFloat(),
+				'string' => $valueType->toString(),
+				'array' => $valueType->toArray(),
+				'object' => new ObjectType(stdClass::class),
+				'null' => new NullType(),
+				default => new ErrorType()
+			};
+		}
+
+		return new SpecifiedTypes(['$value' => [$value, TypeCombinator::union(...$types)]], [], true);
+	}
+
+}

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -1431,6 +1431,7 @@ public function dataFileAsserts(): iterable
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/assert-inheritance.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-9123.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-10037.php');
+		yield from $this->gatherAssertTypes(__DIR__ . '/data/set-type-type-specifying.php');
 	}
 
 	/**