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 +1,91 @@
+<?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\TypeSpecifier;
+use PHPStan\Analyser\TypeSpecifierAwareExtension;
+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, TypeSpecifierAwareExtension
+{
+
+	private TypeSpecifier $typeSpecifier;
+
+	public function isFunctionSupported(FunctionReflection $functionReflection, FuncCall $node, TypeSpecifierContext $context): bool
+	{
+		return strtolower($functionReflection->getName()) === 'settype'
+			&& count($node->getArgs()) > 1
+			&& $context->null();
+	}
+
+	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 = $castType->getConstantStrings();
+		if (count($constantStrings) < 1) {
+			return new SpecifiedTypes();
+		}
+
+		$types = [];
+
+		foreach ($constantStrings as $constantString) {
+			switch ($constantString->getValue()) {
+				case 'bool':
+				case 'boolean':
+					$types[] = $valueType->toBoolean();
+					break;
+				case 'int':
+				case 'integer':
+					$types[] = $valueType->toInteger();
+					break;
+				case 'float':
+				case 'double':
+					$types[] = $valueType->toFloat();
+					break;
+				case 'string':
+					$types[] = $valueType->toString();
+					break;
+				case 'array':
+					$types[] = $valueType->toArray();
+					break;
+				case 'object':
+					$types[] = new ObjectType(stdClass::class);
+					break;
+				case 'null':
+					$types[] = new NullType();
+					break;
+				default:
+					$types[] = new ErrorType();
+			}
+		}
+
+		return $this->typeSpecifier->create(
+			$value,
+			TypeCombinator::union(...$types),
+			TypeSpecifierContext::createTruthy(),
+			true,
+			$scope,
+		);
+	}
+
+	public function setTypeSpecifier(TypeSpecifier $typeSpecifier): void
+	{
+		$this->typeSpecifier = $typeSpecifier;
+	}
+
+}

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');
 	}
 
 	/**