Get closer to how type inference in TypeScript works

See https://javascript.xgqfrms.xyz/pdfs/TypeScript%20Language%20Specification.pdf
- section 3.11.7 Type Inference about union types

Otherwise, if T is a union or intersection type:
* First, inferences are made from S to each constituent type in T that isn't simply one of the
type parameters for which inferences are being made.
* If the first step produced no inferences then if T is a union type and exactly one
constituent type in T is simply a type parameter for which inferences are being made,
inferences are made from S to that type parameter.

src/Type/UnionType.php

@@ -12,6 +12,8 @@
 use PHPStan\Reflection\Type\UnresolvedPropertyPrototypeReflection;
 use PHPStan\TrinaryLogic;
 use PHPStan\Type\Constant\ConstantBooleanType;
+use PHPStan\Type\Generic\GenericClassStringType;
+use PHPStan\Type\Generic\TemplateType;
 use PHPStan\Type\Generic\TemplateTypeMap;
 use PHPStan\Type\Generic\TemplateTypeVariance;
 
@@ -599,6 +601,23 @@ public function inferTemplateTypes(Type $receivedType): TemplateTypeMap
 			$myTypes = $this->types;
 		}
 
+		$myTemplateTypes = [];
+		foreach ($myTypes as $type) {
+			if ($type instanceof TemplateType || ($type instanceof GenericClassStringType && $type->getGenericType() instanceof TemplateType)) {
+				$myTemplateTypes[] = $type;
+				continue;
+			}
+			$types = $types->union($type->inferTemplateTypes($receivedType));
+		}
+
+		if (!$types->isEmpty()) {
+			return $types;
+		}
+
+		if (count($myTemplateTypes) === 1) {
+			return $types->union($myTemplateTypes[0]->inferTemplateTypes($receivedType));
+		}
+
 		foreach ($myTypes as $type) {
 			$types = $types->union($type->inferTemplateTypes($receivedType));
 		}

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -10340,6 +10340,7 @@ public function dataFileAsserts(): iterable
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/phpdoc-in-closure-bind.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/multi-assign.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/generics-reduce-types-first.php');
+		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-4803.php');
 	}
 
 	/**

tests/PHPStan/Analyser/data/bug-4803.php

@@ -0,0 +1,78 @@
+<?php
+
+namespace Bug4803;
+
+use function PHPStan\Analyser\assertType;
+
+/**
+ * @template T of object
+ */
+interface Proxy {}
+
+class Foo
+{
+
+	/**
+	 * @template T of object
+	 * @param Proxy<T>|T $proxyOrObject
+	 * @return T
+	 */
+	public function doFoo($proxyOrObject)
+	{
+		assertType('Bug4803\Proxy<T of object (method Bug4803\Foo::doFoo(), argument)>|T of object (method Bug4803\Foo::doFoo(), argument)', $proxyOrObject);
+	}
+
+	/** @param Proxy<\stdClass> $proxy */
+	public function doBar($proxy): void
+	{
+		assertType('stdClass', $this->doFoo($proxy));
+	}
+
+	/** @param \stdClass $std */
+	public function doBaz($std): void
+	{
+		assertType('stdClass', $this->doFoo($std));
+	}
+
+	/** @param Proxy<\stdClass>|\stdClass $proxyOrStd */
+	public function doLorem($proxyOrStd): void
+	{
+		assertType('stdClass', $this->doFoo($proxyOrStd));
+	}
+
+}
+
+interface ProxyClassResolver
+{
+	/**
+	 * @template T of object
+	 * @param class-string<Proxy<T>>|class-string<T> $className
+	 * @return class-string<T>
+	 */
+	public function resolveClassName(string $className): string;
+}
+
+final class Client
+{
+	private ProxyClassResolver $proxyClassResolver;
+
+	public function __construct(ProxyClassResolver $proxyClassResolver)
+	{
+		$this->proxyClassResolver = $proxyClassResolver;
+	}
+
+	/**
+	 * @template T of object
+	 * @param class-string<Proxy<T>>|class-string<T> $className
+	 * @return class-string<T>
+	 */
+	public function getRealClass(string $className): string
+	{
+		assertType('class-string<Bug4803\Proxy<T of object (method Bug4803\Client::getRealClass(), argument)>>|class-string<T of object (method Bug4803\Client::getRealClass(), argument)>', $className);
+
+		$result = $this->proxyClassResolver->resolveClassName($className);
+		assertType('class-string<T of object (method Bug4803\Client::getRealClass(), argument)>', $result);
+
+		return $result;
+	}
+}

tests/PHPStan/Rules/Methods/ReturnTypeRuleTest.php

@@ -485,4 +485,9 @@ public function testBug4795(): void
 		$this->analyse([__DIR__ . '/data/bug-4795.php'], []);
 	}
 
+	public function testBug4803(): void
+	{
+		$this->analyse([__DIR__ . '/../../Analyser/data/bug-4803.php'], []);
+	}
+
 }