Union normalization - do not take template types apart

src/Type/TypeCombinator.php

@@ -185,6 +185,9 @@ public static function union(Type ...$types): Type
 			if (!($types[$i] instanceof UnionType)) {
 				continue;
 			}
+			if ($types[$i] instanceof TemplateType) {
+				continue;
+			}
 
 			array_splice($types, $i, 1, $types[$i]->getTypes());
 		}

src/Type/UnionType.php

@@ -46,6 +46,9 @@ public function __construct(array $types)
 			if (!($type instanceof UnionType)) {
 				continue;
 			}
+			if ($type instanceof TemplateType) {
+				continue;
+			}
 
 			$throwException();
 		}

tests/PHPStan/Analyser/data/generics.php

@@ -1437,3 +1437,28 @@ function (float $f): void {
 	assertType('1.0', floatBound(1.0));
 	assertType('float', floatBound($f));
 };
+
+/**
+ * @template T of string|int|float|bool
+ */
+class UnionT
+{
+
+	/**
+	 * @param T|null $t
+	 * @return T|null
+	 */
+	public function doFoo($t)
+	{
+		return $t;
+	}
+
+}
+
+/**
+ * @param UnionT<string> $foo
+ */
+function foooo(UnionT $foo): void
+{
+	assertType('string|null', $foo->doFoo('a'));
+}

tests/PHPStan/Type/TypeCombinatorTest.php

@@ -1804,6 +1804,24 @@ public function dataUnion(): array
 				UnionType::class,
 				'string|false',
 			],
+			[
+				[
+					TemplateTypeFactory::create(
+						TemplateTypeScope::createWithFunction('doFoo'),
+						'T',
+						new UnionType([
+							new StringType(),
+							new IntegerType(),
+							new FloatType(),
+							new BooleanType(),
+						]),
+						TemplateTypeVariance::createInvariant()
+					),
+					new NullType(),
+				],
+				UnionType::class,
+				'T of bool|float|int|string (function doFoo(), parameter)|null',
+			],
 		];
 	}