Throw exception on invalid array start

When the current type index is null, no type was resolved yet. So
an array start is invalid. An `[]` array operator should always be
prefixed with a type notation.

Author: jaapio

src/TypeResolver.php

@@ -38,6 +38,7 @@
 use function class_exists;
 use function class_implements;
 use function count;
+use function current;
 use function end;
 use function in_array;
 use function key;
@@ -273,6 +274,10 @@ private function parseTypes(ArrayIterator $tokens, Context $context, int $parser
             } elseif ($token === self::OPERATOR_ARRAY) {
                 end($types);
                 $last = key($types);
+                if ($last === null) {
+                    throw new InvalidArgumentException('Unexpected array operator');
+                }
+
                 $lastItem = $types[$last];
                 if ($lastItem instanceof Expression) {
                     $lastItem = $lastItem->getValueType();
@@ -317,7 +322,7 @@ private function parseTypes(ArrayIterator $tokens, Context $context, int $parser
                 );
             }
         } elseif (count($types) === 1) {
-            return $types[0];
+            return current($types);
         }
 
         if ($compoundToken === '|') {

tests/unit/TypeResolverTest.php

@@ -714,6 +714,19 @@ public function testExceptionIsThrownIfTypeIsEmpty(): void
         $fixture->resolve(' ', new Context(''));
     }
 
+    /**
+     * @uses \phpDocumentor\Reflection\Types\Context
+     *
+     * @covers ::__construct
+     * @covers ::resolve
+     */
+    public function testInvalidArrayOperator(): void
+    {
+        $this->expectException('InvalidArgumentException');
+        $fixture = new TypeResolver();
+        $fixture->resolve('[]', new Context(''));
+    }
+
     /**
      * Returns a list of keywords and expected classes that are created from them.
      *