Validate variable name starts with a $

generator/config/expressions.php

@@ -13,7 +13,6 @@
 use MongoDB\Model\BSONArray;
 use stdClass;
 
-use function in_array;
 use function ucfirst;
 
 $bsonTypes = [
@@ -46,11 +45,6 @@
 $expressions = [];
 $resolvesToInterfaces = [];
 foreach ($bsonTypes as $name => $acceptedTypes) {
-    // an expression can be a string with a $-prefixed field name
-    if (! in_array('string', $acceptedTypes)) {
-        $acceptedTypes[] = 'string';
-    }
-
     $expressions[$name] = ['acceptedTypes' => $acceptedTypes];
 
     $resolvesTo = 'resolvesTo' . ucfirst($name);

generator/src/OperatorClassGenerator.php

@@ -169,6 +169,18 @@ public function createClass(GeneratorDefinition $definition, OperatorDefinition
 
                     PHP);
                 }
+
+                if ($type->dollarPrefixedString) {
+                    $namespace->addUseFunction('is_string');
+                    $namespace->addUseFunction('str_starts_with');
+                    $namespace->addUse(InvalidArgumentException::class);
+                    $constructor->addBody(<<<PHP
+                    if (is_string(\${$argument->propertyName}) && ! str_starts_with(\${$argument->propertyName}, '$')) {
+                        throw new InvalidArgumentException('Argument \${$argument->propertyName} can be an expression, field paths and variable names must be prefixed by "$" or "$$".');
+                    }
+
+                    PHP);
+                }
             }
 
             // Set property from constructor argument

generator/src/OperatorGenerator.php

@@ -26,6 +26,7 @@
 use function ltrim;
 use function sort;
 use function sprintf;
+use function str_starts_with;
 use function ucfirst;
 use function usort;
 
@@ -71,13 +72,19 @@ final protected function getType(string $type): ExpressionDefinition
      * Expression types can contain class names, interface, native types or "list".
      * PHPDoc types are more precise than native types, so we use them systematically even if redundant.
      *
-     * @return object{native:string,doc:string,use:list<class-string>,list:bool,query:bool,javascript:bool}
+     * @return object{native:string,doc:string,use:list<class-string>,list:bool,query:bool,javascript:bool,dollarPrefixedString:bool}
      */
     final protected function getAcceptedTypes(ArgumentDefinition $arg): stdClass
     {
         $nativeTypes = [];
 
+        $dollarPrefixedString = false;
+
         foreach ($arg->type as $type) {
+            if (str_starts_with($type, 'resolvesTo')) {
+                $dollarPrefixedString = true;
+            }
+
             $type = $this->getType($type);
             $nativeTypes = array_merge($nativeTypes, $type->acceptedTypes);
 
@@ -91,6 +98,14 @@ final protected function getAcceptedTypes(ArgumentDefinition $arg): stdClass
             $nativeTypes[] = Optional::class;
         }
 
+        // If the argument accepts an expression, a $-prefixed string is accepted (field path or variable)
+        // Checked only if the argument does not already accept a string
+        if (in_array('string', $nativeTypes, true)) {
+            $dollarPrefixedString = false;
+        } elseif ($dollarPrefixedString) {
+            $nativeTypes[] = 'string';
+        }
+
         $docTypes = $nativeTypes = array_unique($nativeTypes);
         $use = [];
 
@@ -131,6 +146,7 @@ final protected function getAcceptedTypes(ArgumentDefinition $arg): stdClass
             'list' => $listCheck,
             'query' => $isQuery,
             'javascript' => $isJavascript,
+            'dollarPrefixedString' => $dollarPrefixedString,
         ];
     }