feat(resolver): add error for this outside of class

src/interpreter/resolver.py

@@ -26,12 +26,18 @@ class LoopType(Enum):
     FOR = auto()
 
 
+class ClassType(Enum):
+    NONE = auto()
+    CLASS = auto()
+
+
 class Resolver(Expr.Visitor[None], Stmt.Visitor[None]):
     def __init__(self, interpreter: Interpreter):
         self.interpreter: Interpreter = interpreter
         self.scopes: list[dict[str, bool]] = []
         self.current_func: FunctionType = FunctionType.NONE
         self.current_loop: LoopType = LoopType.NONE
+        self.current_class: ClassType = ClassType.NONE
 
     def resolve(self, *objects: Expr | Stmt) -> None:
         for obj in objects:
@@ -111,6 +117,8 @@ class Resolver(Expr.Visitor[None], Stmt.Visitor[None]):
         self.resolve(expr.object)
 
     def visit_this_expr(self, expr: ThisExpr) -> None:
+        if self.current_class == ClassType.NONE:
+            Pebble.token_error(expr.keyword, "Cannot use 'this' outside of a class.")
         self.resolve_local(expr, expr.keyword)
 
     def visit_block_stmt(self, stmt: BlockStmt) -> None:
@@ -119,6 +127,8 @@ class Resolver(Expr.Visitor[None], Stmt.Visitor[None]):
         self.end_scope()
 
     def visit_class_stmt(self, stmt: ClassStmt) -> None:
+        enclosing_class: ClassType = self.current_class
+        self.current_class = ClassType.CLASS
         self.declare(stmt.name)
         self.define(stmt.name)
 
@@ -129,6 +139,7 @@ class Resolver(Expr.Visitor[None], Stmt.Visitor[None]):
             self.resolve_function(method, FunctionType.METHOD)
 
         self.end_scope()
+        self.current_class = enclosing_class
 
     def visit_expression_stmt(self, stmt: ExpressionStmt) -> None:
         self.resolve(stmt.expression)