Add type checking to for-loops

trans/stdlib/__init__.py

@@ -36,6 +36,8 @@ class str:
     def format(self, *args) -> Self: ...
     def encode(self, encoding: Self) -> bytes: ...
     def __len__(self) -> int: ...
+    def __add__(self, other: Self) -> Self: ...
+    def __mul__(self, other: int) -> Self: ...
 
 class bytes:
     def decode(self, encoding: str) -> str: ...

trans/transpiler/phases/typing/block.py

@@ -136,8 +136,18 @@ class ScoperBlockVisitor(ScoperVisitor):
         scope = self.scope.child(ScopeKind.FUNCTION_INNER)
         node.inner_scope = scope
         assert isinstance(node.target, ast.Name)
-        scope.vars[node.target.id] = VarDecl(VarKind.LOCAL, TypeVariable())
-        self.expr().visit(node.iter)
+        var_var = TypeVariable()
+        scope.vars[node.target.id] = VarDecl(VarKind.LOCAL, var_var)
+        seq_type = self.expr().visit(node.iter)
+        try:
+            iter_type = seq_type.methods["__iter__"].return_type
+        except:
+            raise IncompatibleTypesError(f"{seq_type} is not iterable")
+        try:
+            next_type = iter_type.methods["__next__"].return_type
+        except:
+            raise IncompatibleTypesError(f"iter({iter_type}) is not an iterator")
+        var_var.unify(next_type)
         body_scope = scope.child(ScopeKind.FUNCTION_INNER)
         body_visitor = ScoperBlockVisitor(body_scope, self.root_decls)
         body_visitor.visit_block(node.body)