ast: match CPython argument validation order (#8414)

Validate parameter annotations while converting ast.arguments, before
merging positional and keyword-only defaults. This gives invalid annotation
contexts precedence over default-list shape errors, as in CPython.

Merge positional defaults before keyword-only defaults so an excess
positional-default error also takes precedence over a keyword-only default
length mismatch.

Remove the expected-failure markers from the FunctionDef and Lambda AST
validator tests that now pass.

Assisted-by: Codex:gpt-5.6-sol

Lib/test/test_ast/test_ast.py

@@ -2055,7 +2055,6 @@ class ASTValidatorTests(unittest.TestCase):
                           kw_defaults=[None, ast.Name("x", ast.Store())]),
                           "must have Load context")
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON; ValueError not raised
     def test_funcdef(self):
         a = ast.arguments([], [], None, [], [], None, [])
         f = ast.FunctionDef("x", a, [], [], None, None, [])
@@ -2267,7 +2266,6 @@ class ASTValidatorTests(unittest.TestCase):
         u = ast.UnaryOp(ast.Not(), ast.Name("x", ast.Store()))
         self.expr(u, "must have Load context")
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON; ValueError not raised
     def test_lambda(self):
         a = ast.arguments([], [], None, [], [], None, [])
         self.expr(ast.Lambda(a, ast.Name("x", ast.Store())),

crates/vm/src/stdlib/_ast/parameter.rs

@@ -88,14 +88,23 @@ impl Node for ast::Parameters {
             "arguments",
         )?;
 
+        validate_parameter_annotations(
+            vm,
+            &posonlyargs,
+            &args,
+            vararg.as_deref(),
+            &kwonlyargs,
+            kwarg.as_deref(),
+        )?;
+
         let ParameterDefaults {
             runtime_defaults,
             defaults,
             _range: _,
         } = defaults;
-        let kwonlyargs = merge_keyword_parameter_defaults(vm, kwonlyargs, kw_defaults)?;
         let (posonlyargs, args) =
             merge_positional_parameter_defaults(vm, posonlyargs, args, defaults)?;
+        let kwonlyargs = merge_keyword_parameter_defaults(vm, kwonlyargs, kw_defaults)?;
 
         Ok(Self {
             node_index: Default::default(),
@@ -114,6 +123,29 @@ impl Node for ast::Parameters {
     }
 }
 
+fn validate_parameter_annotations(
+    vm: &VirtualMachine,
+    posonlyargs: &PositionalParameters,
+    args: &PositionalParameters,
+    vararg: Option<&ast::Parameter>,
+    kwonlyargs: &KeywordParameters,
+    kwarg: Option<&ast::Parameter>,
+) -> PyResult<()> {
+    for parameter in posonlyargs.args.iter().chain(&args.args) {
+        super::validate::validate_parameter_annotation(vm, parameter)?;
+    }
+    if let Some(parameter) = vararg {
+        super::validate::validate_parameter_annotation(vm, parameter)?;
+    }
+    for parameter in &kwonlyargs.keywords {
+        super::validate::validate_parameter_annotation(vm, parameter)?;
+    }
+    if let Some(parameter) = kwarg {
+        super::validate::validate_parameter_annotation(vm, parameter)?;
+    }
+    Ok(())
+}
+
 // product
 impl Node for ast::Parameter {
     fn ast_to_object(self, vm: &VirtualMachine, source_file: &SourceFile) -> PyObjectRef {

crates/vm/src/stdlib/_ast/validate.rs

@@ -56,7 +56,10 @@ fn validate_keywords(vm: &VirtualMachine, keywords: &[ast::Keyword]) -> PyResult
     Ok(())
 }
 
-fn validate_parameter_annotation(vm: &VirtualMachine, parameter: &ast::Parameter) -> PyResult<()> {
+pub(super) fn validate_parameter_annotation(
+    vm: &VirtualMachine,
+    parameter: &ast::Parameter,
+) -> PyResult<()> {
     if let Some(annotation) = &parameter.annotation {
         validate_expr(vm, annotation, ast::ExprContext::Load)?;
     }