Prohibit starred arguments after double-starred arguments

CPython prohibits ‘f(**kwargs, *args)’; we should too.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

parser/src/error.rs

@@ -22,6 +22,7 @@ pub enum LexicalErrorType {
     TabsAfterSpaces,
     DefaultArgumentError,
     PositionalArgumentError,
+    UnpackedArgumentError,
     DuplicateKeywordArgumentError,
     UnrecognizedToken { tok: char },
     FStringError(FStringErrorType),
@@ -55,6 +56,12 @@ impl fmt::Display for LexicalErrorType {
             LexicalErrorType::PositionalArgumentError => {
                 write!(f, "positional argument follows keyword argument")
             }
+            LexicalErrorType::UnpackedArgumentError => {
+                write!(
+                    f,
+                    "iterable argument unpacking follows keyword argument unpacking"
+                )
+            }
             LexicalErrorType::UnrecognizedToken { tok } => {
                 write!(f, "Got unexpected token {tok}")
             }

parser/src/function.rs

@@ -55,6 +55,7 @@ pub fn parse_args(func_args: Vec<FunctionArgument>) -> Result<ArgumentList, Lexi
 
     let mut keyword_names =
         FxHashSet::with_capacity_and_hasher(func_args.len(), Default::default());
+    let mut double_starred = false;
     for (name, value) in func_args {
         match name {
             Some((start, end, name)) => {
@@ -67,6 +68,8 @@ pub fn parse_args(func_args: Vec<FunctionArgument>) -> Result<ArgumentList, Lexi
                     }
 
                     keyword_names.insert(keyword_name.clone());
+                } else {
+                    double_starred = true;
                 }
 
                 keywords.push(ast::Keyword::new(
@@ -76,12 +79,18 @@ pub fn parse_args(func_args: Vec<FunctionArgument>) -> Result<ArgumentList, Lexi
                 ));
             }
             None => {
-                // Allow starred args after keyword arguments.
+                // Allow starred arguments after keyword arguments but
+                // not after double-starred arguments.
                 if !keywords.is_empty() && !is_starred(&value) {
                     return Err(LexicalError {
                         error: LexicalErrorType::PositionalArgumentError,
                         location: value.location,
                     });
+                } else if double_starred {
+                    return Err(LexicalError {
+                        error: LexicalErrorType::UnpackedArgumentError,
+                        location: value.location,
+                    });
                 }
 
                 args.push(value);