fix: allow JSX generic trailing comma iff required (#316)

Author: davo-canva

src/parsing/parser.rs

@@ -5301,13 +5301,18 @@ fn parse_type_parameters<'a>(node: TypeParamNode<'a>, context: &mut Context<'a>)
     // trailing commas should be allowed in type parameters only—not arguments
     if let Some(type_params) = node.parent().get_type_parameters() {
       if type_params.lo() == node.lo() {
-        // Always use trailing commas for arrow function expressions in a JSX file
-        // if one exists as it may be used to assist with parsing ambiguity.
+        // Use trailing commas for function expressions in a JSX file
+        // if the absence of one would lead to a parsing ambiguity.
         if context.is_jsx && (node.parent().kind() == NodeKind::ArrowExpr || node.parent().parent().unwrap().kind() == NodeKind::FnExpr) {
-          let comma_count = type_params.tokens_fast(context.module).iter().filter(|t| t.token == Token::Comma).count();
-          let has_trailing_comma = comma_count >= type_params.params.len();
-          if has_trailing_comma {
-            return TrailingCommas::Always;
+          let children = type_params.children();
+          // It is not ambiguous if there are multiple type parameters.
+          if children.len() == 1 && children[0].kind() == NodeKind::TsTypeParam {
+            let type_param = children[0];
+            let children = type_param.children();
+            // We have a possible ambiguity iff this type parameter is just an identifier.
+            if children.len() == 1 && children[0].kind() == NodeKind::Ident {
+              return TrailingCommas::Always;
+            }
           }
         }
         return trailing_commas;

tests/specs/general/Jsx_TrailingCommas.txt

@@ -15,6 +15,10 @@ const Test3 = <div></div>;
 function test<T,>() {
 }
 
+// generic constraints are not ambiguous either
+const Test4 = <P extends R<S, T>,>() => {};
+const Test5 = <P extends R<S, T>>() => {};
+
 [expect]
 const Test1 = <T,>() => false;
 const Test2 = function<T,>() {};
@@ -23,3 +27,7 @@ const Test3 = <div></div>;
 // not for declarations though
 function test<T>() {
 }
+
+// generic constraints are not ambiguous either
+const Test4 = <P extends R<S, T>>() => {};
+const Test5 = <P extends R<S, T>>() => {};

tests/specs/issues/issue0303.txt

@@ -1,23 +1,24 @@
+-- test.tsx --
 == should format with comments ==
-const t = <X>() => (
+const t = <X,>() => (
     <>
         {/* Comment 1 */}
         {/* Comment 2 */}
         {/* Comment 3 */}
     </>
 );
-const u = <X>() => {
+const u = <X,>() => {
     // Hello world
 };
 
 [expect]
-const t = <X>() => (
+const t = <X,>() => (
     <>
         {/* Comment 1 */}
         {/* Comment 2 */}
         {/* Comment 3 */}
     </>
 );
-const u = <X>() => {
+const u = <X,>() => {
     // Hello world
 };