Don't add a trailing comma in lists that shouldn't have one.

CHANGELOG.md

@@ -1,3 +1,8 @@
+## 3.0.2-wip
+
+* Don't add a trailing comma in lists that don't allow it, even when there is
+  a trailing comment (#1639).
+
 ## 3.0.1
 
 * Handle trailing commas in for-loop updaters (#1354).

lib/src/front_end/delimited_list_builder.dart

@@ -62,8 +62,21 @@ final class DelimitedListBuilder {
       });
     }
 
-    var piece =
-        ListPiece(_leftBracket, _elements, _blanksAfter, _rightBracket, _style);
+    // Find the index of the last non-comment element. The list may have
+    // trailing elements that are just comments. In that case, those are
+    // not considered the last element when it comes to deciding whether to add
+    // a trailing comma on the last element.
+    var lastNonCommentElement = -1;
+    for (var i = _elements.length - 1; i >= 0; i--) {
+      if (!_elements[i].isComment) {
+        lastNonCommentElement = i;
+        break;
+      }
+    }
+
+    var piece = ListPiece(
+        _leftBracket, _elements, _blanksAfter, _rightBracket, _style,
+        lastNonCommentElement: lastNonCommentElement);
     if (_mustSplit || forceSplit) piece.pin(State.split);
     return piece;
   }

lib/src/piece/list.dart

@@ -55,6 +55,10 @@ final class ListPiece extends Piece {
   /// The details of how this particular list should be formatted.
   final ListStyle _style;
 
+  /// The index of the last element in [_elements] that has content and isn't
+  /// a comment, or `-1` if all elements are comments.
+  final int _lastNonCommentElement;
+
   /// Whether any element in this argument list can be block formatted.
   bool get hasBlockElement =>
       _elements.any((element) => element.allowNewlinesWhenUnsplit);
@@ -64,8 +68,10 @@ final class ListPiece extends Piece {
   /// [_elements] must not be empty. (If there are no elements, just concatenate
   /// the brackets directly.)
   ListPiece(
-      this._before, this._elements, this._blanksAfter, this._after, this._style)
-      : assert(_elements.isNotEmpty) {
+      this._before, this._elements, this._blanksAfter, this._after, this._style,
+      {required int lastNonCommentElement})
+      : assert(_elements.isNotEmpty),
+        _lastNonCommentElement = lastNonCommentElement {
     // For most elements, we know whether or not it will have a comma based
     // only on the comma style and its position in the list, so pin those here.
     for (var i = 0; i < _elements.length; i++) {
@@ -80,13 +86,13 @@ final class ListPiece extends Piece {
           // Always has a comma after every element except the last. The last
           // will be constrained to have one or not depending on whether the
           // list splits. See applyConstraints().
-          if (i < _elements.length - 1) {
+          if (i < _lastNonCommentElement) {
             element.pin(ListElementPiece._appendComma);
           }
 
         case Commas.nonTrailing:
           // Never a trailing comma after the last element.
-          element.pin(i < _elements.length - 1
+          element.pin(i < _lastNonCommentElement
               ? ListElementPiece._appendComma
               : State.unsplit);
 
@@ -103,8 +109,8 @@ final class ListPiece extends Piece {
   @override
   void applyConstraints(State state, Constrain constrain) {
     // Give the last element a trailing comma only if the list is split.
-    if (_style.commas == Commas.trailing) {
-      constrain(_elements.last,
+    if (_style.commas == Commas.trailing && _lastNonCommentElement != -1) {
+      constrain(_elements[_lastNonCommentElement],
           state == State.split ? ListElementPiece._appendComma : State.unsplit);
     }
   }
@@ -319,6 +325,15 @@ final class ListElementPiece extends Piece {
     _hangingComments.add(comment);
   }
 
+  /// Whether this piece is a comment-only element, like the second element in:
+  ///
+  ///     [
+  ///       real,
+  ///       // Comment.
+  ///       anotherReal,
+  ///     ]
+  bool get isComment => _content == null;
+
   void addComment(Piece comment, {bool beforeDelimiter = false}) {
     _hangingComments.add(comment);
     if (beforeDelimiter) _commentsBeforeDelimiter++;

pubspec.yaml

@@ -1,6 +1,6 @@
 name: dart_style
 # Note: See tool/grind.dart for how to bump the version.
-version: 3.0.1
+version: 3.0.2-wip
 description: >-
   Opinionated, automatic Dart source code formatter.
   Provides an API and a CLI tool.

test/tall/regression/1600/1639.unit

@@ -0,0 +1,33 @@
+>>>
+class C<T
+// 1
+// 2
+// 3
+> {}
+<<<
+class C<
+  T
+  // 1
+  // 2
+  // 3
+> {}
+>>> (indent 2)
+  void Function<
+    Y extends A<FcovCyclicCoBound<Function(Function(FutureOr<Object>))>>
+    //          ^
+    // [cfe] Type argument 'dynamic Function(dynamic Function(FutureOr<Object>))' doesn't conform to the bound 'dynamic Function(X)' of the type variable 'X' on 'FcovCyclicCoBound'.
+    //                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    // [analyzer] COMPILE_TIME_ERROR.TYPE_ARGUMENT_NOT_MATCHING_BOUNDS
+  >()
+  x264;
+  void Function<Y extends A<CFcov<Object>>>() x265;
+<<<
+  void Function<
+    Y extends A<FcovCyclicCoBound<Function(Function(FutureOr<Object>))>>
+    //          ^
+    // [cfe] Type argument 'dynamic Function(dynamic Function(FutureOr<Object>))' doesn't conform to the bound 'dynamic Function(X)' of the type variable 'X' on 'FcovCyclicCoBound'.
+    //                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    // [analyzer] COMPILE_TIME_ERROR.TYPE_ARGUMENT_NOT_MATCHING_BOUNDS
+  >()
+  x264;
+  void Function<Y extends A<CFcov<Object>>>() x265;

test/tall/type/function_comment.stmt

@@ -443,4 +443,16 @@ Function(
   int c, [
   direction,
 ])
+x;
+>>> After type parameter on its own line.
+Function<
+Y
+// Comment.
+>()
+x;
+<<<
+Function<
+  Y
+  // Comment.
+>()
 x;