Proposed resolution (approved by CWG 2025-09-12):

Change in 8.7 [stmt.expand] paragraph 1 as follows:

  expansion-init-list :
       { expression-list ,opt }
       { }

(From submission #754.)

Brace-enclosed lists generally allow trailing commas. But an expansion-init-list does not; consider:

           for (int x : { 1, }) { }   // OK
  template for (int x : { 1, }) { }   // syntax error