CWG3061 [stmt.expand] Trailing commas in an expansion-init-list should be permitted #754

Reference (section label): [stmt.expand]

Issue description

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

Unlike any other brace-enclosed list, an expression-list in an expansion-init-list does not permit the use of trailing commas. This is almost certainly a cplusplus/papers#156 oversight given obvious inconsistencies and that the paper does not discuss this as a design choice.

Furthermore, existing implementations of expansion statements do not implement the standard behavior; see https://godbolt.org/z/czYqPG3r6.

Suggested resolution

Change the syntax of expansion-init-list as follows:

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