Possible resolution:

Change in 7.5.6.2 [expr.prim.lambda.closure] paragraph 4 as follows:

The closure type is not an aggregate type (9.5.2 [dcl.init.aggr]) and is not final (11.1 [class.pre]); it is a structural type (13.2 [temp.param]) if and only if the lambda has no lambda-capture.

(From submission #832.)

The current specification leaves open the possibility that an implementation declares closure types as final. That is undesirable.