PR13657 (and duplicates):

When a comma occurs in a default argument or default initializer within a
class, disambiguate whether it is part of the initializer or whether it ends
the initializer.

The way this works (which I will be proposing for standardization) is to treat
the comma as ending the default argument or default initializer if the
following token sequence matches the syntactic constraints of a
parameter-declaration-clause or init-declarator-list (respectively).

This is both consistent with the disambiguation rules elsewhere (where entities
are treated as declarations if they can be), and should have no regressions
over our old behavior. I think it might also disambiguate all cases correctly,
but I don't have a proof of that.

There is an annoyance here: because we're performing a tentative parse in a
situation where we may not have seen declarations of all relevant entities (if
the comma is part of the initializer, lookup may find entites declared later in
the class), we need to turn off typo-correction and diagnostics during the
tentative parse, and in the rare case that we decide the comma is part of the
initializer, we need to revert all token annotations we performed while
disambiguating.

Any diagnostics that occur outside of the immediate context of the tentative
parse (for instance, if we trigger the implicit instantiation of a class
template) are *not* suppressed, mirroring the usual rules for a SFINAE context.