Can we use sizeof(this) in a default argument? If so, "subexpression" is not the right term. Maybe we're looking at "potentially-evaluated" or "odr-usable". (We should be correct in notes, but not necessarily exhaustive.)

Can we use sizeof(this) in a default argument? If so, "subexpression" is not the right term. Maybe we're looking at "potentially-evaluated" or "odr-usable". (We should be correct in notes, but not necessarily exhaustive.)

No, we cannot. [expr.prim.this] p3 defines this only between the optional cv-qualifier-seq and the end of the declaration/definition. Default arguments are in the parameter-declaration-clause of parameters-and-qualifiers, which precedes the cv-qualifier-seq.