Thanks to @fpvandoorn for noticing this issue in Lean 2! We encountered
this situation when the inferred type of the projection argument did not
reduce to the structure type with the current transparency setting of
the type context.
Now, eta and impredicativity are assumed to be always true.
Motivation: the rest of the system assumes eta.
Regarding impredicativity, we decided to support only the standard library.