Erased instance arguments are not considered for erased instance goals

[jespercockx]

Minimal example:

postulate
  A B : Set
  f : @0 {{A}} → B

g : @0 {{A}} → B
g = f

I'm getting the following error:

No instance of type A was found in scope.
when checking that the expression f has type B

even though defining g {{x}} = f {{x}} works perfectly fine.