Note that test for issue #1200 broke.
The bug fixed by this commit was allowing the example to be elaborated
correctly :(
Initially, the type of the discriminant is not available, and
`.none (α:=α)` can only be elaborated when the expected type is of the
form `C ...`. Lean then tries to elaborate the alternatives, learn
that the discriminant should be `Option ?m`, and fails because the
patterns still have metavariables after elaboration. Before the bug
fix, `resumePostpone` was **not** restoring the metavariable context,
and the assingnment would stay there. With this information, Lean
can now elaborate `.none (α:=α)`.
Although the bug had a positive impact in this case, it produced
incorrect behavior in other examples.
The fixed example looks reasonable. Thus, we will not reopen
issue #1200
