prove_goal_async fails when there are instances in the context

Prerequisites

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  • Reduced the issue to a self-contained, reproducible test case.

Description

The first thing prove_goal_async tries to do is revert everything in the context, but if there are frozen instances then this fails.

Steps to Reproduce

example {α : Type} [inhabited α] (x : α) : x = x :=
begin
do
  -- tactic.unfreeze_local_instances,
  tactic.prove_goal_async `[refl]
end

Expected behavior: The goal is solved

Actual behavior:

/home/rwbarton/lean/mathlib/obviously/bug.lean:3:0: error: failed to revert '_inst_1', it is a frozen local instance (possible solution: use tactic `tactic.unfreeze_local_instances` to reset the set of local instances)
state:
α : Type,
_inst_1 : inhabited α,
x : α
⊢ x = x

Uncommenting tactic.unfreeze_local_instances fixes this, but probably prove_goal_async should take care of this? The fact that it uses revert is an implementation detail.

In theory maybe should "refreeze" the originally frozen instances in the context in which the inner tactic will eventually run? Maybe it's not worth worrying about.

Reproduces how often: Always

Versions

Lean (version 3.11.0, commit fb3f3e88be8a, Release)

Start with the implementation of tactic.prove_goal_async and reproduce the provided Lean example with a frozen local instance. Trace the initial context reversion and verify that the example succeeds without requiring tactic.unfreeze_local_instances, while preserving the intended behavior for asynchronous tactics.