This PR ensures `isDefEq` does not increase the transparency mode to
`.default` when checking whether implicit arguments are definitionally
equal. The previous behavior was creating scalability problems in
Mathlib. That said, this is a very disruptive change. The previous
behavior can be restored using the command
```
set_option backward.isDefEq.respectTransparency false
```
This PR ensures `simp` does not "simplify" instances by default. The old
behavior can be retrieved by using `simp +instances`. This PR is similar
to #12195, but for `dsimp`.
The backward compatibility flag for `dsimp` also deactivates this new
feature.
```
set_option backward.dsimp.instances true
```
Applying `simp` (and `dsimp`) to instances creates non-standard
instances, and this creates all sorts of problems in Mathlib.
---------
Co-authored-by: Henrik Böving <hargonix@gmail.com>
Co-authored-by: Sebastian Graf <sgraf1337@gmail.com>
Co-authored-by: Kim Morrison <kim@tqft.net>
This PR moves the `PredTrans.apply` structure field into a separate
`def`. Doing so improves kernel reduction speed because the kernel is
less likely to unfold definitions compared to structure field
projections. This causes minor shifts in `simp` normal forms.
This PR makes `mvcgen` reduce through `let`s, so that it progresses over
`(have t := 42; fun _ => foo t) 23` by reduction to `have t := 42; foo
t` and then introducing `t`.
