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>
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