Commit 78a5c4ce authored by Ryan Scott's avatar Ryan Scott Committed by Marge Bot

Check for duplicate variables in associated default equations

A follow-up to !696's, which attempted to clean up the error messages
for ill formed associated type family default equations. The previous
attempt, !696, forgot to account for the possibility of duplicate
kind variable arguments, as in the following example:

```hs
class C (a :: j) where
  type T (a :: j) (b :: k)
  type T (a :: k) (b :: k) = k
```

This patch addresses this shortcoming by adding an additional check
for this. Fixes #13971 (hopefully for good this time).
parent 96197961
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