GHC thinks type variables are not bound by data family constructor in LHS of type synonym instance
## Summary
Today I tried building GHC HEAD for the first time, so disclaimer: this could be user error. But assuming it isn’t, I seem to be getting an error on this fairly benign program:
```haskell
{-# LANGUAGE TypeFamilies #-}
module DataFamilySynonymInstance where
data family D a
type family F a
type instance F (D a) = a
```
```
$ ghc DataFamilySynonymInstance.hs
[1 of 1] Compiling DataFamilySynonymInstance ( DataFamilySynonymInstance.hs, DataFamilySynonymInstance.o )
DataFamilySynonymInstance.hs:5:20: error:
• Type variable ‘a’ is mentioned in the RHS,
but not bound on the LHS of the family instance
The real LHS (expanding synonyms) is: F (D a)
• In the type instance declaration for ‘F’
|
5 | type instance F (D a) = a
| ^
```
GHC 8.6.5 accepts this program, as I am fairly certain it ought to.
## Environment
* GHC version used: 8.9.0.20190816 (built with Hadrian `--flavour=prof`)
* Operating System: macOS 10.14.5
* System Architecture: x86_64
issue