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We weren't linting the types used in `newFamInst`, which
might have been why #15012 went undiscovered for so long. Let's fix
that.
One has to be surprisingly careful with expanding type synonyms in
`lintType`, since in the offending program (simplified):
```lang=haskell
type FakeOut a = Int
type family TF a
type instance TF Int = FakeOut a
```
If one expands type synonyms, then `FakeOut a` will expand to
`Int`, which masks the issue (that `a` is unbound). I added an
extra Lint flag to configure whether type synonyms should be
expanded or not in Lint, and disabled this when calling `lintTypes`
from `newFamInst`.
As evidence that this works, I ran it on the offending program
from #15012, and voilà:
```
$ ghc3/inplace/bin/ghc-stage2 Bug.hs -dcore-lint
[1 of 1] Compiling Foo ( Bug.hs, Bug.o )
ghc-stage2: panic! (the 'impossible' happened)
(GHC version 8.5.20180417 for x86_64-unknown-linux):
Core Lint error
<no location info>: warning:
In the type ‘... (Rec0 (FakeOut b_a1Qt))))’
@ b_a1Qt is out of scope
```
Test Plan: make test TEST=T15057
Reviewers: simonpj, goldfire, bgamari
Reviewed By: bgamari
Subscribers: thomie, carter
GHC Trac Issues: #15057
Differential Revision: https://phabricator.haskell.org/D4611
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