9.8.4 breakage inventory

Closes:

* #139
* #140

breakage-inventory-ghc-9.8.4.md

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+# GHC 9.8.4 breakage inventory
+
+A catalogue of all the non-trivial fixes (that is, not just bounds
+bumps) required to upgrade Haskell code that worked with an earlier
+version of GHC 9.8 to GHC 9.8.4.  I am not aware of any migration
+guide for these changes.
+
+## `ghc-lib-parser`
+
+### Type of `getkey`
+
+In `ghc-lib-parser` 9.8.3...., [`getKey :: Unique ->
+Int`](https://hackage.haskell.org/package/ghc-lib-parser-9.8.3.20241103/docs/GHC-Types-Unique.html#v:getKey).
+In 9.8.4...., [`getKey :: Unique ->
+Word64`](https://hackage.haskell.org/package/ghc-lib-parser-9.8.4.20241130/docs/GHC-Types-Unique.html#v:getKey).
+This appears to be a PVP violation.  The type of an exported
+identifier should not change with a minor version bump.
+
+#### Examples
+
+* [`clash-lib`](https://github.com/clash-lang/clash-compiler/issues/2852)
+
+* [`calligraphy`](https://github.com/jonascarpay/calligraphy/issues/40)
+
+* [`ghc-typelits-presburger`](https://github.com/konn/ghc-typelits-presburger/issues/30)
+
+## `deepseq`
+
+### Added `Control.DeepSeq.Unit`
+
+`deepseq` added `Control.DeepSeq.Unit` which collides with
+`GHC.Unit.Types.Unit` when `Control.DeepSeq` is imported unqualified.
+
+#### Examples
+
+* [`ghc`](https://gitlab.haskell.org/ghc/ghc/-/issues/25411)
+
+* [`haskell-language-server`](https://github.com/haskell/haskell-language-server/pull/4459)