Commit fa25c8c4 authored by Richard Eisenberg's avatar Richard Eisenberg Committed by Marge Bot

Update release notes about #16512 / #17405.

parent 6f4c1250
......@@ -141,6 +141,18 @@ Language
inferred type is higher-rank. GHC 8.10 will now reject this, as one must now
enable :ghc-flag:`-XRankNTypes` in ``B`` to accept the inferred type signature.
- Type family dependencies (also known as injective type families)
sometimes now need ``-XUndecidableInstances`` in order to be
accepted. Here is an example::
type family F1 a = r | r -> a
type family F2 a = r | r -> a
type instance F2 [a] = Maybe (F1 a)
Because GHC needs to look under a type family to see that ``a`` is determined
by the right-hand side of ``F2``\'s equation, this now needs ``-XUndecidableInstances``.
The problem is very much akin to its need to detect some functional dependencies.
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