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Summary:
Implement the "Embrace Type :: Type" GHC proposal,
.../ghc-proposals/blob/master/proposals/0020-no-type-in-type.rst
GHC 8.0 included a major change to GHC's type system: the Type :: Type
axiom. Though casual users were protected from this by hiding its
features behind the -XTypeInType extension, all programs written in GHC
8+ have the axiom behind the scenes. In order to preserve backward
compatibility, various legacy features were left unchanged. For example,
with -XDataKinds but not -XTypeInType, GADTs could not be used in types.
Now these restrictions are lifted and -XTypeInType becomes a redundant
flag that will be eventually deprecated.
* Incorporate the features currently in -XTypeInType into the
-XPolyKinds and -XDataKinds extensions.
* Introduce a new extension -XStarIsType to control how to parse * in
code and whether to print it in error messages.
Test Plan: Validate
Reviewers: goldfire, hvr, bgamari, alanz, simonpj
Reviewed By: goldfire, simonpj
Subscribers: rwbarton, thomie, mpickering, carter
GHC Trac Issues: #15195
Differential Revision: https://phabricator.haskell.org/D4748
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