Include -fwarn-star-is-type in -Wcompat

According to the deprecation schedule in the accepted proposal,
the first step is to include `-fwarn-star-is-type` in `-Wcompat`.

Test Plan: Validate

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: #15476

Differential Revision: https://phabricator.haskell.org/D5044

6 files changed, 19 insertions, 2 deletions

diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index f9ccc25225..2c8f13418e 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -4614,6 +4614,7 @@ minusWcompatOpts
       , Opt_WarnSemigroup
       , Opt_WarnNonCanonicalMonoidInstances
       , Opt_WarnImplicitKindVars
+      , Opt_WarnStarIsType
       ]
 
 enableUnusedBinds :: DynP ()
diff --git a/compiler/parser/RdrHsSyn.hs b/compiler/parser/RdrHsSyn.hs
index 1015319986..1e89d5a459 100644
--- a/compiler/parser/RdrHsSyn.hs
+++ b/compiler/parser/RdrHsSyn.hs
@@ -2049,7 +2049,8 @@ warnStarIsType span = addWarning Opt_WarnStarIsType span msg
     msg =  text "Using" <+> quotes (text "*")
            <+> text "(or its Unicode variant) to mean"
            <+> quotes (text "Data.Kind.Type")
-        $$ text "relies on the StarIsType extension."
+        $$ text "relies on the StarIsType extension, which will become"
+        $$ text "deprecated in the future."
         $$ text "Suggested fix: use" <+> quotes (text "Type")
            <+> text "from" <+> quotes (text "Data.Kind") <+> text "instead."
 
diff --git a/docs/users_guide/8.8.1-notes.rst b/docs/users_guide/8.8.1-notes.rst
index a27aee721e..d3da37e9eb 100644
--- a/docs/users_guide/8.8.1-notes.rst
+++ b/docs/users_guide/8.8.1-notes.rst
@@ -59,6 +59,8 @@ Compiler
 
 - New :ghc-flag:`-keep-hscpp-files` to keep the output of the CPP pre-processor.
 
+- The :ghc-flag:`-Wcompat` warning group now includes :ghc-flag:`-Wstar-is-type`.
+
 Runtime system
 ~~~~~~~~~~~~~~
 
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index dba30dbf1e..aeabbe9713 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -117,6 +117,7 @@ The following flags are simple ways to select standard "packages" of warnings:
         * :ghc-flag:`-Wsemigroup`
         * :ghc-flag:`-Wnoncanonical-monoid-instances`
         * :ghc-flag:`-Wimplicit-kind-vars`
+        * :ghc-flag:`-Wstar-is-type`
 
 .. ghc-flag:: -Wno-compat
     :shortdesc: Disables all warnings enabled by :ghc-flag:`-Wcompat`.
@@ -1194,6 +1195,9 @@ of ``-W(no-)*``.
      breaking change takes place. The recommended fix is to replace ``*`` with
      ``Type`` imported from ``Data.Kind``.
 
+     Being part of the :ghc-flag:`-Wcompat` option group, this warning is off by
+     default, but will be switched on in a future GHC release.
+
 .. ghc-flag:: -Wstar-binder
      :shortdesc: warn about binding the ``(*)`` type operator despite
          :ghc-flag:`-XStarIsType`
diff --git a/testsuite/tests/wcompat-warnings/Template.hs b/testsuite/tests/wcompat-warnings/Template.hs
index e3423c8b9f..03f9a4957e 100644
--- a/testsuite/tests/wcompat-warnings/Template.hs
+++ b/testsuite/tests/wcompat-warnings/Template.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE NoMonadFailDesugaring #-}
+{-# LANGUAGE NoMonadFailDesugaring, KindSignatures #-}
 
 module WCompatWarningsOnOff where
 
@@ -21,3 +21,6 @@ instance Monoid S where
   S a `mappend` S b = S (a+b)
   mempty = S 0
 
+-- -fwarn-star-is-type
+b :: (Bool :: *)
+b = True
diff --git a/testsuite/tests/wcompat-warnings/WCompatWarningsOn.stderr b/testsuite/tests/wcompat-warnings/WCompatWarningsOn.stderr
index 5c2d9c5428..3c3e73ddf6 100644
--- a/testsuite/tests/wcompat-warnings/WCompatWarningsOn.stderr
+++ b/testsuite/tests/wcompat-warnings/WCompatWarningsOn.stderr
@@ -33,3 +33,9 @@ Template.hs:21:3: warning: [-Wnoncanonical-monoid-instances (in -Wcompat)]
     Noncanonical ‘mappend’ definition detected
     in the instance declaration for ‘Monoid S’.
     Define as ‘mappend = (<>)’
+
+Template.hs:25:15: warning: [-Wstar-is-type (in -Wcompat)]
+    Using ‘*’ (or its Unicode variant) to mean ‘Data.Kind.Type’
+    relies on the StarIsType extension, which will become
+    deprecated in the future.
+    Suggested fix: use ‘Type’ from ‘Data.Kind’ instead.