Introduce -Wcompat-unqualified-imports

This implements the warning proposed in option (B) of the
Data.List.singleton CLC [discussion][].
This warning, which is included in `-Wcompat` is intended to help users
identify imports of modules that will change incompatibly in future GHC
releases. This currently only includes `Data.List` due to the expected
specialisation and addition of `Data.List.singleton`.

Fixes #17244.

[discussion]: https://groups.google.com/d/msg/haskell-core-libraries/q3zHLmzBa5E/PmlAs_kYAQAJ

11 files changed, 102 insertions, 5 deletions

diff --git a/compiler/GHC/Rename/Names.hs b/compiler/GHC/Rename/Names.hs
index ecf82fffa0..b666b89875 100644
--- a/compiler/GHC/Rename/Names.hs
+++ b/compiler/GHC/Rename/Names.hs
@@ -380,6 +380,9 @@ rnImportDecl this_mod
           _           -> return ()
      )
 
+    -- Complain about -Wcompat-unqualified-imports violations.
+    warnUnqualifiedImport decl iface
+
     let new_imp_decl = L loc (decl { ideclExt = noExtField, ideclSafe = mod_safe'
                                    , ideclHiding = new_imp_details })
 
@@ -487,6 +490,40 @@ calculateAvails dflags iface mod_safe' want_boot imported_by =
      }
 
 
+-- | Issue a warning if the user imports Data.List without either an import
+-- list or `qualified`. This is part of the migration plan for the
+-- `Data.List.singleton` proposal. See #17244.
+warnUnqualifiedImport :: ImportDecl GhcPs -> ModIface -> RnM ()
+warnUnqualifiedImport decl iface =
+    whenWOptM Opt_WarnCompatUnqualifiedImports
+    $ when bad_import
+    $ addWarnAt (Reason Opt_WarnCompatUnqualifiedImports) loc warning
+  where
+    mod = mi_module iface
+    loc = getLoc $ ideclName decl
+
+    is_qual = isImportDeclQualified (ideclQualified decl)
+    has_import_list =
+      -- We treat a `hiding` clause as not having an import list although
+      -- it's not entirely clear this is the right choice.
+      case ideclHiding decl of
+        Just (False, _) -> True
+        _               -> False
+    bad_import =
+      mod `elemModuleSet` qualifiedMods
+      && not is_qual
+      && not has_import_list
+
+    warning = vcat
+      [ text "To ensure compatibility with future core libraries changes"
+      , text "imports to" <+> ppr (ideclName decl) <+> text "should be"
+      , text "either qualified or have an explicit import list."
+      ]
+
+    -- Modules for which we warn if we see unqualified imports
+    qualifiedMods = mkModuleSet [ dATA_LIST ]
+
+
 warnRedundantSourceImport :: ModuleName -> SDoc
 warnRedundantSourceImport mod_name
   = text "Unnecessary {-# SOURCE #-} in the import of module"
diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index be40ff9e2e..3064c1991d 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -926,6 +926,7 @@ data WarningFlag =
    | Opt_WarnUnusedPackages               -- Since 8.10
    | Opt_WarnInferredSafeImports          -- Since 8.10
    | Opt_WarnMissingSafeHaskellMode       -- Since 8.10
+   | Opt_WarnCompatUnqualifiedImports     -- Since 8.10
    | Opt_WarnDerivingDefaults
    deriving (Eq, Show, Enum)
 
@@ -4163,7 +4164,8 @@ wWarningFlagsDeps = [
   flagSpec "partial-fields"              Opt_WarnPartialFields,
   flagSpec "prepositive-qualified-module"
                                          Opt_WarnPrepositiveQualifiedModule,
-  flagSpec "unused-packages"             Opt_WarnUnusedPackages
+  flagSpec "unused-packages"             Opt_WarnUnusedPackages,
+  flagSpec "compat-unqualified-imports"  Opt_WarnCompatUnqualifiedImports
  ]
 
 -- | These @-\<blah\>@ flags can all be reversed with @-no-\<blah\>@
@@ -4931,6 +4933,7 @@ minusWcompatOpts
       , Opt_WarnSemigroup
       , Opt_WarnNonCanonicalMonoidInstances
       , Opt_WarnStarIsType
+      , Opt_WarnCompatUnqualifiedImports
       ]
 
 enableUnusedBinds :: DynP ()
diff --git a/compiler/prelude/PrelNames.hs b/compiler/prelude/PrelNames.hs
index 17aee23fab..095b853927 100644
--- a/compiler/prelude/PrelNames.hs
+++ b/compiler/prelude/PrelNames.hs
@@ -502,7 +502,7 @@ gHC_PRIM, gHC_TYPES, gHC_GENERICS, gHC_MAGIC,
     gHC_CLASSES, gHC_PRIMOPWRAPPERS, gHC_BASE, gHC_ENUM,
     gHC_GHCI, gHC_GHCI_HELPERS, gHC_CSTRING,
     gHC_SHOW, gHC_READ, gHC_NUM, gHC_MAYBE, gHC_INTEGER_TYPE, gHC_NATURAL,
-    gHC_LIST, gHC_TUPLE, dATA_TUPLE, dATA_EITHER, dATA_STRING,
+    gHC_LIST, gHC_TUPLE, dATA_TUPLE, dATA_EITHER, dATA_LIST, dATA_STRING,
     dATA_FOLDABLE, dATA_TRAVERSABLE,
     gHC_CONC, gHC_IO, gHC_IO_Exception,
     gHC_ST, gHC_IX, gHC_STABLE, gHC_PTR, gHC_ERR, gHC_REAL,
@@ -534,6 +534,7 @@ gHC_LIST        = mkBaseModule (fsLit "GHC.List")
 gHC_TUPLE       = mkPrimModule (fsLit "GHC.Tuple")
 dATA_TUPLE      = mkBaseModule (fsLit "Data.Tuple")
 dATA_EITHER     = mkBaseModule (fsLit "Data.Either")
+dATA_LIST       = mkBaseModule (fsLit "Data.List")
 dATA_STRING     = mkBaseModule (fsLit "Data.String")
 dATA_FOLDABLE   = mkBaseModule (fsLit "Data.Foldable")
 dATA_TRAVERSABLE= mkBaseModule (fsLit "Data.Traversable")
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index 586af57136..aab74d36c2 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -124,6 +124,7 @@ The following flags are simple ways to select standard "packages" of warnings:
         * :ghc-flag:`-Wsemigroup`
         * :ghc-flag:`-Wnoncanonical-monoid-instances`
         * :ghc-flag:`-Wstar-is-type`
+        * :ghc-flag:`-Wcompat-unqualified-imports`
 
 .. ghc-flag:: -Wno-compat
     :shortdesc: Disables all warnings enabled by :ghc-flag:`-Wcompat`.
@@ -219,12 +220,31 @@ of ``-W(no-)*``.
         encountered on the command line.
     :type: dynamic
     :reverse: -Wno-unrecognised-warning-flags
+    :default: on
     :category:
 
     Enables warnings when the compiler encounters a ``-W...`` flag that is not
     recognised.
 
-    This warning is on by default.
+.. ghc-flag:: -Wcompat-unqualified-imports
+    :shortdesc: Report unqualified imports of core libraries which are expected
+      to cause compatibility problems in future releases.
+    :type: dynamic
+    :reverse: -Wno-compat-unqualified-imports
+    :category:
+
+    Warns on qualified imports of core library modules which are subject to
+    change in future GHC releases. Currently the following modules are covered
+    by this warning:
+
+     - ``Data.List`` due to the future addition of ``Data.List.singleton`` and
+       specialisation of exports to the ``[]`` type. See the
+       :ref:`mailing list
+       <https://groups.google.com/forum/#!topic/haskell-core-libraries/q3zHLmzBa5E>`
+       for details.
+
+    This warning can be addressed by either adding an explicit import list or
+    using a ``qualified`` import.
 
 .. ghc-flag:: -Wtyped-holes
     :shortdesc: Report warnings when :ref:`typed hole <typed-holes>` errors are
@@ -232,14 +252,13 @@ of ``-W(no-)*``.
         :ghc-flag:`-fdefer-typed-holes`.
     :type: dynamic
     :reverse: -Wno-typed-holes
+    :default: on
     :category:
 
     Determines whether the compiler reports typed holes warnings. Has no
     effect unless typed holes errors are deferred until runtime. See
     :ref:`typed-holes` and :ref:`defer-type-errors`
 
-    This warning is on by default.
-
 .. ghc-flag:: -Wdeferred-type-errors
     :shortdesc: Report warnings when :ref:`deferred type errors
         <defer-type-errors>` are enabled. This option is enabled by
diff --git a/testsuite/tests/rename/should_compile/T17244A.hs b/testsuite/tests/rename/should_compile/T17244A.hs
new file mode 100644
index 0000000000..290120affd
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T17244A.hs
@@ -0,0 +1,9 @@
+{-# OPTIONS_GHC -Wcompat-unqualified-imports #-}
+
+module T17244A (hello) where
+
+-- This should warn with -Wcompat-unqualified-imports.
+import Data.List
+
+hello :: [Int] -> Int
+hello = sum
diff --git a/testsuite/tests/rename/should_compile/T17244A.stderr b/testsuite/tests/rename/should_compile/T17244A.stderr
new file mode 100644
index 0000000000..621e9439f1
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T17244A.stderr
@@ -0,0 +1,5 @@
+
+T17244A.hs:6:8: warning: [-Wcompat-unqualified-imports (in -Wcompat)]
+    To ensure compatibility with future core libraries changes
+    imports to Data.List should be
+    either qualified or have an explicit import list.
diff --git a/testsuite/tests/rename/should_compile/T17244B.hs b/testsuite/tests/rename/should_compile/T17244B.hs
new file mode 100644
index 0000000000..61cbde3bee
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T17244B.hs
@@ -0,0 +1,10 @@
+{-# OPTIONS_GHC -Wcompat-unqualified-imports #-}
+
+module T17244B (hello) where
+
+-- This should not warn with -Wcompat-unqualified-imports.
+import qualified Data.List as List
+
+hello :: [Int] -> Int
+hello = List.sum
+
diff --git a/testsuite/tests/rename/should_compile/T17244B.stderr b/testsuite/tests/rename/should_compile/T17244B.stderr
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T17244B.stderr
diff --git a/testsuite/tests/rename/should_compile/T17244C.hs b/testsuite/tests/rename/should_compile/T17244C.hs
new file mode 100644
index 0000000000..3da92dddd6
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T17244C.hs
@@ -0,0 +1,10 @@
+{-# OPTIONS_GHC -Wcompat-unqualified-imports #-}
+
+module T17244C (hello) where
+
+-- This should not warn with -Wcompat-unqualified-imports.
+import Data.List (sum)
+
+hello :: [Int] -> Int
+hello = sum
+
diff --git a/testsuite/tests/rename/should_compile/T17244C.stderr b/testsuite/tests/rename/should_compile/T17244C.stderr
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T17244C.stderr
diff --git a/testsuite/tests/rename/should_compile/all.T b/testsuite/tests/rename/should_compile/all.T
index 51684f1eb3..b461d09076 100644
--- a/testsuite/tests/rename/should_compile/all.T
+++ b/testsuite/tests/rename/should_compile/all.T
@@ -169,3 +169,6 @@ test('T15798b', normal, compile, [''])
 test('T15798c', normal, compile, [''])
 test('T16116a', normal, compile, [''])
 test('T15957', normal, compile, ['-Werror -Wredundant-record-wildcards -Wunused-record-wildcards'])
+test('T17244A', normal, compile, ['-Wno-error=compat-unqualified-imports'])
+test('T17244B', normal, compile, [''])
+test('T17244C', normal, compile, [''])