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-rw-r--r--compiler/main/DynFlags.hs2
-rw-r--r--compiler/rename/RnSource.hs25
-rw-r--r--compiler/typecheck/TcDerivUtils.hs2
-rw-r--r--docs/users_guide/8.8.1-notes.rst4
-rw-r--r--docs/users_guide/extending_ghc.rst1
-rw-r--r--docs/users_guide/using-warnings.rst21
-rw-r--r--testsuite/tests/rename/should_compile/T15798a.hs12
-rw-r--r--testsuite/tests/rename/should_compile/T15798a.stderr3
-rw-r--r--testsuite/tests/rename/should_compile/T15798b.hs9
-rw-r--r--testsuite/tests/rename/should_compile/T15798b.stderr4
-rw-r--r--testsuite/tests/rename/should_compile/T15798c.hs6
-rw-r--r--testsuite/tests/rename/should_compile/T15798c.stderr4
-rw-r--r--testsuite/tests/rename/should_compile/all.T4
13 files changed, 96 insertions, 1 deletions
diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index 02ad366338..7296809155 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -834,6 +834,7 @@ data WarningFlag =
| Opt_WarnStarBinder -- Since 8.6
| Opt_WarnImplicitKindVars -- Since 8.6
| Opt_WarnSpaceAfterBang
+ | Opt_WarnMissingDerivingStrategies -- Since 8.8
deriving (Eq, Show, Enum)
data Language = Haskell98 | Haskell2010
@@ -4022,6 +4023,7 @@ wWarningFlagsDeps = [
flagSpec "wrong-do-bind" Opt_WarnWrongDoBind,
flagSpec "missing-pattern-synonym-signatures"
Opt_WarnMissingPatternSynonymSignatures,
+ flagSpec "missing-deriving-strategies" Opt_WarnMissingDerivingStrategies,
flagSpec "simplifiable-class-constraints" Opt_WarnSimplifiableClassConstraints,
flagSpec "missing-home-modules" Opt_WarnMissingHomeModules,
flagSpec "unrecognised-warning-flags" Opt_WarnUnrecognisedWarningFlags,
diff --git a/compiler/rename/RnSource.hs b/compiler/rename/RnSource.hs
index 5ec4e05cd1..ca35e941fb 100644
--- a/compiler/rename/RnSource.hs
+++ b/compiler/rename/RnSource.hs
@@ -1004,6 +1004,7 @@ rnSrcDerivDecl (DerivDecl _ ty mds overlap)
<- rnLDerivStrategy DerivDeclCtx mds $ \strat_tvs ppr_via_ty ->
rnAndReportFloatingViaTvs strat_tvs loc ppr_via_ty "instance" $
rnHsSigWcType BindUnlessForall DerivDeclCtx ty
+ ; warnNoDerivStrat mds' loc
; return (DerivDecl noExt ty' mds' overlap, fvs) }
where
loc = getLoc $ hsib_body $ hswc_body ty
@@ -1705,6 +1706,29 @@ rnDataDefn doc (HsDataDefn { dd_ND = new_or_data, dd_cType = cType
; return (cL loc ds', fvs) }
rnDataDefn _ (XHsDataDefn _) = panic "rnDataDefn"
+warnNoDerivStrat :: Maybe (LDerivStrategy GhcRn)
+ -> SrcSpan
+ -> RnM ()
+warnNoDerivStrat mds loc
+ = do { dyn_flags <- getDynFlags
+ ; when (wopt Opt_WarnMissingDerivingStrategies dyn_flags) $
+ case mds of
+ Nothing -> addWarnAt
+ (Reason Opt_WarnMissingDerivingStrategies)
+ loc
+ (if xopt LangExt.DerivingStrategies dyn_flags
+ then no_strat_warning
+ else no_strat_warning $+$ deriv_strat_nenabled
+ )
+ _ -> pure ()
+ }
+ where
+ no_strat_warning :: SDoc
+ no_strat_warning = text "No deriving strategy specified. Did you want stock"
+ <> text ", newtype, or anyclass?"
+ deriv_strat_nenabled :: SDoc
+ deriv_strat_nenabled = text "Use DerivingStrategies to specify a strategy."
+
rnLHsDerivingClause :: HsDocContext -> LHsDerivingClause GhcPs
-> RnM (LHsDerivingClause GhcRn, FreeVars)
rnLHsDerivingClause doc
@@ -1715,6 +1739,7 @@ rnLHsDerivingClause doc
= do { (dcs', dct', fvs)
<- rnLDerivStrategy doc dcs $ \strat_tvs ppr_via_ty ->
mapFvRn (rn_deriv_ty strat_tvs ppr_via_ty) dct
+ ; warnNoDerivStrat dcs' loc
; pure ( cL loc (HsDerivingClause { deriv_clause_ext = noExt
, deriv_clause_strategy = dcs'
, deriv_clause_tys = cL loc' dct' })
diff --git a/compiler/typecheck/TcDerivUtils.hs b/compiler/typecheck/TcDerivUtils.hs
index 86205de5fd..5f48e5f003 100644
--- a/compiler/typecheck/TcDerivUtils.hs
+++ b/compiler/typecheck/TcDerivUtils.hs
@@ -62,7 +62,7 @@ import ListSetOps (assocMaybe)
-- is a simple reader around 'TcRn'.
type DerivM = ReaderT DerivEnv TcRn
--- | Is GHC processing a stanalone deriving declaration?
+-- | Is GHC processing a standalone deriving declaration?
isStandaloneDeriv :: DerivM Bool
isStandaloneDeriv = asks (go . denv_ctxt)
where
diff --git a/docs/users_guide/8.8.1-notes.rst b/docs/users_guide/8.8.1-notes.rst
index 69d539748d..cd4c00d1a4 100644
--- a/docs/users_guide/8.8.1-notes.rst
+++ b/docs/users_guide/8.8.1-notes.rst
@@ -84,6 +84,10 @@ Compiler
- The deprecated ghc-flag ``-Wamp`` has been removed.
+- Add new :ghc-flag:`-Wmissing-deriving-strategies` flag that warns users when they are not
+ taking advantage of :extension:`DerivingStrategies`. The warning is supplied at each
+ ``deriving`` site.
+
Runtime system
~~~~~~~~~~~~~~
diff --git a/docs/users_guide/extending_ghc.rst b/docs/users_guide/extending_ghc.rst
index a913684a15..02847c9b86 100644
--- a/docs/users_guide/extending_ghc.rst
+++ b/docs/users_guide/extending_ghc.rst
@@ -851,6 +851,7 @@ In general, the ``pluginRecompile`` field has the following type::
The ``PluginRecompile`` data type is an enumeration determining how the plugin
should affect recompilation. ::
+
data PluginRecompile = ForceRecompile | NoForceRecompile | MaybeRecompile Fingerprint
A plugin which declares itself impure using ``ForceRecompile`` will always
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index 6a6166bf0d..03ca184531 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -904,6 +904,27 @@ of ``-W(no-)*``.
This option isn't enabled by default because it can be very noisy,
and it often doesn't indicate a bug in the program.
+.. ghc-flag:: -Wmissing-deriving-strategies
+ :shortdesc: warn when a deriving clause is missing a deriving strategy
+ :type: dynamic
+ :reverse: -Wno-missing-deriving-strategies
+ :category:
+
+ :since: 8.8.1
+
+ The datatype below derives the ``Eq`` typeclass, but doesn't specify a
+ strategy. When :ghc-flag:`-Wmissing-deriving-strategies` is enabled,
+ the compiler will emit a warning about this. ::
+
+ data Foo a = Foo a
+ deriving (Eq)
+
+ The compiler will warn here that the deriving clause doesn't specify a
+ strategy. If the warning is enabled, but :extension:`DerivingStrategies` is
+ not enabled, the compiler will suggest turning on the
+ :extension:`DerivingStrategies` extension. This option is not on by default,
+ having to be turned on manually or with :ghc-flag:`-Weverything`.
+
.. ghc-flag:: -Wmissing-fields
:shortdesc: warn when fields of a record are uninitialised
:type: dynamic
diff --git a/testsuite/tests/rename/should_compile/T15798a.hs b/testsuite/tests/rename/should_compile/T15798a.hs
new file mode 100644
index 0000000000..d34e55bdab
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T15798a.hs
@@ -0,0 +1,12 @@
+{-# LANGUAGE DerivingStrategies #-}
+
+{-# OPTIONS_GHC -Wmissing-deriving-strategies #-}
+
+module T15798a () where
+
+data Foo a = Foo a
+ deriving stock (Eq)
+
+data Bar a = Bar a
+ deriving (Eq, Show)
+ deriving stock (Ord)
diff --git a/testsuite/tests/rename/should_compile/T15798a.stderr b/testsuite/tests/rename/should_compile/T15798a.stderr
new file mode 100644
index 0000000000..6832205228
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T15798a.stderr
@@ -0,0 +1,3 @@
+
+T15798a.hs:11:3: warning: [-Wmissing-deriving-strategies]
+ No deriving strategy specified. Did you want stock, newtype, or anyclass?
diff --git a/testsuite/tests/rename/should_compile/T15798b.hs b/testsuite/tests/rename/should_compile/T15798b.hs
new file mode 100644
index 0000000000..8504d6fee1
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T15798b.hs
@@ -0,0 +1,9 @@
+{-# LANGUAGE StandaloneDeriving #-}
+
+{-# OPTIONS_GHC -Wmissing-deriving-strategies #-}
+
+module T15798b () where
+
+data Foo a = Foo a
+
+deriving instance Eq a => Eq (Foo a)
diff --git a/testsuite/tests/rename/should_compile/T15798b.stderr b/testsuite/tests/rename/should_compile/T15798b.stderr
new file mode 100644
index 0000000000..de1b09ee48
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T15798b.stderr
@@ -0,0 +1,4 @@
+
+T15798b.hs:9:19: warning: [-Wmissing-deriving-strategies]
+ No deriving strategy specified. Did you want stock, newtype, or anyclass?
+ Use DerivingStrategies to specify a strategy.
diff --git a/testsuite/tests/rename/should_compile/T15798c.hs b/testsuite/tests/rename/should_compile/T15798c.hs
new file mode 100644
index 0000000000..f3048eded5
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T15798c.hs
@@ -0,0 +1,6 @@
+{-# OPTIONS_GHC -Wmissing-deriving-strategies #-}
+
+module T15798c () where
+
+data Foo a = Foo a
+ deriving (Eq)
diff --git a/testsuite/tests/rename/should_compile/T15798c.stderr b/testsuite/tests/rename/should_compile/T15798c.stderr
new file mode 100644
index 0000000000..868266aa3a
--- /dev/null
+++ b/testsuite/tests/rename/should_compile/T15798c.stderr
@@ -0,0 +1,4 @@
+
+T15798c.hs:6:3: warning: [-Wmissing-deriving-strategies]
+ No deriving strategy specified. Did you want stock, newtype, or anyclass?
+ Use DerivingStrategies to specify a strategy.
diff --git a/testsuite/tests/rename/should_compile/all.T b/testsuite/tests/rename/should_compile/all.T
index b6c06c151c..0bcd25ccf1 100644
--- a/testsuite/tests/rename/should_compile/all.T
+++ b/testsuite/tests/rename/should_compile/all.T
@@ -163,3 +163,7 @@ test('T14747', [], multimod_compile, ['T14747', '-v0'])
test('T15149', [], multimod_compile, ['T15149', '-v0'])
test('T13064', normal, compile, [''])
test('T15994', [], run_command, ['$MAKE -s --no-print-directory T15994'])
+test('T15798a', normal, compile, [''])
+test('T15798b', normal, compile, [''])
+test('T15798c', normal, compile, [''])
+
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