From b6698360c4d663cd3597975a68d4790005cd17e0 Mon Sep 17 00:00:00 2001
From: Kari Pahula <kari.pahula@vincit.fi>
Date: Wed, 11 Sep 2019 14:00:23 +0300
Subject: [PATCH] Add -Wderiving-defaults (#15839)
Combining DeriveAnyClass and GeneralizedNewtypeDeriving can cause
warnings about defaulting with a deriving strategy. This change adds
an enable/suppress flag for it.
---
compiler/main/DynFlags.hs | 3 +++
compiler/typecheck/TcDeriv.hs | 6 ++++--
docs/users_guide/using-warnings.rst | 15 +++++++++++++++
3 files changed, 22 insertions(+), 2 deletions(-)
diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index c2d0322cd9e5..24d527e56fe7 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -919,6 +919,7 @@ data WarningFlag =
| Opt_WarnUnusedPackages -- Since 8.10
| Opt_WarnInferredSafeImports -- Since 8.10
| Opt_WarnMissingSafeHaskellMode -- Since 8.10
+ | Opt_WarnDerivingDefaults
deriving (Eq, Show, Enum)
data Language = Haskell98 | Haskell2010
@@ -4025,6 +4026,7 @@ wWarningFlagsDeps = [
Opt_WarnDeferredOutOfScopeVariables,
flagSpec "deprecations" Opt_WarnWarningsDeprecations,
flagSpec "deprecated-flags" Opt_WarnDeprecatedFlags,
+ flagSpec "deriving-defaults" Opt_WarnDerivingDefaults,
flagSpec "deriving-typeable" Opt_WarnDerivingTypeable,
flagSpec "dodgy-exports" Opt_WarnDodgyExports,
flagSpec "dodgy-foreign-imports" Opt_WarnDodgyForeignImports,
@@ -4822,6 +4824,7 @@ standardWarnings -- see Note [Documenting warning flags]
Opt_WarnPartialTypeSignatures,
Opt_WarnUnrecognisedPragmas,
Opt_WarnDuplicateExports,
+ Opt_WarnDerivingDefaults,
Opt_WarnOverflowedLiterals,
Opt_WarnEmptyEnumerations,
Opt_WarnMissingFields,
diff --git a/compiler/typecheck/TcDeriv.hs b/compiler/typecheck/TcDeriv.hs
index 0863e22cb9c0..52417fbe8203 100644
--- a/compiler/typecheck/TcDeriv.hs
+++ b/compiler/typecheck/TcDeriv.hs
@@ -1755,8 +1755,10 @@ mkNewTypeEqn
-- enabled, we take the diplomatic approach of defaulting to
-- DeriveAnyClass, but emitting a warning about the choice.
-- See Note [Deriving strategies]
- when (newtype_deriving && deriveAnyClass) $
- lift $ addWarnTc NoReason $ sep
+ when (newtype_deriving && deriveAnyClass) $ do
+ dyn_flags <- getDynFlags
+ when (wopt Opt_WarnDerivingDefaults dyn_flags) $
+ lift $ addWarnTc NoReason $ sep
[ text "Both DeriveAnyClass and"
<+> text "GeneralizedNewtypeDeriving are enabled"
, text "Defaulting to the DeriveAnyClass strategy"
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index dda7bb656c8d..6271f5c8924a 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -26,6 +26,7 @@ generally likely to indicate bugs in your program. These are:
* :ghc-flag:`-Wdeprecated-flags`
* :ghc-flag:`-Wunrecognised-pragmas`
* :ghc-flag:`-Wduplicate-exports`
+ * :ghc-flag:`-Wderiving-defaults`
* :ghc-flag:`-Woverflowed-literals`
* :ghc-flag:`-Wempty-enumerations`
* :ghc-flag:`-Wmissing-fields`
@@ -640,6 +641,20 @@ of ``-W(no-)*``.
Causes a warning to be emitted if an enumeration is empty, e.g.
``[5 .. 3]``.
+.. ghc-flag:: -Wderiving-defaults
+ :shortdesc: warn about default deriving when using both
+ DeriveAnyClass and GeneralizedNewtypeDeriving
+ :type: dynamic
+ :reverse: -Wno-deriving-defaults
+ :category:
+
+ Causes a warning when using both :ref:`DeriveAnyClass` and
+ :ref:`GeneralizedNewtypeDeriving` and the compiler defaults to
+ using the first one of them. For example ::
+
+ class C a
+ newtype T a = MkT a deriving C
+
.. ghc-flag:: -Wduplicate-constraints
:shortdesc: warn when a constraint appears duplicated in a type signature
:type: dynamic
--
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