Implement `-Wpartial-fields` warning (#7169)

Warning on declaring a partial record selector.
However, disable warn with field names that start with underscore.

Test Plan: Added 1 test case.

Reviewers: austin, bgamari, simonpj

Reviewed By: bgamari, simonpj

Subscribers: goldfire, simonpj, duog, rwbarton, thomie

GHC Trac Issues: #7169

Differential Revision: https://phabricator.haskell.org/D4083

2 files changed, 45 insertions, 1 deletions

diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index 4c62a0d464..7602b719cc 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -674,6 +674,7 @@ data WarningFlag =
    | Opt_WarnCPPUndef                     -- Since 8.2
    | Opt_WarnUnbangedStrictPatterns       -- Since 8.2
    | Opt_WarnMissingHomeModules           -- Since 8.2
+   | Opt_WarnPartialFields                -- Since 8.4
    deriving (Eq, Show, Enum)
 
 data Language = Haskell98 | Haskell2010
@@ -3665,7 +3666,8 @@ wWarningFlagsDeps = [
                                     Opt_WarnMissingPatternSynonymSignatures,
   flagSpec "simplifiable-class-constraints" Opt_WarnSimplifiableClassConstraints,
   flagSpec "missing-home-modules"        Opt_WarnMissingHomeModules,
-  flagSpec "unrecognised-warning-flags"  Opt_WarnUnrecognisedWarningFlags ]
+  flagSpec "unrecognised-warning-flags"  Opt_WarnUnrecognisedWarningFlags,
+  flagSpec "partial-fields"              Opt_WarnPartialFields ]
 
 -- | These @-\<blah\>@ flags can all be reversed with @-no-\<blah\>@
 negatableFlagsDeps :: [(Deprecation, FlagSpec GeneralFlag)]
diff --git a/compiler/typecheck/TcTyClsDecls.hs b/compiler/typecheck/TcTyClsDecls.hs
index b4b31e3d50..cf92638b6c 100644
--- a/compiler/typecheck/TcTyClsDecls.hs
+++ b/compiler/typecheck/TcTyClsDecls.hs
@@ -2355,6 +2355,7 @@ checkValidTyCon tc
                ; let ex_ok = existential_ok || gadt_ok
                      -- Data cons can have existential context
                ; mapM_ (checkValidDataCon dflags ex_ok tc) data_cons
+               ; mapM_ (checkPartialRecordField data_cons) (tyConFieldLabels tc)
 
                 -- Check that fields with the same name share a type
                ; mapM_ check_fields groups }}
@@ -2401,6 +2402,29 @@ checkValidTyCon tc
                 (_, _, _, res2) = dataConSig con2
                 fty2 = dataConFieldType con2 lbl
 
+checkPartialRecordField :: [DataCon] -> FieldLabel -> TcM ()
+-- Check the partial record field selector, and warns.
+-- See Note [Checking partial record field]
+checkPartialRecordField all_cons fld
+  = setSrcSpan loc $
+      warnIfFlag Opt_WarnPartialFields
+        (not is_exhaustive && not (startsWithUnderscore occ_name))
+        (sep [text "Use of partial record field selector" <> colon,
+              nest 2 $ quotes (ppr occ_name)])
+  where
+    sel_name = flSelector fld
+    loc    = getSrcSpan sel_name
+    occ_name = getOccName sel_name
+
+    (cons_with_field, cons_without_field) = partition has_field all_cons
+    has_field con = fld `elem` (dataConFieldLabels con)
+    is_exhaustive = all (dataConCannotMatch inst_tys) cons_without_field
+
+    con1 = ASSERT( not (null cons_with_field) ) head cons_with_field
+    (univ_tvs, _, eq_spec, _, _, _) = dataConFullSig con1
+    eq_subst = mkTvSubstPrs (map eqSpecPair eq_spec)
+    inst_tys = substTyVars eq_subst univ_tvs
+
 checkFieldCompat :: FieldLabelString -> DataCon -> DataCon
                  -> Type -> Type -> Type -> Type -> TcM ()
 checkFieldCompat fld con1 con2 res1 res2 fty1 fty2
@@ -2958,6 +2982,24 @@ tcSplitSigmaTy. tcSplitNestedSigmaTys will always split any foralls that it
 sees until it can't go any further, so if you called it on the default type
 signature for `each`, it would return (a -> f b) -> s -> f t like we desired.
 
+Note [Checking partial record field]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+This check checks the partial record field selector, and warns (Trac #7169).
+
+For example:
+
+  data T a = A { m1 :: a, m2 :: a } | B { m1 :: a }
+
+The function 'm2' is partial record field, and will fail when it is applied to
+'B'. The warning identifies such partial fields. The check is performed at the
+declaration of T, not at the call-sites of m2.
+
+The warning can be suppressed by prefixing the field-name with an underscore.
+For example:
+
+  data T a = A { m1 :: a, _m2 :: a } | B { m1 :: a }
+
+
 ************************************************************************
 *                                                                      *
                 Checking role validity