Implement -Wambiguous-fields

Fixes #18966. Adds a new warning -Wambiguous-fields for uses of field selectors
or record updates that will be rejected in the future, when the DuplicateRecordFields
extension is simplified per https://github.com/ghc-proposals/ghc-proposals/pull/366.

4 files changed, 49 insertions, 4 deletions

diff --git a/compiler/GHC/Driver/Flags.hs b/compiler/GHC/Driver/Flags.hs
index 3d0908caa0..7867d4bd04 100644
--- a/compiler/GHC/Driver/Flags.hs
+++ b/compiler/GHC/Driver/Flags.hs
@@ -505,6 +505,7 @@ data WarningFlag =
    | Opt_WarnInvalidHaddock               -- Since 8.12
    | Opt_WarnOperatorWhitespaceExtConflict  -- Since 9.2
    | Opt_WarnOperatorWhitespace             -- Since 9.2
+   | Opt_WarnAmbiguousFields                -- Since 9.2
    deriving (Eq, Show, Enum)
 
 -- | Used when outputting warnings: if a reason is given, it is
diff --git a/compiler/GHC/Driver/Session.hs b/compiler/GHC/Driver/Session.hs
index 7afcf7309c..a0a6153b0b 100644
--- a/compiler/GHC/Driver/Session.hs
+++ b/compiler/GHC/Driver/Session.hs
@@ -3055,6 +3055,7 @@ wWarningFlagsDeps = [
 -- Please keep the list of flags below sorted alphabetically
   flagSpec "alternative-layout-rule-transitional"
                                       Opt_WarnAlternativeLayoutRuleTransitional,
+  flagSpec "ambiguous-fields"            Opt_WarnAmbiguousFields,
   depFlagSpec "auto-orphans"             Opt_WarnAutoOrphans
     "it has no effect",
   flagSpec "cpp-undef"                   Opt_WarnCPPUndef,
@@ -3909,6 +3910,7 @@ standardWarnings -- see Note [Documenting warning flags]
         Opt_WarnDerivingDefaults,
         Opt_WarnOverflowedLiterals,
         Opt_WarnEmptyEnumerations,
+        Opt_WarnAmbiguousFields,
         Opt_WarnMissingFields,
         Opt_WarnMissingMethods,
         Opt_WarnWrongDoBind,
diff --git a/compiler/GHC/Tc/Gen/Expr.hs b/compiler/GHC/Tc/Gen/Expr.hs
index 8ad1e59796..dfb9d6abd9 100644
--- a/compiler/GHC/Tc/Gen/Expr.hs
+++ b/compiler/GHC/Tc/Gen/Expr.hs
@@ -1216,13 +1216,16 @@ disambiguateRecordBinds record_expr record_rho rbnds res_ty
 
         -- Multiple possible parents: try harder to disambiguate
         -- Can we get a parent TyCon from the pushed-in type?
-        _:_ | Just p <- tyConOfET fam_inst_envs res_ty -> return (RecSelData p)
+        _:_ | Just p <- tyConOfET fam_inst_envs res_ty ->
+              do { reportAmbiguousField p
+                 ; return (RecSelData p) }
 
         -- Does the expression being updated have a type signature?
         -- If so, try to extract a parent TyCon from it
             | Just {} <- obviousSig (unLoc record_expr)
             , Just tc <- tyConOf fam_inst_envs record_rho
-            -> return (RecSelData tc)
+            -> do { reportAmbiguousField tc
+                  ; return (RecSelData tc) }
 
         -- Nothing else we can try...
         _ -> failWithTc badOverloadedUpdate
@@ -1263,6 +1266,18 @@ disambiguateRecordBinds record_expr record_rho rbnds res_ty
            ; return $ L l upd { hsRecFieldLbl
                                   = L loc (Unambiguous i (L loc lbl)) } }
 
+    -- See Note [Deprecating ambiguous fields] in GHC.Tc.Gen.Head
+    reportAmbiguousField :: TyCon -> TcM ()
+    reportAmbiguousField parent_type =
+        setSrcSpan loc $ warnIfFlag Opt_WarnAmbiguousFields True $
+          vcat [ text "The record update" <+> ppr rupd
+                   <+> text "with type" <+> ppr parent_type
+                   <+> text "is ambiguous."
+               , text "This will not be supported by -XDuplicateRecordFields in future releases of GHC."
+               ]
+      where
+        rupd = RecordUpd { rupd_expr = record_expr, rupd_flds = rbnds, rupd_ext = noExtField }
+        loc  = getLoc (head rbnds)
 
 {-
 Game plan for record bindings
diff --git a/compiler/GHC/Tc/Gen/Head.hs b/compiler/GHC/Tc/Gen/Head.hs
index 7dc993d8cc..faf5e0f2f2 100644
--- a/compiler/GHC/Tc/Gen/Head.hs
+++ b/compiler/GHC/Tc/Gen/Head.hs
@@ -435,8 +435,25 @@ add_head_ctxt fun args thing_inside
 *                                                                      *
 ********************************************************************* -}
 
-{- Note [Disambiguating record fields]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+{-
+Note [Deprecating ambiguous fields]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In the future, the -XDuplicateRecordFields extension will no longer support
+disambiguating record fields during type-checking (as described in Note
+[Disambiguating record fields]).  For now, the -Wambiguous-fields option will
+emit a warning whenever an ambiguous field is resolved using type information.
+In a subsequent GHC release, this functionality will be removed and the warning
+will turn into an ambiguity error in the renamer.
+
+For background information, see GHC proposal #366
+(https://github.com/ghc-proposals/ghc-proposals/pull/366).
+
+
+Note [Disambiguating record fields]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+NB. The following is going to be removed: see
+Note [Deprecating ambiguous fields].
+
 When the -XDuplicateRecordFields extension is used, and the renamer
 encounters a record selector or update that it cannot immediately
 disambiguate (because it involves fields that belong to multiple
@@ -601,6 +618,16 @@ finish_ambiguous_selector lr@(L _ rdr) parent_type
     -- See Note [Unused name reporting and HasField] in GHC.Tc.Instance.Class
     do { addUsedGRE True gre
        ; keepAlive (greMangledName gre)
+         -- See Note [Deprecating ambiguous fields]
+       ; warnIfFlag Opt_WarnAmbiguousFields True $
+          vcat [ text "The field" <+> quotes (ppr rdr)
+                   <+> text "belonging to type" <+> ppr parent_type
+                   <+> text "is ambiguous."
+               , text "This will not be supported by -XDuplicateRecordFields in future releases of GHC."
+               , if isLocalGRE gre
+                 then text "You can use explicit case analysis to resolve the ambiguity."
+                 else text "You can use a qualified import or explicit case analysis to resolve the ambiguity."
+               ]
        ; return (greMangledName gre) } } } } }
 
 -- This field name really is ambiguous, so add a suitable "ambiguous