Fix for Trac #15611: Scope errors lie about what modules are imported.

Summary:
For the error message:
    Not in scope X.Y
    Module X does not export Y
    No module named ‘X’ is imported:
there are 2 cases, where we don't show the last "no module named is imported" line:
1. If the module X has been imported.
2. If the module X is the current module. There are 2 subcases:

   2.1 If the unknown module name is in a input source file,
       then we can use the getModule function to get the current module name.

   2.2 If the unknown module name has been entered by the user in GHCi,
       then the getModule function returns something like "interactive:Ghci1",
       and we have to check the current module in the last added entry of
       the HomePackageTable.

Test Plan: make test TESTS="T15611a T15611b"

Reviewers: monoidal, hvr, thomie, dfeuer, bgamari, DavidEichmann

Reviewed By: monoidal, DavidEichmann

Subscribers: rwbarton, carter

GHC Trac Issues: #15611

Differential Revision: https://phabricator.haskell.org/D5284

1 files changed, 47 insertions, 6 deletions

diff --git a/compiler/rename/RnUnbound.hs b/compiler/rename/RnUnbound.hs
index ce5d0dc315..97d3fd7279 100644
--- a/compiler/rename/RnUnbound.hs
+++ b/compiler/rename/RnUnbound.hs
@@ -24,13 +24,14 @@ import Name
 import Module
 import SrcLoc
 import Outputable
-import PrelNames ( mkUnboundName, forall_tv_RDR, isUnboundName )
+import PrelNames ( mkUnboundName, forall_tv_RDR, isUnboundName, getUnique)
 import Util
 import Maybes
 import DynFlags
 import FastString
 import Data.List
 import Data.Function ( on )
+import UniqDFM (udfmToList)
 
 {-
 ************************************************************************
@@ -67,8 +68,11 @@ unboundNameX where_look rdr_name extra
           else do { local_env  <- getLocalRdrEnv
                   ; global_env <- getGlobalRdrEnv
                   ; impInfo <- getImports
+                  ; currmod <- getModule
+                  ; hpt <- getHpt
                   ; let suggestions = unknownNameSuggestions_ where_look
-                                        dflags global_env local_env impInfo rdr_name
+                          dflags hpt currmod global_env local_env impInfo
+                          rdr_name
                   ; addErr (err $$ suggestions) }
         ; return (mkUnboundNameRdr rdr_name) }
 
@@ -89,16 +93,19 @@ type HowInScope = Either SrcSpan ImpDeclSpec
 
 -- | Called from the typechecker (TcErrors) when we find an unbound variable
 unknownNameSuggestions :: DynFlags
+                       -> HomePackageTable -> Module
                        -> GlobalRdrEnv -> LocalRdrEnv -> ImportAvails
                        -> RdrName -> SDoc
 unknownNameSuggestions = unknownNameSuggestions_ WL_Any
 
 unknownNameSuggestions_ :: WhereLooking -> DynFlags
+                       -> HomePackageTable -> Module
                        -> GlobalRdrEnv -> LocalRdrEnv -> ImportAvails
                        -> RdrName -> SDoc
-unknownNameSuggestions_ where_look dflags global_env local_env imports tried_rdr_name =
+unknownNameSuggestions_ where_look dflags hpt curr_mod global_env local_env
+                          imports tried_rdr_name =
     similarNameSuggestions where_look dflags global_env local_env tried_rdr_name $$
-    importSuggestions where_look  imports tried_rdr_name $$
+    importSuggestions where_look hpt curr_mod imports tried_rdr_name $$
     extensionSuggestions tried_rdr_name
 
 
@@ -223,12 +230,15 @@ similarNameSuggestions where_look dflags global_env
         | i <- is, let ispec = is_decl i, is_qual ispec ]
 
 -- | Generate helpful suggestions if a qualified name Mod.foo is not in scope.
-importSuggestions :: WhereLooking -> ImportAvails -> RdrName -> SDoc
-importSuggestions where_look imports rdr_name
+importSuggestions :: WhereLooking
+                  -> HomePackageTable -> Module
+                  -> ImportAvails -> RdrName -> SDoc
+importSuggestions where_look hpt currMod imports rdr_name
   | WL_LocalOnly <- where_look                 = Outputable.empty
   | not (isQual rdr_name || isUnqual rdr_name) = Outputable.empty
   | null interesting_imports
   , Just name <- mod_name
+  , showNotImportedLine (fromJust mod_name)
   = hsep
       [ text "No module named"
       , quotes (ppr name)
@@ -245,6 +255,7 @@ importSuggestions where_look imports rdr_name
       ]
   | is_qualified
   , null helpful_imports
+  , not (null interesting_imports)
   , mods <- map fst interesting_imports
   = hsep
       [ text "Neither"
@@ -330,6 +341,19 @@ importSuggestions where_look imports rdr_name
   (helpful_imports_hiding, helpful_imports_non_hiding)
     = partition (imv_is_hiding . snd) helpful_imports
 
+  -- See note [showNotImportedLine]
+  showNotImportedLine :: ModuleName -> Bool                           -- #15611
+  showNotImportedLine modnam
+      | modnam `elem`
+          fmap moduleName (moduleEnvKeys (imp_mods imports)) = False   -- 1
+      | moduleName currMod == modnam       = False                     -- 2.1
+      | isLastLoadedMod modnam hptUniques  = False                     -- 2.2
+      | otherwise                          = True
+    where
+      hptUniques = map fst (udfmToList hpt)
+      isLastLoadedMod _ []         = False
+      isLastLoadedMod modnam uniqs = last uniqs == getUnique modnam
+
 extensionSuggestions :: RdrName -> SDoc
 extensionSuggestions rdrName
   | rdrName == mkUnqual varName (fsLit "mdo") ||
@@ -341,3 +365,20 @@ perhapsForallMsg :: SDoc
 perhapsForallMsg
   = vcat [ text "Perhaps you intended to use ExplicitForAll or similar flag"
          , text "to enable explicit-forall syntax: forall <tvs>. <type>"]
+
+{- Note [showNotImportedLine]                                   -- #15611
+For the error message:
+    Not in scope X.Y
+    Module X does not export Y
+    No module named ‘X’ is imported:
+there are 2 cases, where we hide the last "no module is imported" line:
+1. If the module X has been imported.
+2. If the module X is the current module. There are 2 subcases:
+   2.1 If the unknown module name is in a input source file,
+       then we can use the getModule function to get the current module name.
+       (See test T15611a)
+   2.2 If the unknown module name has been entered by the user in GHCi,
+       then the getModule function returns something like "interactive:Ghci1",
+       and we have to check the current module in the last added entry of
+       the HomePackageTable. (See test T15611b)
+-}