path: root/compiler/GHC/Tc/Utils/Backpack.hs

{-# LANGUAGE NondecreasingIndentation #-}
{-# LANGUAGE ScopedTypeVariables      #-}
{-# LANGUAGE TypeFamilies             #-}

module GHC.Tc.Utils.Backpack (
    findExtraSigImports,
    implicitRequirements,
    implicitRequirementsShallow,
    checkUnit,
    tcRnCheckUnit,
    tcRnMergeSignatures,
    mergeSignatures,
    tcRnInstantiateSignature,
    instantiateSignature,
) where

import GHC.Prelude


import GHC.Driver.Env
import GHC.Driver.Ppr
import GHC.Driver.Session

import GHC.Types.Basic (TypeOrKind(..))
import GHC.Types.Fixity (defaultFixity)
import GHC.Types.Fixity.Env
import GHC.Types.TypeEnv
import GHC.Types.Name.Reader
import GHC.Types.Name
import GHC.Types.Name.Env
import GHC.Types.Name.Set
import GHC.Types.Avail
import GHC.Types.SrcLoc
import GHC.Types.SourceFile
import GHC.Types.Var
import GHC.Types.Id( idType )
import GHC.Types.Unique.DSet
import GHC.Types.Name.Shape
import GHC.Types.PkgQual

import GHC.Unit
import GHC.Unit.Finder
import GHC.Unit.Module.Warnings
import GHC.Unit.Module.ModIface
import GHC.Unit.Module.ModDetails
import GHC.Unit.Module.Imported
import GHC.Unit.Module.Deps

import GHC.Tc.Errors
import GHC.Tc.Errors.Types
import {-# SOURCE #-} GHC.Tc.Module
import GHC.Tc.Gen.Export
import GHC.Tc.Solver
import GHC.Tc.TyCl.Utils
import GHC.Tc.Types.Constraint
import GHC.Tc.Types.Origin
import GHC.Tc.Utils.Env
import GHC.Tc.Utils.Monad
import GHC.Tc.Utils.Instantiate
import GHC.Tc.Utils.Unify
import GHC.Tc.Utils.TcMType
import GHC.Tc.Utils.TcType

import GHC.Hs

import GHC.Core.InstEnv
import GHC.Core.FamInstEnv

import GHC.IfaceToCore
import GHC.Iface.Load
import GHC.Iface.Rename
import GHC.Iface.Syntax

import GHC.Rename.Names
import GHC.Rename.Fixity ( lookupFixityRn )

import GHC.Utils.Error
import GHC.Utils.Misc ( HasDebugCallStack )
import GHC.Utils.Outputable
import GHC.Utils.Panic
import GHC.Utils.Panic.Plain

import GHC.Data.FastString
import GHC.Data.Maybe

import Control.Monad
import Data.List (find)

import GHC.Iface.Errors.Types

checkHsigDeclM :: ModIface -> TyThing -> TyThing -> TcRn ()
checkHsigDeclM sig_iface sig_thing real_thing = do
    let name = getName real_thing
    -- TODO: Distinguish between signature merging and signature
    -- implementation cases.
    checkBootDeclM Hsig sig_thing real_thing
    real_fixity <- lookupFixityRn name
    let sig_fixity = case mi_fix_fn (mi_final_exts sig_iface) (occName name) of
                        Nothing -> defaultFixity
                        Just f -> f
    when (real_fixity /= sig_fixity) $
      addErrAt (nameSrcSpan name)
        (TcRnHsigFixityMismatch real_thing real_fixity sig_fixity)

-- | Given a 'ModDetails' of an instantiated signature (note that the
-- 'ModDetails' must be knot-tied consistently with the actual implementation)
-- and a 'GlobalRdrEnv' constructed from the implementor of this interface,
-- verify that the actual implementation actually matches the original
-- interface.
--
-- Note that it is already assumed that the implementation *exports*
-- a sufficient set of entities, since otherwise the renaming and then
-- typechecking of the signature 'ModIface' would have failed.
checkHsigIface :: TcGblEnv -> GlobalRdrEnv -> ModIface -> ModDetails -> TcRn ()
checkHsigIface tcg_env gre_env sig_iface
  ModDetails { md_insts = sig_insts, md_fam_insts = sig_fam_insts,
               md_types = sig_type_env, md_exports = sig_exports   } = do
    traceTc "checkHsigIface" $ vcat
        [ ppr sig_type_env, ppr sig_insts, ppr sig_exports ]
    mapM_ check_export (map availName sig_exports)
    failIfErrsM -- See Note [Fail before checking instances in checkHsigIface]
    unless (null sig_fam_insts) $
        panic ("GHC.Tc.Utils.Backpack.checkHsigIface: " ++
               "Cannot handle family instances in hsig files yet...")
    -- Delete instances so we don't look them up when
    -- checking instance satisfiability
    -- TODO: this should not be necessary
    tcg_env <- getGblEnv
    setGblEnv tcg_env { tcg_inst_env = emptyInstEnv,
                        tcg_fam_inst_env = emptyFamInstEnv,
                        tcg_insts = [],
                        tcg_fam_insts = [] } $ do
    mapM_ check_inst (instEnvElts sig_insts)
    failIfErrsM
  where
    -- NB: the Names in sig_type_env are bogus.  Let's say we have H.hsig
    -- in package p that defines T; and we implement with himpl:H.  Then the
    -- Name is p[himpl:H]:H.T, NOT himplH:H.T.  That's OK but we just
    -- have to look up the right name.
    sig_type_occ_env = mkOccEnv
                     . map (\t -> (nameOccName (getName t), t))
                     $ nonDetNameEnvElts sig_type_env
    dfun_names = map getName (instEnvElts sig_insts)
    check_export name
      -- Skip instances, we'll check them later
      -- TODO: Actually this should never happen, because DFuns are
      -- never exported...
      | name `elem` dfun_names = return ()
      -- See if we can find the type directly in the hsig ModDetails
      -- TODO: need to special case wired in names
      | Just sig_thing <- lookupOccEnv sig_type_occ_env (nameOccName name) = do
        -- NB: We use tcLookupImported_maybe because we want to EXCLUDE
        -- tcg_env (TODO: but maybe this isn't relevant anymore).
        r <- tcLookupImported_maybe name
        case r of
          Failed err           -> addErr (TcRnInterfaceError err)
          Succeeded real_thing -> checkHsigDeclM sig_iface sig_thing real_thing

      -- The hsig did NOT define this function; that means it must
      -- be a reexport.  In this case, make sure the 'Name' of the
      -- reexport matches the 'Name' exported here.
      | [gre] <- lookupGRE_OccName (AllNameSpaces WantNormal) gre_env (nameOccName name) = do
        let name' = greName gre
        when (name /= name') $ do
            -- See Note [Error reporting bad reexport]
            -- TODO: Actually this error swizzle doesn't work
            let p (L _ ie) = name `elem` ieNames ie
                loc = case tcg_rn_exports tcg_env of
                       Just es | Just e <- find p (map fst es)
                         -- TODO: maybe we can be a little more
                         -- precise here and use the Located
                         -- info for the *specific* name we matched.
                         -> getLocA e
                       _ -> nameSrcSpan name
            addErrAt loc
              (TcRnBootMismatch Hsig $ BadReexportedBootThing name name')
      -- This should actually never happen, but whatever...
      | otherwise =
        addErrAt (nameSrcSpan name)
            (missingBootThing Hsig name MissingBootExport)

-- Note [Fail before checking instances in checkHsigIface]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- We need to be careful about failing before checking instances if there happens
-- to be an error in the exports.
-- Otherwise, we might proceed with typechecking (and subsequently panic-ing) on
-- ill-kinded types that are constructed while checking instances.
-- This lead to #19244

-- Note [Error reporting bad reexport]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- NB: You want to be a bit careful about what location you report on reexports.
-- If the name was declared in the hsig file, 'nameSrcSpan name' is indeed the
-- correct source location.  However, if it was *reexported*, obviously the name
-- is not going to have the right location.  In this case, we need to grovel in
-- tcg_rn_exports to figure out where the reexport came from.



-- | Checks if a 'ClsInst' is "defined". In general, for hsig files we can't
-- assume that the implementing file actually implemented the instances (they
-- may be reexported from elsewhere).  Where should we look for the instances?
-- We do the same as we would otherwise: consult the EPS.  This isn't perfect
-- (we might conclude the module exports an instance when it doesn't, see
-- #9422), but we will never refuse to compile something.
check_inst :: ClsInst -> TcM ()
check_inst sig_inst@(ClsInst { is_dfun = dfun_id }) = do
    -- TODO: This could be very well generalized to support instance
    -- declarations in boot files.
    tcg_env <- getGblEnv
    -- NB: Have to tug on the interface, not necessarily
    -- tugged... but it didn't work?
    mapM_ tcLookupImported_maybe (nameSetElemsStable (orphNamesOfClsInst sig_inst))

    -- Based off of 'simplifyDeriv'
    let origin = InstProvidedOrigin (tcg_semantic_mod tcg_env) sig_inst
    (skol_info, tvs_skols, inst_theta, cls, inst_tys) <- tcSkolDFunType (idType dfun_id)
    (tclvl,cts) <- pushTcLevelM $ do
       wanted <- newWanted origin (Just TypeLevel) (mkClassPred cls inst_tys)
       givens <- forM inst_theta $ \given -> do
           loc <- getCtLocM origin (Just TypeLevel)
           new_ev <- newEvVar given
           return CtGiven { ctev_pred = given
                          -- Doesn't matter, make something up
                          , ctev_evar = new_ev
                          , ctev_loc = loc
                          }
       return $ wanted : givens
    unsolved <- simplifyWantedsTcM cts

    (implic, _) <- buildImplicationFor tclvl skol_info tvs_skols [] unsolved
    reportAllUnsolved (mkImplicWC implic)

-- | For a module @modname@ of type 'HscSource', determine the list
-- of extra "imports" of other requirements which should be considered part of
-- the import of the requirement, because it transitively depends on those
-- requirements by imports of modules from other packages.  The situation
-- is something like this:
--
--      unit p where
--          signature X
--          signature Y
--              import X
--
--      unit q where
--          dependency p[X=\<A>,Y=\<B>]
--          signature A
--          signature B
--
-- Although q's B does not directly import A, we still have to make sure we
-- process A first, because the merging process will cause B to indirectly
-- import A.  This function finds the TRANSITIVE closure of all such imports
-- we need to make.
findExtraSigImports :: HscEnv
                    -> HscSource
                    -> ModuleName
                    -> IO [ModuleName]
findExtraSigImports hsc_env HsigFile modname = do
    let
      dflags     = hsc_dflags hsc_env
      ctx        = initSDocContext dflags defaultUserStyle
      unit_state = hsc_units hsc_env
      reqs       = requirementMerges unit_state modname
    holes <- forM reqs $ \(Module iuid mod_name) -> do
        initIfaceLoad hsc_env
            . withIfaceErr ctx
            $ moduleFreeHolesPrecise (text "findExtraSigImports")
                (mkModule (VirtUnit iuid) mod_name)
    return (uniqDSetToList (unionManyUniqDSets holes))

findExtraSigImports _ _ _ = return []

-- Given a list of 'import M' statements in a module, figure out
-- any extra implicit requirement imports they may have.  For
-- example, if they 'import M' and M resolves to p[A=<B>,C=D], then
-- they actually also import the local requirement B.
implicitRequirements :: HscEnv
                     -> [(PkgQual, Located ModuleName)]
                     -> IO [ModuleName]
implicitRequirements hsc_env normal_imports
  = fmap concat $
    forM normal_imports $ \(mb_pkg, L _ imp) -> do
        found <- findImportedModule hsc_env imp mb_pkg
        case found of
            Found _ mod | notHomeModuleMaybe mhome_unit mod ->
                return (uniqDSetToList (moduleFreeHoles mod))
            _ -> return []
  where
    mhome_unit = hsc_home_unit_maybe hsc_env

-- | Like @implicitRequirements'@, but returns either the module name, if it is
-- a free hole, or the instantiated unit the imported module is from, so that
-- that instantiated unit can be processed and via the batch mod graph (rather
-- than a transitive closure done here) all the free holes are still reachable.
implicitRequirementsShallow
  :: HscEnv
  -> [(PkgQual, Located ModuleName)]
  -> IO ([ModuleName], [InstantiatedUnit])
implicitRequirementsShallow hsc_env normal_imports = go ([], []) normal_imports
 where
  mhome_unit = hsc_home_unit_maybe hsc_env

  go acc [] = pure acc
  go (accL, accR) ((mb_pkg, L _ imp):imports) = do
    found <- findImportedModule hsc_env imp mb_pkg
    let acc' = case found of
          Found _ mod | notHomeModuleMaybe mhome_unit mod ->
              case moduleUnit mod of
                  HoleUnit -> (moduleName mod : accL, accR)
                  RealUnit _ -> (accL, accR)
                  VirtUnit u -> (accL, u:accR)
          _ -> (accL, accR)
    go acc' imports

-- | Given a 'Unit', make sure it is well typed.  This is because
-- unit IDs come from Cabal, which does not know if things are well-typed or
-- not; a component may have been filled with implementations for the holes
-- that don't actually fulfill the requirements.
checkUnit :: Unit -> TcM ()
checkUnit HoleUnit         = return ()
checkUnit (RealUnit _)     = return () -- if it's already compiled, must be well-typed
checkUnit (VirtUnit indef) = do
   let insts = instUnitInsts indef
   forM_ insts $ \(mod_name, mod) ->
       -- NB: direct hole instantiations are well-typed by construction
       -- (because we FORCE things to be merged in), so don't check them
       when (not (isHoleModule mod)) $ do
           checkUnit (moduleUnit mod)
           _ <- mod `checkImplements` Module indef mod_name
           return ()

-- | Top-level driver for signature instantiation (run when compiling
-- an @hsig@ file.)
tcRnCheckUnit ::
    HscEnv -> Unit ->
    IO (Messages TcRnMessage, Maybe ())
tcRnCheckUnit hsc_env uid =
   withTiming logger
              (text "Check unit id" <+> ppr uid)
              (const ()) $
   initTc hsc_env
          HsigFile -- bogus
          False
          (mainModIs (hsc_HUE hsc_env))
          (realSrcLocSpan (mkRealSrcLoc (fsLit loc_str) 0 0)) -- bogus
    $ checkUnit uid
  where
   dflags = hsc_dflags hsc_env
   logger = hsc_logger hsc_env
   loc_str = "Command line argument: -unit-id " ++ showSDoc dflags (ppr uid)

-- TODO: Maybe lcl_iface0 should be pre-renamed to the right thing? Unclear...

-- | Top-level driver for signature merging (run after typechecking
-- an @hsig@ file).
tcRnMergeSignatures :: HscEnv -> HsParsedModule -> TcGblEnv {- from local sig -} -> ModIface
                    -> IO (Messages TcRnMessage, Maybe TcGblEnv)
tcRnMergeSignatures hsc_env hpm orig_tcg_env iface =
  withTiming logger
             (text "Signature merging" <+> brackets (ppr this_mod))
             (const ()) $
  initTc hsc_env HsigFile False this_mod real_loc $
    mergeSignatures hpm orig_tcg_env iface
 where
  logger   = hsc_logger hsc_env
  this_mod = mi_module iface
  real_loc = tcg_top_loc orig_tcg_env

thinModIface :: [AvailInfo] -> ModIface -> ModIface
thinModIface avails iface =
    iface {
        mi_exports = avails,
        -- mi_fixities = ...,
        -- mi_warns = ...,
        -- mi_anns = ...,
        -- TODO: The use of nameOccName here is a bit dodgy, because
        -- perhaps there might be two IfaceTopBndr that are the same
        -- OccName but different Name.  Requires better understanding
        -- of invariants here.
        mi_decls = exported_decls ++ non_exported_decls ++ dfun_decls
        -- mi_insts = ...,
        -- mi_fam_insts = ...,
    }
  where
    decl_pred occs decl = nameOccName (ifName decl) `elemOccSet` occs
    filter_decls occs = filter (decl_pred occs . snd) (mi_decls iface)

    exported_occs = mkOccSet [ nameOccName n
                             | a <- avails
                             , n <- availNames a ]
    exported_decls = filter_decls exported_occs

    non_exported_occs = mkOccSet [ occName n
                                 | (_, d) <- exported_decls
                                 , n <- ifaceDeclNeverExportedRefs d ]
    non_exported_decls = filter_decls non_exported_occs

    dfun_pred IfaceId{ ifIdDetails = IfDFunId } = True
    dfun_pred _ = False
    dfun_decls = filter (dfun_pred . snd) (mi_decls iface)

-- | The list of 'Name's of *non-exported* 'IfaceDecl's which this
-- 'IfaceDecl' may refer to.  A non-exported 'IfaceDecl' should be kept
-- after thinning if an *exported* 'IfaceDecl' (or 'mi_insts', perhaps)
-- refers to it; we can't decide to keep it by looking at the exports
-- of a module after thinning.  Keep this synchronized with
-- 'rnIfaceDecl'.
ifaceDeclNeverExportedRefs :: IfaceDecl -> [Name]
ifaceDeclNeverExportedRefs d@IfaceFamily{} =
    case ifFamFlav d of
        IfaceClosedSynFamilyTyCon (Just (n, _))
            -> [n]
        _   -> []
ifaceDeclNeverExportedRefs _ = []


-- Note [Blank hsigs for all requirements]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- One invariant that a client of GHC must uphold is that there
-- must be an hsig file for every requirement (according to
-- @-this-unit-id@); this ensures that for every interface
-- file (hi), there is a source file (hsig), which helps grease
-- the wheels of recompilation avoidance which assumes that
-- source files always exist.

{-
inheritedSigPvpWarning :: WarningTxt
inheritedSigPvpWarning =
    WarningTxt (noLoc NoSourceText) [noLoc (StringLiteral NoSourceText (fsLit msg))]
  where
    msg = "Inherited requirements from non-signature libraries (libraries " ++
          "with modules) should not be used, as this mode of use is not " ++
          "compatible with PVP-style version bounds.  Instead, copy the " ++
          "declaration to the local hsig file or move the signature to a " ++
          "library of its own and add that library as a dependency."
-}

-- Note [Handling never-exported TyThings under Backpack]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
--   DEFINITION: A "never-exported TyThing" is a TyThing whose 'Name' will
--   never be mentioned in the export list of a module (mi_avails).
--   Unlike implicit TyThings (Note [Implicit TyThings]), non-exported
--   TyThings DO have a standalone IfaceDecl declaration in their
--   interface file.
--
-- Originally, Backpack was designed under the assumption that anything
-- you could declare in a module could also be exported; thus, merging
-- the export lists of two signatures is just merging the declarations
-- of two signatures writ small.  Of course, in GHC Haskell, there are a
-- few important things which are not explicitly exported but still can
-- be used:  in particular, dictionary functions for instances, Typeable
-- TyCon bindings, and coercion axioms for type families also count.
--
-- When handling these non-exported things, there two primary things
-- we need to watch out for:
--
--  * Signature matching/merging is done by comparing each
--    of the exported entities of a signature and a module.  These exported
--    entities may refer to non-exported TyThings which must be tested for
--    consistency.  For example, an instance (ClsInst) will refer to a
--    non-exported DFunId.  In this case, 'checkBootDeclM' directly compares the
--    embedded 'DFunId' in 'is_dfun'.
--
--    For this to work at all, we must ensure that pointers in 'is_dfun' refer
--    to DISTINCT 'DFunId's, even though the 'Name's (may) be the same.
--    Unfortunately, this is the OPPOSITE of how we treat most other references
--    to 'Name's, so this case needs to be handled specially.
--
--    The details are in the documentation for 'typecheckIfacesForMerging'.
--    and the Note [Resolving never-exported Names] in GHC.IfaceToCore.
--
--  * When we rename modules and signatures, we use the export lists to
--    decide how the declarations should be renamed.  However, this
--    means we don't get any guidance for how to rename non-exported
--    entities.  Fortunately, we only need to rename these entities
--    *consistently*, so that 'typecheckIfacesForMerging' can wire them
--    up as needed.
--
--    The details are in Note [rnIfaceNeverExported] in 'GHC.Iface.Rename'.
--
-- The root cause for all of these complications is the fact that these
-- logically "implicit" entities are defined indirectly in an interface
-- file.  #13151 gives a proposal to make these *truly* implicit.

merge_msg :: ModuleName -> [InstantiatedModule] -> SDoc
merge_msg mod_name [] =
    text "while checking the local signature" <+> ppr mod_name <+>
    text "for consistency"
merge_msg mod_name reqs =
  hang (text "while merging the signatures from" <> colon)
   2 (vcat [ bullet <+> ppr req | req <- reqs ] $$
      bullet <+> text "...and the local signature for" <+> ppr mod_name)

-- | Given a local 'ModIface', merge all inherited requirements
-- from 'requirementMerges' into this signature, producing
-- a final 'TcGblEnv' that matches the local signature and
-- all required signatures.
mergeSignatures :: HasDebugCallStack => HsParsedModule -> TcGblEnv -> ModIface -> TcRn TcGblEnv
mergeSignatures
  (HsParsedModule { hpm_module = L loc (HsModule { hsmodExports = mb_exports }),
                    hpm_src_files = src_files })
  orig_tcg_env lcl_iface0 = setSrcSpan loc $ do
    -- The lcl_iface0 is the ModIface for the local hsig
    -- file, which is guaranteed to exist, see
    -- Note [Blank hsigs for all requirements]
    hsc_env <- getTopEnv

    -- Copy over some things from the original TcGblEnv that
    -- we want to preserve
    updGblEnv (\env -> env {
        -- Renamed imports/declarations are often used
        -- by programs that use the GHC API, e.g., Haddock.
        -- These won't get filled by the merging process (since
        -- we don't actually rename the parsed module again) so
        -- we need to take them directly from the previous
        -- typechecking.
        --
        -- NB: the export declarations aren't in their final
        -- form yet.  We'll fill those in when we reprocess
        -- the export declarations.
        tcg_rn_imports = tcg_rn_imports orig_tcg_env,
        tcg_rn_decls   = tcg_rn_decls   orig_tcg_env,
        -- Annotations
        tcg_ann_env    = tcg_ann_env    orig_tcg_env,
        -- Documentation header
        tcg_doc_hdr    = tcg_doc_hdr orig_tcg_env
        -- tcg_dus?
        -- tcg_th_used           = tcg_th_used orig_tcg_env,
        -- tcg_th_splice_used    = tcg_th_splice_used orig_tcg_env
       }) $ do
    tcg_env <- getGblEnv

    let outer_mod  = tcg_mod tcg_env
    let inner_mod  = tcg_semantic_mod tcg_env
    let mod_name   = moduleName outer_mod
    let unit_state = hsc_units hsc_env
    let dflags     = hsc_dflags hsc_env

    -- STEP 1: Figure out all of the external signature interfaces
    -- we are going to merge in.
    let reqs = requirementMerges unit_state mod_name

    addErrCtxt (pprWithUnitState unit_state $ merge_msg mod_name reqs) $ do

    -- STEP 2: Read in the RAW forms of all of these interfaces
    ireq_ifaces0 <- liftIO $ forM reqs $ \(Module iuid mod_name) -> do
        let m = mkModule (VirtUnit iuid) mod_name
            im = fst (getModuleInstantiation m)
            ctx = initSDocContext dflags defaultUserStyle
        fmap fst
         . withIfaceErr ctx
         $ findAndReadIface hsc_env (text "mergeSignatures") im m NotBoot

    -- STEP 3: Get the unrenamed exports of all these interfaces,
    -- thin it according to the export list, and do shaping on them.
    let extend_ns nsubst as = liftIO $ extendNameShape hsc_env nsubst as
        -- This function gets run on every inherited interface, and
        -- it's responsible for:
        --
        --  1. Merging the exports of the interface into @nsubst@,
        --  2. Adding these exports to the "OK to import" set (@oks@)
        --  if they came from a package with no exposed modules
        --  (this means we won't report a PVP error in this case), and
        --  3. Thinning the interface according to an explicit export
        --  list.
        --
        gen_subst (nsubst,oks,ifaces) (imod@(Module iuid _), ireq_iface) = do
            let insts = instUnitInsts iuid
                isFromSignaturePackage =
                    let inst_uid = instUnitInstanceOf iuid
                        pkg = unsafeLookupUnitId unit_state inst_uid
                    in null (unitExposedModules pkg)
            -- 3(a). Rename the exports according to how the dependency
            -- was instantiated.  The resulting export list will be accurate
            -- except for exports *from the signature itself* (which may
            -- be subsequently updated by exports from other signatures in
            -- the merge.
            as1 <- tcRnModExports insts ireq_iface
            -- 3(b). Thin the interface if it comes from a signature package.
            (thinned_iface, as2) <- case mb_exports of
                    Just (L loc _)
                      -- Check if the package containing this signature is
                      -- a signature package (i.e., does not expose any
                      -- modules.)  If so, we can thin it.
                      | isFromSignaturePackage
                      -> setSrcSpanA loc $ do
                        -- Suppress missing errors; they might be used to refer
                        -- to entities from other signatures we are merging in.
                        -- If an identifier truly doesn't exist in any of the
                        -- signatures that are merged in, we will discover this
                        -- when we run exports_from_avail on the final merged
                        -- export list.
                        (mb_r, msgs) <- tryTc $ do
                            -- Suppose that we have written in a signature:
                            --  signature A ( module A ) where {- empty -}
                            -- If I am also inheriting a signature from a
                            -- signature package, does 'module A' scope over
                            -- all of its exports?
                            --
                            -- There are two possible interpretations:
                            --
                            --  1. For non self-reexports, a module reexport
                            --  is interpreted only in terms of the local
                            --  signature module, and not any of the inherited
                            --  ones.  The reason for this is because after
                            --  typechecking, module exports are completely
                            --  erased from the interface of a file, so we
                            --  have no way of "interpreting" a module reexport.
                            --  Thus, it's only useful for the local signature
                            --  module (where we have a useful GlobalRdrEnv.)
                            --
                            --  2. On the other hand, a common idiom when
                            --  you want to "export everything, plus a reexport"
                            --  in modules is to say module A ( module A, reex ).
                            --  This applies to signature modules too; and in
                            --  particular, you probably still want the entities
                            --  from the inherited signatures to be preserved
                            --  too.
                            --
                            -- We think it's worth making a special case for
                            -- self reexports to make use case (2) work.  To
                            -- do this, we take the exports of the inherited
                            -- signature @as1@, and bundle them into a
                            -- GlobalRdrEnv where we treat them as having come
                            -- from the import @import A@.  Thus, we will
                            -- pick them up if they are referenced explicitly
                            -- (@foo@) or even if we do a module reexport
                            -- (@module A@).
                            let ispec = ImpSpec ImpDeclSpec{
                                            -- NB: This needs to be mod name
                                            -- of the local signature, not
                                            -- the (original) module name of
                                            -- the inherited signature,
                                            -- because we need module
                                            -- LocalSig (from the local
                                            -- export list) to match it!
                                            is_mod  = mod_name,
                                            is_as   = mod_name,
                                            is_qual = False,
                                            is_dloc = locA loc
                                          } ImpAll
                                rdr_env = mkGlobalRdrEnv $ gresFromAvails hsc_env (Just ispec) as1
                            setGblEnv tcg_env {
                                tcg_rdr_env = rdr_env
                            } $ exports_from_avail mb_exports rdr_env
                                    -- NB: tcg_imports is also empty!
                                    emptyImportAvails
                                    (tcg_semantic_mod tcg_env)
                        case mb_r of
                            Just (_, as2) -> return (thinModIface as2 ireq_iface, as2)
                            Nothing -> addMessages msgs >> failM
                    -- We can't thin signatures from non-signature packages
                    _ -> return (ireq_iface, as1)
            -- 3(c). Only identifiers from signature packages are "ok" to
            -- import (that is, they are safe from a PVP perspective.)
            -- (NB: This code is actually dead right now.)
            let oks' | isFromSignaturePackage
                     = extendOccSetList oks (exportOccs as2)
                     | otherwise
                     = oks
            -- 3(d). Extend the name substitution (performing shaping)
            mb_r <- extend_ns nsubst as2
            case mb_r of
                Left err -> failWithTc (TcRnHsigShapeMismatch err)
                Right nsubst' -> return (nsubst',oks',(imod, thinned_iface):ifaces)
        nsubst0 = mkNameShape (moduleName inner_mod) (mi_exports lcl_iface0)
        ok_to_use0 = mkOccSet (exportOccs (mi_exports lcl_iface0))
    -- Process each interface, getting the thinned interfaces as well as
    -- the final, full set of exports @nsubst@ and the exports which are
    -- "ok to use" (we won't attach 'inheritedSigPvpWarning' to them.)
    (nsubst, ok_to_use, rev_thinned_ifaces)
        <- foldM gen_subst (nsubst0, ok_to_use0, []) (zip reqs ireq_ifaces0)
    let thinned_ifaces = reverse rev_thinned_ifaces
        exports        = nameShapeExports nsubst
        rdr_env        = mkGlobalRdrEnv (gresFromAvails hsc_env Nothing exports)
        _warn_occs     = filter (not . (`elemOccSet` ok_to_use)) (exportOccs exports)
        warns          = NoWarnings
        {-
        -- TODO: Warnings are transitive, but this is not what we want here:
        -- if a module reexports an entity from a signature, that should be OK.
        -- Not supported in current warning framework
        warns | null warn_occs = NoWarnings
              | otherwise = WarnSome $ map (\o -> (o, inheritedSigPvpWarning)) warn_occs
        -}
    setGblEnv tcg_env {
        -- The top-level GlobalRdrEnv is quite interesting.  It consists
        -- of two components:
        --  1. First, we reuse the GlobalRdrEnv of the local signature.
        --     This is very useful, because it means that if we have
        --     to print a message involving some entity that the local
        --     signature imported, we'll qualify it accordingly.
        --  2. Second, we need to add all of the declarations we are
        --     going to merge in (as they need to be in scope for the
        --     final test of the export list.)
        tcg_rdr_env = rdr_env `plusGlobalRdrEnv` tcg_rdr_env orig_tcg_env,
        -- Inherit imports from the local signature, so that module
        -- reexports are picked up correctly
        tcg_imports = tcg_imports orig_tcg_env,
        tcg_exports = exports,
        tcg_dus     = usesOnly (availsToNameSet exports),
        tcg_warns   = warns
        } $ do
    tcg_env <- getGblEnv

    -- Make sure we didn't refer to anything that doesn't actually exist
    -- pprTrace "mergeSignatures: exports_from_avail" (ppr exports) $ return ()
    (mb_lies, _) <- exports_from_avail mb_exports rdr_env
                        (tcg_imports tcg_env) (tcg_semantic_mod tcg_env)

    {- -- NB: This is commented out, because warns above is disabled.
    -- If you tried to explicitly export an identifier that has a warning
    -- attached to it, that's probably a mistake.  Warn about it.
    case mb_lies of
      Nothing -> return ()
      Just lies ->
        forM_ (concatMap (\(L loc x) -> map (L loc) (ieNames x)) lies) $ \(L loc n) ->
          setSrcSpan loc $
            unless (nameOccName n `elemOccSet` ok_to_use) $
                addWarn NoReason $ vcat [
                    text "Exported identifier" <+> quotes (ppr n) <+> text "will cause warnings if used.",
                    parens (text "To suppress this warning, remove" <+> quotes (ppr n) <+> text "from the export list of this signature.")
                    ]
    -}

    failIfErrsM

    -- Save the exports
    setGblEnv tcg_env { tcg_rn_exports = mb_lies } $ do
    tcg_env <- getGblEnv

    let home_unit = hsc_home_unit hsc_env

    -- STEP 4: Rename the interfaces
    ext_ifaces <- forM thinned_ifaces $ \((Module iuid _), ireq_iface) ->
        tcRnModIface (instUnitInsts iuid) (Just nsubst) ireq_iface
    lcl_iface <- tcRnModIface (homeUnitInstantiations home_unit) (Just nsubst) lcl_iface0
    let ifaces = lcl_iface : ext_ifaces

    -- STEP 4.1: Merge fixities (we'll verify shortly) tcg_fix_env
    let fix_env = mkNameEnv [ (greName rdr_elt, FixItem occ f)
                            | (occ, f) <- concatMap mi_fixities ifaces
                            , rdr_elt <- lookupGRE_OccName (AllNameSpaces WantBoth) rdr_env occ ]

    -- STEP 5: Typecheck the interfaces
    let type_env_var = tcg_type_env_var tcg_env

    -- typecheckIfacesForMerging does two things:
    --      1. It merges the all of the ifaces together, and typechecks the
    --      result to type_env.
    --      2. It typechecks each iface individually, but with their 'Name's
    --      resolving to the merged type_env from (1).
    -- See typecheckIfacesForMerging for more details.
    (type_env, detailss) <- initIfaceTcRn $
                            typecheckIfacesForMerging inner_mod ifaces type_env_var
    let infos = zip ifaces detailss

    -- Test for cycles
    checkSynCycles (homeUnitAsUnit home_unit) (typeEnvTyCons type_env) []

    -- NB on type_env: it contains NO dfuns.  DFuns are recorded inside
    -- detailss, and given a Name that doesn't correspond to anything real.  See
    -- also Note [Signature merging DFuns]

    -- Add the merged type_env to TcGblEnv, so that it gets serialized
    -- out when we finally write out the interface.
    --
    -- NB: Why do we set tcg_tcs/tcg_patsyns/tcg_type_env directly,
    -- rather than use tcExtendGlobalEnv (the normal method to add newly
    -- defined types to TcGblEnv?)  tcExtendGlobalEnv adds these
    -- TyThings to 'tcg_type_env_var', which is consulted when
    -- we read in interfaces to tie the knot.  But *these TyThings themselves
    -- come from interface*, so that would result in deadlock.  Don't
    -- update it!
    setGblEnv tcg_env {
        tcg_tcs = typeEnvTyCons type_env,
        tcg_patsyns = typeEnvPatSyns type_env,
        tcg_type_env = type_env,
        tcg_fix_env = fix_env
        } $ do
    tcg_env <- getGblEnv

    -- STEP 6: Check for compatibility/merge things
    tcg_env <- (\x -> foldM x tcg_env infos)
             $ \tcg_env (iface, details) -> do

        let check_export name
              | Just sig_thing <- lookupTypeEnv (md_types details) name
              = case lookupTypeEnv type_env (getName sig_thing) of
                  Just thing -> checkHsigDeclM iface sig_thing thing
                  Nothing -> panic "mergeSignatures: check_export"
              -- Oops! We're looking for this export but it's
              -- not actually in the type environment of the signature's
              -- ModDetails.
              --
              -- NB: This case happens because the we're iterating
              -- over the union of all exports, so some interfaces
              -- won't have everything.  Note that md_exports is nonsense
              -- (it's the same as exports); maybe we should fix this
              -- eventually.
              | otherwise
              = return ()
        mapM_ check_export (map availName exports)

        -- Note [Signature merging instances]
        -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        -- Merge instances into the global environment.  The algorithm here is
        -- dumb and simple: if an instance has exactly the same DFun type
        -- (tested by 'memberInstEnv') as an existing instance, we drop it;
        -- otherwise, we add it even, even if this would cause overlap.
        --
        -- Why don't we deduplicate instances with identical heads?  There's no
        -- good choice if they have premises:
        --
        --      instance K1 a => K (T a)
        --      instance K2 a => K (T a)
        --
        -- Why not eagerly error in this case?  The overlapping head does not
        -- necessarily mean that the instances are unimplementable: in fact,
        -- they may be implemented without overlap (if, for example, the
        -- implementing module has 'instance K (T a)'; both are implemented in
        -- this case.)  The implements test just checks that the wanteds are
        -- derivable assuming the givens.
        --
        -- Still, overlapping instances with hypotheses like above are going
        -- to be a bad deal, because instance resolution when we're typechecking
        -- against the merged signature is going to have a bad time when
        -- there are overlapping heads like this: we never backtrack, so it
        -- may be difficult to see that a wanted is derivable.  For now,
        -- we hope that we get lucky / the overlapping instances never
        -- get used, but it is not a very good situation to be in.
        --
        let merge_inst (insts, inst_env) inst
                | memberInstEnv inst_env inst -- test DFun Type equality
                = (insts, inst_env)
                | otherwise
                -- NB: is_dfun_name inst is still nonsense here,
                -- see Note [Signature merging DFuns]
                = (inst:insts, extendInstEnv inst_env inst)
            (insts, inst_env) = foldl' merge_inst
                                    (tcg_insts tcg_env, tcg_inst_env tcg_env)
                                    (instEnvElts $ md_insts details)
            -- Use mi_deps directly rather than calculateAvails.
            -- because a module is NOT
            -- supposed to include itself in its dep_orphs/dep_finsts.  See #13214
            avails = tcg_imports tcg_env
            deps = mi_deps iface
            avails_with_trans = addTransitiveDepInfo avails deps

        return tcg_env {
            tcg_inst_env = inst_env,
            tcg_insts    = insts,
            tcg_imports  = avails_with_trans,
            tcg_merged   =
                if outer_mod == mi_module iface
                    -- Don't add ourselves!
                    then tcg_merged tcg_env
                    else (mi_module iface, mi_mod_hash (mi_final_exts iface)) : tcg_merged tcg_env
            }

    -- Note [Signature merging DFuns]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    -- Once we know all of instances which will be defined by this merged
    -- signature, we go through each of the DFuns and rename them with a fresh,
    -- new, unique DFun Name, and add these DFuns to tcg_type_env (thus fixing
    -- up the "bogus" names that were setup in 'typecheckIfacesForMerging'.
    --
    -- We can't do this fixup earlier, because we need a way to identify each
    -- source DFun (from each of the signatures we are merging in) so that
    -- when we have a ClsInst, we can pull up the correct DFun to check if
    -- the types match.
    --
    -- See also Note [rnIfaceNeverExported] in GHC.Iface.Rename
    dfun_insts <- forM (tcg_insts tcg_env) $ \inst -> do
        n <- newDFunName (is_cls inst) (is_tys inst) (nameSrcSpan (is_dfun_name inst))
        let dfun = setVarName (is_dfun inst) n
        return (dfun, inst { is_dfun_name = n, is_dfun = dfun })

    tcg_env <- return $
      tcg_env { tcg_insts    = map snd dfun_insts
              , tcg_type_env = extendTypeEnvWithIds (tcg_type_env tcg_env) (map fst dfun_insts) }

    addDependentFiles src_files

    return tcg_env

-- | Add on the necessary transitive information from the merged signature to
-- the 'ImportAvails' of the result of merging. This propagates the orphan instances
-- which were in the transitive closure of the signature through the merge.
addTransitiveDepInfo :: ImportAvails -- ^ From the signature resulting from the merge
                     -> Dependencies -- ^ From the original signature
                     -> ImportAvails
addTransitiveDepInfo avails deps =
  -- Avails for the merged in signature
  -- Add on transitive information from the signature but nothing else..
  -- because we do not "import" the signature.
  avails { imp_orphs = imp_orphs avails ++ dep_orphs deps
         , imp_finsts = imp_finsts avails ++ dep_finsts deps
         , imp_sig_mods = imp_sig_mods avails ++ dep_sig_mods deps }

-- | Top-level driver for signature instantiation (run when compiling
-- an @hsig@ file.)
tcRnInstantiateSignature ::
    HscEnv -> Module -> RealSrcSpan ->
    IO (Messages TcRnMessage, Maybe TcGblEnv)
tcRnInstantiateSignature hsc_env this_mod real_loc =
   withTiming logger
              (text "Signature instantiation"<+>brackets (ppr this_mod))
              (const ()) $
   initTc hsc_env HsigFile False this_mod real_loc $ instantiateSignature
  where
   logger = hsc_logger hsc_env

exportOccs :: [AvailInfo] -> [OccName]
exportOccs = concatMap (map nameOccName . availNames)

impl_msg :: UnitState -> Module -> InstantiatedModule -> SDoc
impl_msg unit_state impl_mod (Module req_uid req_mod_name)
   = pprWithUnitState unit_state $
      text "While checking that" <+> quotes (ppr impl_mod) <+>
      text "implements signature" <+> quotes (ppr req_mod_name) <+>
      text "in" <+> quotes (ppr req_uid) <> dot

-- | Check if module implements a signature.  (The signature is
-- always un-hashed, which is why its components are specified
-- explicitly.)
checkImplements :: HasDebugCallStack => Module -> InstantiatedModule -> TcRn TcGblEnv
checkImplements impl_mod req_mod@(Module uid mod_name) = do
  hsc_env <- getTopEnv
  let unit_state = hsc_units hsc_env
      home_unit  = hsc_home_unit hsc_env
      other_home_units = hsc_all_home_unit_ids hsc_env
  addErrCtxt (impl_msg unit_state impl_mod req_mod) $ do
    let insts = instUnitInsts uid

    -- STEP 1: Load the implementing interface, and make a RdrEnv
    -- for its exports.  Also, add its 'ImportAvails' to 'tcg_imports',
    -- so that we treat all orphan instances it provides as visible
    -- when we verify that all instances are checked (see #12945), and so that
    -- when we eventually write out the interface we record appropriate
    -- dependency information.
    impl_iface <- initIfaceTcRn $
        loadSysInterface (text "checkImplements 1") impl_mod
    let impl_gr = mkGlobalRdrEnv
                    (gresFromAvails hsc_env Nothing (mi_exports impl_iface))
        nsubst = mkNameShape (moduleName impl_mod) (mi_exports impl_iface)

    -- Load all the orphans, so the subsequent 'checkHsigIface' sees
    -- all the instances it needs to
    loadModuleInterfaces (text "Loading orphan modules (from implementor of hsig)")
                         (dep_orphs (mi_deps impl_iface))

    let avails = calculateAvails home_unit other_home_units
                    impl_iface False{- safe -} NotBoot ImportedBySystem
        fix_env = mkNameEnv [ (greName rdr_elt, FixItem occ f)
                            | (occ, f) <- mi_fixities impl_iface
                            , rdr_elt <- lookupGRE_OccName (AllNameSpaces WantBoth) impl_gr occ ]
    updGblEnv (\tcg_env -> tcg_env {
        -- Setting tcg_rdr_env to treat all exported entities from
        -- the implementing module as in scope improves error messages,
        -- as it reduces the amount of qualification we need.  Unfortunately,
        -- we still end up qualifying references to external modules
        -- (see bkpfail07 for an example); we'd need to record more
        -- information in ModIface to solve this.
        tcg_rdr_env = tcg_rdr_env tcg_env `plusGlobalRdrEnv` impl_gr,
        tcg_imports = tcg_imports tcg_env `plusImportAvails` avails,
        -- This is here so that when we call 'lookupFixityRn' for something
        -- directly implemented by the module, we grab the right thing
        tcg_fix_env = fix_env
        }) $ do

    -- STEP 2: Load the *unrenamed, uninstantiated* interface for
    -- the ORIGINAL signature.  We are going to eventually rename it,
    -- but we must proceed slowly, because it is NOT known if the
    -- instantiation is correct.
    let sig_mod = mkModule (VirtUnit uid) mod_name
        isig_mod = fst (getModuleInstantiation sig_mod)
    hsc_env <- getTopEnv
    mb_isig_iface <- liftIO $ findAndReadIface hsc_env
                                               (text "checkImplements 2")
                                               isig_mod sig_mod NotBoot
    isig_iface <- case mb_isig_iface of
        Succeeded (iface, _) -> return iface
        Failed err ->
          failWithTc $ TcRnInterfaceError $
          Can'tFindInterface err (LookingForSig isig_mod)

    -- STEP 3: Check that the implementing interface exports everything
    -- we need.  (Notice we IGNORE the Modules in the AvailInfos.)
    forM_ (exportOccs (mi_exports isig_iface)) $ \occ ->
        case lookupGRE_OccName SameOccName impl_gr occ of
            [] -> addErr $ TcRnHsigMissingModuleExport occ unit_state impl_mod
            _ -> return ()
    failIfErrsM

    -- STEP 4: Now that the export is complete, rename the interface...
    sig_iface <- tcRnModIface insts (Just nsubst) isig_iface

    -- STEP 5: ...and typecheck it.  (Note that in both cases, the nsubst
    -- lets us determine how top-level identifiers should be handled.)
    sig_details <- initIfaceTcRn $ typecheckIfaceForInstantiate nsubst sig_iface

    -- STEP 6: Check that it's sufficient
    tcg_env <- getGblEnv
    checkHsigIface tcg_env impl_gr sig_iface sig_details

    -- STEP 7: Return the updated 'TcGblEnv' with the signature exports,
    -- so we write them out.
    return tcg_env {
        tcg_exports = mi_exports sig_iface
        }

-- | Given 'tcg_mod', instantiate a 'ModIface' from the indefinite
-- library to use the actual implementations of the relevant entities,
-- checking that the implementation matches the signature.
instantiateSignature :: TcRn TcGblEnv
instantiateSignature = do
    hsc_env <- getTopEnv
    tcg_env <- getGblEnv
    let outer_mod = tcg_mod tcg_env
        inner_mod = tcg_semantic_mod tcg_env
        home_unit = hsc_home_unit hsc_env
    -- TODO: setup the local RdrEnv so the error messages look a little better.
    -- But this information isn't stored anywhere. Should we RETYPECHECK
    -- the local one just to get the information?  Hmm...
    massert (isHomeModule home_unit outer_mod )
    massert (isHomeUnitInstantiating home_unit)
    let uid = homeUnitInstanceOf home_unit
    inner_mod `checkImplements`
        Module
            (mkInstantiatedUnit uid (homeUnitInstantiations home_unit))
            (moduleName outer_mod)