Pass -this-component-id to GHC when necessary.

Previously GHC inferred it off of -this-unit-id but we've
modified things so that GHC makes no assumptions about the
format of instantiated unit ids. In that case, pass it in
explicitly!
Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Cabal/Distribution/Simple/GHC.hs

@@ -736,6 +736,13 @@ buildOrReplLib forRepl verbosity numJobs pkg_descr lbi lib clbi = do
                     LibComponentLocalBuildInfo { componentCompatPackageKey = pk }
                       -> toFlag pk
                     _ -> mempty,
+                ghcOptThisComponentId = case clbi of
+                    LibComponentLocalBuildInfo { componentUnitId = lib_uid
+                                               , componentInstantiatedWith = insts } ->
+                        if null insts
+                            then mempty
+                            else toFlag (unitIdComponentId lib_uid)
+                    _ -> mempty,
                 ghcOptInstantiatedWith = case clbi of
                     LibComponentLocalBuildInfo { componentInstantiatedWith = insts }
                       -> insts

Cabal/Distribution/Simple/GHC/Internal.hs

@@ -289,6 +289,13 @@ componentGhcOptions verbosity lbi bi clbi odir =
         LibComponentLocalBuildInfo { componentCompatPackageKey = pk }
           -> toFlag pk
         _ -> mempty,
+      ghcOptThisComponentId = case clbi of
+          LibComponentLocalBuildInfo { componentUnitId = uid
+                                     , componentInstantiatedWith = insts } ->
+              if null insts
+                  then mempty
+                  else toFlag (unitIdComponentId uid)
+          _ -> mempty,
       ghcOptInstantiatedWith = case clbi of
         LibComponentLocalBuildInfo { componentInstantiatedWith = insts }
           -> insts

Cabal/Distribution/Simple/Program/GHC.hs

@@ -19,6 +19,7 @@ module Distribution.Simple.Program.GHC (
 import Prelude ()
 import Distribution.Compat.Prelude
 
+import Distribution.Package
 import Distribution.Backpack
 import Distribution.Simple.GHC.ImplInfo
 import Distribution.PackageDescription hiding (Flag)
@@ -83,6 +84,12 @@ data GhcOptions = GhcOptions {
   -- (we need to handle backwards compatibility.)
   ghcOptThisUnitId   :: Flag String,
 
+  -- | GHC doesn't make any assumptions about the format of
+  -- definite unit ids, so when we are instantiating a package it
+  -- needs to be told explicitly what the component being instantiated
+  -- is.  This only gets set when 'ghcOptInstantiatedWith' is non-empty
+  ghcOptThisComponentId :: Flag ComponentId,
+
   ghcOptInstantiatedWith :: [(ModuleName, OpenModule)],
 
   -- | No code? (But we turn on interface writing
@@ -403,6 +410,9 @@ renderGhcOptions comp _platform@(Platform _arch os) opts
              , this_arg ]
              | this_arg <- flag ghcOptThisUnitId ]
 
+  , concat [ ["-this-component-id", display this_cid ]
+           | this_cid <- flag ghcOptThisComponentId ]
+
   , if null (ghcOptInstantiatedWith opts)
         then []
         else "-instantiated-with"