Avoid find_tycon panic if datacon is not in scope

When using TH to splice expressions involving record field construction,
the parent datacon may not be in scope.  We shouldn't panic about this,
because we will be renaming Exact RdrNames which don't require any
disambiguation.

Test Plan: new test th/T12130

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2321

GHC Trac Issues: #12130

compiler/rename/RnPat.hs

@@ -635,7 +635,7 @@ rnHsRecFields ctxt mk_arg (HsRecFields { rec_flds = flds, rec_dotdot = dotdot })
     find_tycon :: GlobalRdrEnv -> Name {- DataCon -} -> Maybe Name {- TyCon -}
     -- Return the parent *type constructor* of the data constructor
     -- (that is, the parent of the data constructor),
-    -- or 'Nothing' if it is a pattern synonym.
+    -- or 'Nothing' if it is a pattern synonym or not in scope.
     -- That's the parent to use for looking up record fields.
     find_tycon env con
       | Just (AConLike (RealDataCon dc)) <- wiredInNameTyThing_maybe con
@@ -647,8 +647,9 @@ rnHsRecFields ctxt mk_arg (HsRecFields { rec_flds = flds, rec_dotdot = dotdot })
           ParentIs p -> Just p
           _          -> Nothing
 
-      | otherwise
-      = pprPanic "find_tycon" (ppr con $$ ppr (lookupGRE_Name env con))
+      | otherwise = Nothing
+        -- This can happen if the datacon is not in scope
+        -- and we are in a TH splice (Trac #12130)
 
     dup_flds :: [[RdrName]]
         -- Each list represents a RdrName that occurred more than once

testsuite/tests/th/T12130.hs

+{-# Language TemplateHaskell #-}
+{-# Language DisambiguateRecordFields #-}
+
+module T12130 where
+
+import T12130a hiding (Block)
+
+b = $(block)

testsuite/tests/th/T12130a.hs

+{-# LANGUAGE TemplateHaskell #-}
+
+module T12130a where
+
+import Language.Haskell.TH
+
+data Block = Block
+    { blockSelector :: ()
+    }
+
+block :: Q Exp
+block =
+    [| Block {
+         -- Using record syntax is neccesary to trigger the bug.
+         blockSelector = ()
+       }
+    |]

testsuite/tests/th/all.T

@@ -407,3 +407,5 @@ test('T11941', normal, compile_fail, ['-v0'])
 test('T11484', normal, compile, ['-v0'])
 test('T8761', unless(ghc_dynamic(), expect_broken(12077)), compile,
      ['-v0 -ddump-splices -dsuppress-uniques'])
+test('T12130', extra_clean(['T12130a.hi','T12130a.o']),
+              multimod_compile, ['T12130', '-v0 ' + config.ghc_th_way_flags])