From 4d71cc89b4e9648f3fbb29c8fcd25d725616e265 Mon Sep 17 00:00:00 2001
From: Adam Gundry <adam@well-typed.com>
Date: Sat, 18 Jun 2016 12:27:47 +0200
Subject: [PATCH] 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      |  7 ++++---
 testsuite/tests/th/T12130.hs  |  8 ++++++++
 testsuite/tests/th/T12130a.hs | 17 +++++++++++++++++
 testsuite/tests/th/all.T      |  2 ++
 4 files changed, 31 insertions(+), 3 deletions(-)
 create mode 100644 testsuite/tests/th/T12130.hs
 create mode 100644 testsuite/tests/th/T12130a.hs

diff --git a/compiler/rename/RnPat.hs b/compiler/rename/RnPat.hs
index 98ca38bf66..d252f7fef0 100644
--- a/compiler/rename/RnPat.hs
+++ b/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
diff --git a/testsuite/tests/th/T12130.hs b/testsuite/tests/th/T12130.hs
new file mode 100644
index 0000000000..7ab7492655
--- /dev/null
+++ b/testsuite/tests/th/T12130.hs
@@ -0,0 +1,8 @@
+{-# Language TemplateHaskell #-}
+{-# Language DisambiguateRecordFields #-}
+
+module T12130 where
+
+import T12130a hiding (Block)
+
+b = $(block)
diff --git a/testsuite/tests/th/T12130a.hs b/testsuite/tests/th/T12130a.hs
new file mode 100644
index 0000000000..f393967e84
--- /dev/null
+++ b/testsuite/tests/th/T12130a.hs
@@ -0,0 +1,17 @@
+{-# 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 = ()
+       }
+    |]
diff --git a/testsuite/tests/th/all.T b/testsuite/tests/th/all.T
index 75364dca98..bd59c4ee30 100644
--- a/testsuite/tests/th/all.T
+++ b/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])
-- 
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