Fix #13968 by consulting isBuiltInOcc_maybe

Summary:
We were unconditionally reporting `Illegal binding of built-in syntax`
in an error message, but this error doesn't make sense in certain Template
Haskell scenarios which can trigger it. Let's give a more sensible error
message by first checking if the name we're binding really is built-in syntax,
using the handy `isBuiltInOcc_maybe` function.

Test Plan: make test TEST=T13968

Reviewers: bgamari, austin, goldfire

Reviewed By: goldfire

Subscribers: goldfire, rwbarton, thomie

GHC Trac Issues: #13968

Differential Revision: https://phabricator.haskell.org/D3789

4 files changed, 25 insertions, 3 deletions

diff --git a/compiler/rename/RnEnv.hs b/compiler/rename/RnEnv.hs
index 617b3556bb..298de54168 100644
--- a/compiler/rename/RnEnv.hs
+++ b/compiler/rename/RnEnv.hs
@@ -53,7 +53,7 @@ import HscTypes
 import TcEnv
 import TcRnMonad
 import RdrHsSyn         ( setRdrNameSpace )
-import TysWiredIn       ( starKindTyConName, unicodeStarKindTyConName )
+import TysWiredIn
 import Name
 import NameSet
 import NameEnv
@@ -1573,5 +1573,17 @@ opDeclErr n
 
 badOrigBinding :: RdrName -> SDoc
 badOrigBinding name
-  = text "Illegal binding of built-in syntax:" <+> ppr (rdrNameOcc name)
-        -- The rdrNameOcc is because we don't want to print Prelude.(,)
+  | Just _ <- isBuiltInOcc_maybe occ
+  = text "Illegal binding of built-in syntax:" <+> ppr occ
+    -- Use an OccName here because we don't want to print Prelude.(,)
+  | otherwise
+  = text "Cannot redefine a Name retrieved by a Template Haskell quote:"
+    <+> ppr name
+    -- This can happen when one tries to use a Template Haskell splice to
+    -- define a top-level identifier with an already existing name, e.g.,
+    --
+    --   $(pure [ValD (VarP 'succ) (NormalB (ConE 'True)) []])
+    --
+    -- (See Trac #13968.)
+  where
+    occ = rdrNameOcc name
diff --git a/testsuite/tests/th/T13968.hs b/testsuite/tests/th/T13968.hs
new file mode 100644
index 0000000000..1e54ef12f3
--- /dev/null
+++ b/testsuite/tests/th/T13968.hs
@@ -0,0 +1,6 @@
+{-# LANGUAGE TemplateHaskell #-}
+module T13968 where
+
+import Language.Haskell.TH
+
+$(pure [ValD (VarP 'succ) (NormalB (ConE 'True)) []])
diff --git a/testsuite/tests/th/T13968.stderr b/testsuite/tests/th/T13968.stderr
new file mode 100644
index 0000000000..2850dae0c5
--- /dev/null
+++ b/testsuite/tests/th/T13968.stderr
@@ -0,0 +1,3 @@
+
+T13968.hs:6:3: error:
+    Cannot redefine a Name retrieved by a Template Haskell quote: succ
diff --git a/testsuite/tests/th/all.T b/testsuite/tests/th/all.T
index f89be6e0bb..df31162c3e 100644
--- a/testsuite/tests/th/all.T
+++ b/testsuite/tests/th/all.T
@@ -390,3 +390,4 @@ test('T13642', normal, compile_fail, ['-v0'])
 test('T13781', normal, compile, ['-v0'])
 test('T13782', normal, compile, [''])
 test('T13856', normal, compile, ['-v0 -ddump-splices -dsuppress-uniques'])
+test('T13968', normal, compile_fail, ['-v0'])