Fix #13947 by checking for unbounded names more

Commit 2484d4da accidentally dropped a
call to `isUnboundName` in an important location. This re-adds it.

Fixes #13947.

Test Plan: make test TEST=T13947

Reviewers: adamgundry, austin, bgamari

Reviewed By: adamgundry

Subscribers: rwbarton, thomie

GHC Trac Issues: #13947

Differential Revision: https://phabricator.haskell.org/D3718

compiler/rename/RnTypes.hs

@@ -1431,8 +1431,9 @@ sectionPrecErr op@(n1,_) arg_op@(n2,_) section
          nest 4 (text "in the section:" <+> quotes (ppr section))]
 
 is_unbound :: OpName -> Bool
-is_unbound UnboundOp{} = True
-is_unbound _           = False
+is_unbound (NormalOp n) = isUnboundName n
+is_unbound UnboundOp{}  = True
+is_unbound _            = False
 
 ppr_opfix :: (OpName, Fixity) -> SDoc
 ppr_opfix (op, fixity) = pp_op <+> brackets (ppr fixity)

testsuite/tests/rename/should_fail/T13947.hs

+{-# LANGUAGE TypeOperators #-}
+module T13947 where
+
+f :: () -> Int :~: Int
+f = undefined

testsuite/tests/rename/should_fail/T13947.stderr

+
+T13947.hs:4:12: error:
+    Not in scope: type constructor or class ‘:~:’

testsuite/tests/rename/should_fail/all.T

@@ -127,3 +127,4 @@ test('T11592', normal, compile_fail, [''])
 test('T12879', normal, compile_fail, [''])
 test('T13644', expect_broken(13644), multimod_compile_fail, ['T13644','-v0'])
 test('T13568', normal, multimod_compile_fail, ['T13568','-v0'])
+test('T13947', normal, compile_fail, [''])