Fix #15214 by listing (~) in isBuiltInOcc_maybe

This changes an obscure error (which mistakenly mentions
Template Haskell) to one that makes more sense.

Test Plan: make test TEST=T15214

Reviewers: bgamari, mpickering

Reviewed By: bgamari, mpickering

Subscribers: mpickering, rwbarton, thomie, carter

GHC Trac Issues: #15214

Differential Revision: https://phabricator.haskell.org/D4768

compiler/prelude/TysWiredIn.hs

@@ -679,6 +679,9 @@ isBuiltInOcc_maybe occ =
       "[]" -> Just $ choose_ns listTyConName nilDataConName
       ":"    -> Just consDataConName
 
+      -- equality tycon
+      "~"    -> Just eqTyConName
+
       -- boxed tuple data/tycon
       "()"    -> Just $ tup_name Boxed 0
       _ | Just rest <- "(" `BS.stripPrefix` name

testsuite/tests/rename/should_fail/T15214.hs

+{-# LANGUAGE TypeOperators #-}
+module T15214 where
+
+type (~) = Either

testsuite/tests/rename/should_fail/T15214.stderr

+
+T15214.hs:4:1: error: Illegal binding of built-in syntax: ~

testsuite/tests/rename/should_fail/all.T

@@ -130,3 +130,4 @@ test('T13947', normal, compile_fail, [''])
 test('T13847', normal, multimod_compile_fail, ['T13847','-v0'])
 test('T14307', normal, compile_fail, [''])
 test('T14591', normal, compile_fail, [''])
+test('T15214', normal, compile_fail, [''])