UnboxedTuples can't be used as constraints

Fixes #14740.

Test Plan: make test TEST="14740"

Reviewers: bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, thomie, carter

GHC Trac Issues: #14740

Differential Revision: https://phabricator.haskell.org/D4359

(cherry picked from commit ced9fbd3)

compiler/parser/RdrHsSyn.hs

@@ -779,11 +779,22 @@ checkTyClHdr is_cls ty
       = parseErrorSDoc l (text "Malformed head of type or class declaration:"
                           <+> ppr ty)
 
+-- | Validate the context constraints and break up a context into a list
+-- of predicates.
+--
+-- @
+--     (Eq a, Ord b)        -->  [Eq a, Ord b]
+--     Eq a                 -->  [Eq a]
+--     (Eq a)               -->  [Eq a]
+--     (((Eq a)))           -->  [Eq a]
+-- @
 checkContext :: LHsType GhcPs -> P ([AddAnn],LHsContext GhcPs)
 checkContext (L l orig_t)
   = check [] (L l orig_t)
  where
-  check anns (L lp (HsTupleTy _ ts))   -- (Eq a, Ord b) shows up as a tuple type
+  check anns (L lp (HsTupleTy HsBoxedOrConstraintTuple ts))
+    -- (Eq a, Ord b) shows up as a tuple type. Only boxed tuples can
+    -- be used as context constraints.
     = return (anns ++ mkParensApiAnn lp,L l ts)                -- Ditto ()
 
     -- don't let HsAppsTy get in the way

testsuite/tests/parser/should_fail/T14740.hs

+{-# LANGUAGE UnboxedTuples #-}
+
+module T14740 where
+
+x :: ((##)) => ()
+x = ()

testsuite/tests/parser/should_fail/T14740.stderr

+
+T14740.hs:5:7:
+     Expecting a lifted type, but ‘(# #)’ is unlifted
+     In the type signature: x :: ((# #)) => ()

testsuite/tests/parser/should_fail/all.T

@@ -103,3 +103,4 @@ test('T8501b', normal, compile_fail, [''])
 test('T8501c', normal, compile_fail, [''])
 test('T12610', normal, compile_fail, [''])
 test('InfixAppPatErr', normal, compile_fail, [''])
+test('T14740', normal, compile_fail, [''])