Require PatternSynonyms language flag when encountering a use of pattern synonym

(#8961)

compiler/typecheck/TcPat.lhs

@@ -813,14 +813,12 @@ tcPatSynPat penv (L con_span _) pat_syn pat_ty arg_pats thing_inside
 
         ; prov_dicts' <- newEvVars prov_theta'
 
-          {-
+        -- Using a pattern synonym requires the PatternSynonyms
+        -- language flag to keep consistent with #2905
         ; patsyns_on <- xoptM Opt_PatternSynonyms
 	; checkTc patsyns_on
                   (ptext (sLit "A pattern match on a pattern synonym requires PatternSynonyms"))
-		  -- Trac #2905 decided that a *pattern-match* of a GADT
-		  -- should require the GADT language flag.
-                  -- Re TypeFamilies see also #7156
--}
+
         ; let skol_info = case pe_ctxt penv of
                             LamPat mc -> PatSkol (PatSynCon pat_syn) mc
                             LetPat {} -> UnkSkol -- Doesn't matter

testsuite/tests/patsyn/should_compile/all.T

@@ -7,3 +7,4 @@ test('ex-view', normal, compile, [''])
 test('ex-num', normal, compile, [''])
 test('num', normal, compile, [''])
 test('incomplete', normal, compile, [''])
+test('export', normal, compile, [''])

testsuite/tests/patsyn/should_compile/export.hs

+{-# LANGUAGE PatternSynonyms #-}
+module ShouldCompile (pattern Single) where
+
+pattern Single x <- [x]

testsuite/tests/patsyn/should_fail/T8961.hs

+module ShouldFail where
+
+import T8961a
+
+single :: [a] -> Maybe a
+single (Single x) = Just x
+single _ = Nothing

testsuite/tests/patsyn/should_fail/T8961.stderr

+[1 of 2] Compiling T8961a           ( T8961a.hs, T8961a.o )
+[2 of 2] Compiling ShouldFail       ( T8961.hs, T8961.o )
+
+T8961.hs:6:9:
+    A pattern match on a pattern synonym requires PatternSynonyms
+    In the pattern: Single x
+    In an equation for ‘single’: single (Single x) = Just x

testsuite/tests/patsyn/should_fail/T8961a.hs

+{-# LANGUAGE PatternSynonyms #-}
+module T8961a (pattern Single) where
+
+pattern Single x <- [x]

testsuite/tests/patsyn/should_fail/all.T

@@ -2,3 +2,4 @@
 test('mono', normal, compile_fail, [''])
 test('unidir', normal, compile_fail, [''])
 test('local', normal, compile_fail, [''])
+test('T8961', normal, multimod_compile_fail, ['T8961',''])