Suggest enabling PatternSynonyms (#10943)

Suggest enabling PatternSynonyms if we find an invalid
signature that looks like a pattern synonym.

Reviewed By: austin, thomie

Differential Revision: https://phabricator.haskell.org/D1347

compiler/parser/RdrHsSyn.hs

@@ -947,11 +947,14 @@ checkValSig lhs@(L l _) ty
                        ppr lhs <+> text "::" <+> ppr ty)
                    $$ text hint)
   where
-    hint = if foreign_RDR `looks_like` lhs
-           then "Perhaps you meant to use ForeignFunctionInterface?"
-           else if default_RDR `looks_like` lhs
-                then "Perhaps you meant to use DefaultSignatures?"
-                else "Should be of form <variable> :: <type>"
+    hint | foreign_RDR `looks_like` lhs =
+           "Perhaps you meant to use ForeignFunctionInterface?"
+         | default_RDR `looks_like` lhs =
+           "Perhaps you meant to use DefaultSignatures?"
+         | pattern_RDR `looks_like` lhs =
+           "Perhaps you meant to use PatternSynonyms?"
+         | otherwise =
+           "Should be of form <variable> :: <type>"
     -- A common error is to forget the ForeignFunctionInterface flag
     -- so check for that, and suggest.  cf Trac #3805
     -- Sadly 'foreign import' still barfs 'parse error' because 'import' is a keyword
@@ -961,6 +964,7 @@ checkValSig lhs@(L l _) ty
 
     foreign_RDR = mkUnqual varName (fsLit "foreign")
     default_RDR = mkUnqual varName (fsLit "default")
+    pattern_RDR = mkUnqual varName (fsLit "pattern")
 
 
 checkDoAndIfThenElse :: LHsExpr RdrName

testsuite/tests/parser/should_fail/NoPatternSynonyms.hs

+module NoPatternSynonyms where
+
+pattern P :: G Int

testsuite/tests/parser/should_fail/NoPatternSynonyms.stderr

+
+NoPatternSynonyms.hs:3:1: error:
+    Invalid type signature: pattern P :: G Int
+    Perhaps you meant to use PatternSynonyms?

testsuite/tests/parser/should_fail/all.T

@@ -68,6 +68,7 @@ test('T3811e', normal, compile_fail, [''])
 test('T3811f', normal, compile_fail, [''])
 test('T3811g', normal, compile_fail, [''])
 test('NoDoAndIfThenElse', normal, compile_fail, [''])
+test('NoPatternSynonyms', normal, compile_fail, [''])
 test('NondecreasingIndentationFail', normal, compile_fail, [''])
 test('readFailTraditionalRecords1', normal, compile_fail, [''])
 test('readFailTraditionalRecords2', normal, compile_fail, [''])