Slighly improve infix con app pattern errors

Given this program:

    main = do
      f $ do
        a <- return 3
          c <- do
          return 5

GHC previously gave this error message:

    Main.hs:2:7: error:
        Parse error in pattern: do a <- return 3 c
        Possibly caused by a missing 'do'?
      |
    2 |   f $ do
      |       ^^...

What happened is GHC considered the whole `f $ do a <- return 3 c` as a
pattern. When parsed as an expression it becomes an infix application of
`($)`, and GHC checks left and right hand sides before checking if `($)`
is a valid infix constructor name, and shows the first error it got.

If instead we first check if the infix op is valid in pattern context,
the error message becomes much clearer:

    Main.hs:2:3: error:
        Parse error in pattern: f $ do a <- return 3 c
        Possibly caused by a missing 'do'?
      |
    2 |   f $ do
      |   ^^^^^^...

This may not entirely fix #11188 but I think it's an improvement.

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #11188

Differential Revision: https://phabricator.haskell.org/D4497

4 files changed, 18 insertions, 6 deletions

diff --git a/compiler/parser/RdrHsSyn.hs b/compiler/parser/RdrHsSyn.hs
index e2943c8001..dc35c124cb 100644
--- a/compiler/parser/RdrHsSyn.hs
+++ b/compiler/parser/RdrHsSyn.hs
@@ -984,12 +984,13 @@ checkAPat msg loc e0 = do
                       | extopt LangExt.NPlusKPatterns opts && (plus == plus_RDR)
                       -> return (mkNPlusKPat (L nloc n) (L lloc lit))
 
-   OpApp l op _fix r  -> do l <- checkLPat msg l
-                            r <- checkLPat msg r
-                            case op of
-                               L cl (HsVar (L _ c)) | isDataOcc (rdrNameOcc c)
-                                      -> return (ConPatIn (L cl c) (InfixCon l r))
-                               _ -> patFail msg loc e0
+   OpApp l (L cl (HsVar (L _ c))) _fix r
+     | isDataOcc (rdrNameOcc c) -> do
+         l <- checkLPat msg l
+         r <- checkLPat msg r
+         return (ConPatIn (L cl c) (InfixCon l r))
+
+   OpApp _l _op _fix _r -> patFail msg loc e0
 
    HsPar e            -> checkLPat msg e >>= (return . ParPat)
    ExplicitList _ _ es  -> do ps <- mapM (checkLPat msg) es
diff --git a/testsuite/tests/parser/should_fail/InfixAppPatErr.hs b/testsuite/tests/parser/should_fail/InfixAppPatErr.hs
new file mode 100644
index 0000000000..5a56f711eb
--- /dev/null
+++ b/testsuite/tests/parser/should_fail/InfixAppPatErr.hs
@@ -0,0 +1,5 @@
+main = do
+  f $ do
+    a <- return 3
+      c <- do
+      return 5
diff --git a/testsuite/tests/parser/should_fail/InfixAppPatErr.stderr b/testsuite/tests/parser/should_fail/InfixAppPatErr.stderr
new file mode 100644
index 0000000000..69839e3920
--- /dev/null
+++ b/testsuite/tests/parser/should_fail/InfixAppPatErr.stderr
@@ -0,0 +1,4 @@
+
+InfixAppPatErr.hs:2:3: error:
+    Parse error in pattern: f $ do a <- return 3 c
+    Possibly caused by a missing 'do'?
diff --git a/testsuite/tests/parser/should_fail/all.T b/testsuite/tests/parser/should_fail/all.T
index 2cb9c49de2..d47e0f5796 100644
--- a/testsuite/tests/parser/should_fail/all.T
+++ b/testsuite/tests/parser/should_fail/all.T
@@ -115,3 +115,5 @@ test('NumericUnderscoresFail0',
      grep_errmsg(r'^NumericUnderscoresFail0.hs:'), compile_fail, [''])
 test('NumericUnderscoresFail1',
      grep_errmsg(r'^NumericUnderscoresFail1.hs:'), compile_fail, [''])
+
+test('InfixAppPatErr', normal, compile_fail, [''])