Fix layout of MultiWayIf expressions (#10807)

With this patch we stop generating virtual semicolons in MultiWayIf
guards. Fixes #10807.

Test Plan:

Reviewers: simonmar, austin, bgamari

Reviewed By: simonmar

Subscribers: mpickering, thomie

Differential Revision: https://phabricator.haskell.org/D2524

GHC Trac Issues: #10807

3 files changed, 48 insertions, 0 deletions

diff --git a/testsuite/tests/parser/should_run/T10807.hs b/testsuite/tests/parser/should_run/T10807.hs
new file mode 100644
index 0000000000..8f6546201e
--- /dev/null
+++ b/testsuite/tests/parser/should_run/T10807.hs
@@ -0,0 +1,43 @@
+{-# LANGUAGE MultiWayIf #-}
+
+module Main where
+
+-- This is how we had to use multi-way if previously. Not indenting lines after
+-- `|` was causing a parse error.
+f1 x = if | even x
+           , x /= 0
+           -> True
+          | otherwise
+           -> False
+
+-- This was previously causing a parse error, but actually it should work.
+f2 x = if | even x
+          , x /= 0
+          -> True
+          | otherwise
+          -> False
+
+-- If we don't generate {} in MultiWayIf we get a shift/reduce conflict here:
+-- It's not clear which guards belong to `case` and which ones belong to `if`.
+--
+-- This test is to make sure we parse it correctly.
+--
+-- - If we shift, we get a non-exhaustive pattern error when argument is odd.
+-- - If we reduce, we run the unreachable code when argument is odd.
+f3 x = case x of
+         x' | even x'   -> if | even x' -> 1 | otherwise -> error "should be unreachable"
+            | otherwise -> 3
+
+-- Testing line breaks
+f4 x = case x of
+         x' | even x'   -> if
+             | even x' -> 1
+             | otherwise -> error "should be unreachable"
+            | otherwise -> 3
+
+main :: IO ()
+main = do
+  print (f3 1)
+  print (f3 2)
+  print (f4 1)
+  print (f4 2)
diff --git a/testsuite/tests/parser/should_run/T10807.stdout b/testsuite/tests/parser/should_run/T10807.stdout
new file mode 100644
index 0000000000..9fcb40e7af
--- /dev/null
+++ b/testsuite/tests/parser/should_run/T10807.stdout
@@ -0,0 +1,4 @@
+3
+1
+3
+1
diff --git a/testsuite/tests/parser/should_run/all.T b/testsuite/tests/parser/should_run/all.T
index 8a72c421ab..bb5e4fde39 100644
--- a/testsuite/tests/parser/should_run/all.T
+++ b/testsuite/tests/parser/should_run/all.T
@@ -9,3 +9,4 @@ test('ParserMultiWayIf', [], compile_and_run, [''])
 test('BinaryLiterals0', normal, compile_and_run, [''])
 test('BinaryLiterals1', [], compile_and_run, [''])
 test('BinaryLiterals2', [], compile_and_run, [''])
+test('T10807', normal, compile_and_run, [''])