ApplicativeDo: document behaviour with strict patterns (#13875)

Test Plan: unit tests, built docs

Reviewers: dfeuer, bgamari, simonpj, austin, erikd

Subscribers: rwbarton, thomie

GHC Trac Issues: #13875, #13242

Differential Revision: https://phabricator.haskell.org/D3691

4 files changed, 62 insertions, 1 deletions

diff --git a/testsuite/tests/ado/T13242.hs b/testsuite/tests/ado/T13242.hs
index ccaa93c087..2111b85baf 100644
--- a/testsuite/tests/ado/T13242.hs
+++ b/testsuite/tests/ado/T13242.hs
@@ -1,6 +1,6 @@
--- Panic.hs
 {-# LANGUAGE ApplicativeDo #-}
 {-# LANGUAGE ExistentialQuantification #-}
+{-# LANGUAGE GADTs #-}
 module T13242 where
 
 import Data.STRef
diff --git a/testsuite/tests/ado/T13242a.hs b/testsuite/tests/ado/T13242a.hs
new file mode 100644
index 0000000000..540b041fed
--- /dev/null
+++ b/testsuite/tests/ado/T13242a.hs
@@ -0,0 +1,13 @@
+{-# LANGUAGE ApplicativeDo #-}
+{-# LANGUAGE ExistentialQuantification #-}
+{-# LANGUAGE GADTs #-}
+module T13242a where
+
+data T where A :: forall a . Eq a => a -> T
+
+test :: IO Bool
+test = do
+  A x <- undefined
+  _ <- return 'a'
+  _ <- return 'b'
+  return (x == x)
diff --git a/testsuite/tests/ado/T13242a.stderr b/testsuite/tests/ado/T13242a.stderr
new file mode 100644
index 0000000000..dc4564f168
--- /dev/null
+++ b/testsuite/tests/ado/T13242a.stderr
@@ -0,0 +1,47 @@
+
+T13242a.hs:10:5: error:
+    • Couldn't match expected type ‘a0’ with actual type ‘a’
+        because type variable ‘a’ would escape its scope
+      This (rigid, skolem) type variable is bound by
+        a pattern with constructor: A :: forall a. Eq a => a -> T,
+        in a pattern binding in
+             'do' block
+        at T13242a.hs:10:3-5
+    • In the expression:
+        do A x <- undefined
+           _ <- return 'a'
+           _ <- return 'b'
+           return (x == x)
+      In an equation for ‘test’:
+          test
+            = do A x <- undefined
+                 _ <- return 'a'
+                 _ <- return 'b'
+                 return (x == x)
+    • Relevant bindings include x :: a (bound at T13242a.hs:10:5)
+
+T13242a.hs:13:11: error:
+    • Ambiguous type variable ‘a0’ arising from a use of ‘==’
+      prevents the constraint ‘(Eq a0)’ from being solved.
+      Relevant bindings include x :: a0 (bound at T13242a.hs:10:5)
+      Probable fix: use a type annotation to specify what ‘a0’ should be.
+      These potential instances exist:
+        instance Eq Ordering -- Defined in ‘GHC.Classes’
+        instance Eq Integer
+          -- Defined in ‘integer-gmp-1.0.0.1:GHC.Integer.Type’
+        instance Eq a => Eq (Maybe a) -- Defined in ‘GHC.Base’
+        ...plus 22 others
+        ...plus five instances involving out-of-scope types
+        (use -fprint-potential-instances to see them all)
+    • In a stmt of a 'do' block: return (x == x)
+      In the expression:
+        do A x <- undefined
+           _ <- return 'a'
+           _ <- return 'b'
+           return (x == x)
+      In an equation for ‘test’:
+          test
+            = do A x <- undefined
+                 _ <- return 'a'
+                 _ <- return 'b'
+                 return (x == x)
diff --git a/testsuite/tests/ado/all.T b/testsuite/tests/ado/all.T
index a738c7a6df..bb1cc163d1 100644
--- a/testsuite/tests/ado/all.T
+++ b/testsuite/tests/ado/all.T
@@ -9,4 +9,5 @@ test('T11607', normal, compile_and_run, [''])
 test('ado-optimal', normal, compile_and_run, [''])
 test('T12490', normal, compile, [''])
 test('T13242', normal, compile, [''])
+test('T13242a', normal, compile_fail, [''])
 test('T13875', normal, compile_and_run, [''])