From fe57a5bae3f8cb87637359f615c77f4afae86d46 Mon Sep 17 00:00:00 2001
From: Matthew Yacavone <matthew@yacavone.net>
Date: Thu, 29 Nov 2018 18:42:39 -0500
Subject: Fix #15828, from `More explicit foralls`

Summary:
Fix a bug in commit 12eeb9 which permits the following:

```
class C a where
  type T a b
instance C (Maybe a) where
  type forall a b. T (Maybe a) b = b
```

where instead, the user should write:

```
instance C (Maybe a) where
  type forall b. T (Maybe a) b = b
```

Update the users guide to discuss scoping of type variables in
explicit foralls in type family instances.

Test Plan: validate

Reviewers: bgamari, goldfire, monoidal

Reviewed By: goldfire

Subscribers: monoidal, rwbarton, carter

GHC Trac Issues: #15828

Differential Revision: https://phabricator.haskell.org/D5283
---
 testsuite/tests/rename/should_fail/T15828.hs     | 9 +++++++++
 testsuite/tests/rename/should_fail/T15828.stderr | 9 +++++++++
 testsuite/tests/rename/should_fail/all.T         | 1 +
 3 files changed, 19 insertions(+)
 create mode 100644 testsuite/tests/rename/should_fail/T15828.hs
 create mode 100644 testsuite/tests/rename/should_fail/T15828.stderr

(limited to 'testsuite/tests/rename/should_fail')

diff --git a/testsuite/tests/rename/should_fail/T15828.hs b/testsuite/tests/rename/should_fail/T15828.hs
new file mode 100644
index 0000000000..90c06214c4
--- /dev/null
+++ b/testsuite/tests/rename/should_fail/T15828.hs
@@ -0,0 +1,9 @@
+{-# LANGUAGE TypeFamilies, ExplicitForAll #-}
+
+module T15828 where
+
+class C a where
+  type T a b
+
+instance C (Maybe a) where
+  type forall a b. T (Maybe a) b = b
diff --git a/testsuite/tests/rename/should_fail/T15828.stderr b/testsuite/tests/rename/should_fail/T15828.stderr
new file mode 100644
index 0000000000..aca2542d88
--- /dev/null
+++ b/testsuite/tests/rename/should_fail/T15828.stderr
@@ -0,0 +1,9 @@
+
+T15828.hs:9:20: error:
+    • Type indexes must match class instance head
+      Expected: T (Maybe a1) <tv>
+        Actual: T (Maybe a) b
+      where the `<tv>' arguments are type variables,
+      distinct from each other and from the instance variables
+    • In the type instance declaration for ‘T’
+      In the instance declaration for ‘C (Maybe a)’
\ No newline at end of file
diff --git a/testsuite/tests/rename/should_fail/all.T b/testsuite/tests/rename/should_fail/all.T
index 9ca330f873..6fd014356c 100644
--- a/testsuite/tests/rename/should_fail/all.T
+++ b/testsuite/tests/rename/should_fail/all.T
@@ -140,5 +140,6 @@ test('T15659', normal, compile_fail, [''])
 test('T15607', normal, compile_fail, [''])
 test('T15611a', normal, compile_fail, [''])
 test('T15611b', normal, ghci_script, ['T15611b.script'])
+test('T15828', normal, compile_fail, [''])
 
 test('ExplicitForAllRules2', normal, compile_fail, [''])
-- 
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