Rip out 9-year-old pattern variable hack (#17007)

GHC had an ad hoc validity check in place to rule out pattern
variables bound by type synonyms, such as in the following example:

```hs
type ItemColID a b = Int  -- Discards a,b

get :: ItemColID a b -> ItemColID a b
get (x :: ItemColID a b) = x :: ItemColID a b
```

This hack is wholly unnecessary nowadays, since OutsideIn(X) is more
than capable of instantiating `a` and `b` to `Any`. In light of this,
let's rip out this validity check.

Fixes #17007.

3 files changed, 23 insertions, 9 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T17007.hs b/testsuite/tests/typecheck/should_compile/T17007.hs
new file mode 100644
index 0000000000..21b7639dd0
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T17007.hs
@@ -0,0 +1,13 @@
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeFamilies #-}
+module T17007 where
+
+type ItemColID a b = Int  -- Discards a,b
+
+get :: ItemColID a b -> ItemColID a b
+get (x :: ItemColID a b) = x :: ItemColID a b
+
+type family ItemColID' a b where ItemColID' a b = Int  -- Discards a,b
+
+get' :: ItemColID' a b -> ItemColID' a b
+get' (x :: ItemColID' a b) = x :: ItemColID' a b
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index 9e9d48659d..e393fe41c7 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -685,3 +685,4 @@ test('UnliftedNewtypesLPFamily', normal, compile, [''])
 test('UnliftedNewtypesDifficultUnification', normal, compile, [''])
 test('T16832', normal, ghci_script, ['T16832.script'])
 test('T16946', normal, compile, [''])
+test('T17007', normal, compile, [''])
diff --git a/testsuite/tests/typecheck/should_fail/T3406.stderr b/testsuite/tests/typecheck/should_fail/T3406.stderr
index 4525bba5d6..69834d15f6 100644
--- a/testsuite/tests/typecheck/should_fail/T3406.stderr
+++ b/testsuite/tests/typecheck/should_fail/T3406.stderr
@@ -1,10 +1,10 @@
 
-T3406.hs:11:6:
-    The type variables ‘a, b’
-    should be bound by the pattern signature ‘ItemColID a b’
-    but are actually discarded by a type synonym
-    To fix this, expand the type synonym
-    [Note: I hope to lift this restriction in due course]
-    In the pattern: x :: ItemColID a b
-    In an equation for ‘get’:
-        get (x :: ItemColID a b) = x :: ItemColID a b
+T3406.hs:11:28: error:
+    • Couldn't match type ‘Int’ with ‘a -> ItemColID a b’
+      Expected type: a -> ItemColID a b
+        Actual type: ItemColID a1 b1
+    • In the expression: x :: ItemColID a b
+      In an equation for ‘get’:
+          get (x :: ItemColID a b) = x :: ItemColID a b
+    • Relevant bindings include
+        get :: ItemColID a b -> a -> ItemColID a b (bound at T3406.hs:11:1)