Use HsTyPats in associated type family defaults

Associated type family default declarations behave strangely in a
couple of ways:

1. If one tries to bind the type variables with an explicit `forall`,
   the `forall`'d part will simply be ignored. (#16110)
2. One cannot use visible kind application syntax on the left-hand
   sides of associated default equations, unlike every other form
   of type family equation. (#16356)

Both of these issues have a common solution. Instead of using
`LHsQTyVars` to represent the left-hand side arguments of an
associated default equation, we instead use `HsTyPats`, which is what
other forms of type family equations use. In particular, here are
some highlights of this patch:

* `FamEqn` is no longer parameterized by a `pats` type variable, as
  the `feqn_pats` field is now always `HsTyPats`.
* The new design for `FamEqn` in chronicled in
  `Note [Type family instance declarations in HsSyn]`.
* `TyFamDefltEqn` now becomes the same thing as `TyFamInstEqn`. This
  means that many of `TyFamDefltEqn`'s code paths can now reuse the
  code paths for `TyFamInstEqn`, resulting in substantial
  simplifications to various parts of the code dealing with
  associated type family defaults.

Fixes #16110 and #16356.

20 files changed, 211 insertions, 10 deletions

diff --git a/testsuite/tests/indexed-types/should_compile/T16110_Compile.hs b/testsuite/tests/indexed-types/should_compile/T16110_Compile.hs
new file mode 100644
index 0000000000..f05a4513fb
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_compile/T16110_Compile.hs
@@ -0,0 +1,13 @@
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeFamilies #-}
+module T16110_Compile where
+
+class C a where
+  type T1 a b
+  type forall a b. T1 a b = Either a b
+
+  type T2 a b
+  type forall x y. T2 x y = Either x y
+
+  type T3 a b
+  type forall. T3 _ _ = Int
diff --git a/testsuite/tests/indexed-types/should_compile/T16356_Compile1.hs b/testsuite/tests/indexed-types/should_compile/T16356_Compile1.hs
new file mode 100644
index 0000000000..74dee38ac4
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_compile/T16356_Compile1.hs
@@ -0,0 +1,25 @@
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE TypeFamilies #-}
+module T16356_Compile1 where
+
+import Data.Kind (Type)
+
+data B (a :: k)
+
+type family FClosed :: k -> Type where
+  FClosed @k = B @k
+
+type family FOpen :: k -> Type
+type instance FOpen @k = B @k
+
+class FAssocClass k where
+  type FAssoc :: k -> Type
+
+instance FAssocClass k where
+  type FAssoc @k = B @k
+
+class FAssocDefaultClass k where
+  type FAssocDefault :: k -> Type
+  type FAssocDefault @k = B @k
diff --git a/testsuite/tests/indexed-types/should_compile/T16356_Compile2.hs b/testsuite/tests/indexed-types/should_compile/T16356_Compile2.hs
new file mode 100644
index 0000000000..46c8b00e1f
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_compile/T16356_Compile2.hs
@@ -0,0 +1,16 @@
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# OPTIONS_GHC -Wunused-type-patterns #-}
+module T16356_Compile2 where
+
+class C (a :: j) where
+  type T1 (a :: j) (b :: k)
+  type T1 @j @_ a _ = Int
+
+  type T2 (a :: j) (b :: k)
+  type forall j (a :: j). T2 a _ = Int
+
+  type T3 (a :: j) (b :: k)
+  type forall j (a :: j). T3 @j @_ a _ = Int
diff --git a/testsuite/tests/indexed-types/should_compile/T16356_Compile2.stderr b/testsuite/tests/indexed-types/should_compile/T16356_Compile2.stderr
new file mode 100644
index 0000000000..3aceb43604
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_compile/T16356_Compile2.stderr
@@ -0,0 +1,18 @@
+
+T16356_Compile2.hs:10:12: warning: [-Wunused-type-patterns]
+    Defined but not used on the right hand side: type variable ‘j’
+
+T16356_Compile2.hs:10:17: warning: [-Wunused-type-patterns]
+    Defined but not used on the right hand side: type variable ‘a’
+
+T16356_Compile2.hs:13:15: warning: [-Wunused-type-patterns]
+    Defined but not used on the right hand side: type variable ‘j’
+
+T16356_Compile2.hs:13:18: warning: [-Wunused-type-patterns]
+    Defined but not used on the right hand side: type variable ‘a’
+
+T16356_Compile2.hs:16:15: warning: [-Wunused-type-patterns]
+    Defined but not used on the right hand side: type variable ‘j’
+
+T16356_Compile2.hs:16:18: warning: [-Wunused-type-patterns]
+    Defined but not used on the right hand side: type variable ‘a’
diff --git a/testsuite/tests/indexed-types/should_compile/all.T b/testsuite/tests/indexed-types/should_compile/all.T
index c268f2638d..9142b8edcd 100644
--- a/testsuite/tests/indexed-types/should_compile/all.T
+++ b/testsuite/tests/indexed-types/should_compile/all.T
@@ -286,4 +286,7 @@ test('T15711', normal, compile, ['-ddump-types'])
 test('T15852', normal, compile, ['-ddump-types'])
 test('T15764a', normal, compile, [''])
 test('T15740a', normal, compile, [''])
+test('T16110_Compile', normal, compile, [''])
+test('T16356_Compile1', normal, compile, [''])
+test('T16356_Compile2', normal, compile, [''])
 test('T16632', normal, compile, ['-Wunused-type-patterns -fdiagnostics-show-caret'])
diff --git a/testsuite/tests/indexed-types/should_fail/SimpleFail4.stderr b/testsuite/tests/indexed-types/should_fail/SimpleFail4.stderr
index 8768c66613..b791ea7d82 100644
--- a/testsuite/tests/indexed-types/should_fail/SimpleFail4.stderr
+++ b/testsuite/tests/indexed-types/should_fail/SimpleFail4.stderr
@@ -1,6 +1,7 @@
 
-SimpleFail4.hs:10:11: error:
-    Unexpected type ‘Int’
-    In the default declaration for ‘S2’
-    A default declaration should have form
-      default S2 a = ...
+SimpleFail4.hs:10:3: error:
+    • Illegal argument ‘Int’ in:
+        ‘type S2 Int = Char’
+        The arguments to ‘S2’ must all be distinct type variables
+    • In the default type instance declaration for ‘S2’
+      In the class declaration for ‘C2’
diff --git a/testsuite/tests/indexed-types/should_fail/T16110_Fail1.hs b/testsuite/tests/indexed-types/should_fail/T16110_Fail1.hs
new file mode 100644
index 0000000000..2ec2332660
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T16110_Fail1.hs
@@ -0,0 +1,19 @@
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeFamilies #-}
+module T16110_Fail1 where
+
+import Data.Kind
+
+class C (a :: Type) where
+  type T1 a b
+  type forall. T1 a b = Either a b
+
+  type T2 a b
+  type forall dup dup dup a b. T2 a b = Either a b
+
+  type T3 a b
+  type forall (a :: a) b. T3 a b = Either a b
+
+  type T4 a b
+  type forall (a :: k) k b. T4 a b = Either a b
diff --git a/testsuite/tests/indexed-types/should_fail/T16110_Fail1.stderr b/testsuite/tests/indexed-types/should_fail/T16110_Fail1.stderr
new file mode 100644
index 0000000000..2381655876
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T16110_Fail1.stderr
@@ -0,0 +1,18 @@
+
+T16110_Fail1.hs:10:19: error: Not in scope: type variable ‘a’
+
+T16110_Fail1.hs:10:21: error: Not in scope: type variable ‘b’
+
+T16110_Fail1.hs:10:32: error: Not in scope: type variable ‘a’
+
+T16110_Fail1.hs:10:34: error: Not in scope: type variable ‘b’
+
+T16110_Fail1.hs:13:15: error:
+    Conflicting definitions for ‘dup’
+    Bound at: T16110_Fail1.hs:13:15-17
+              T16110_Fail1.hs:13:19-21
+              T16110_Fail1.hs:13:23-25
+
+T16110_Fail1.hs:16:21: error: Not in scope: type variable ‘a’
+
+T16110_Fail1.hs:19:21: error: Not in scope: type variable ‘k’
diff --git a/testsuite/tests/indexed-types/should_fail/T16110_Fail2.hs b/testsuite/tests/indexed-types/should_fail/T16110_Fail2.hs
new file mode 100644
index 0000000000..fe0a9506ad
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T16110_Fail2.hs
@@ -0,0 +1,9 @@
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE TypeFamilies #-}
+module T16110_Fail2 where
+
+-- Ensure that kind variables don't leak into error messages if they're not
+-- pertitent to the issue at hand
+class C (a :: j) where
+  type T (a :: j) (b :: k) (c :: k)
+  type T a b b = Int
diff --git a/testsuite/tests/indexed-types/should_fail/T16110_Fail2.stderr b/testsuite/tests/indexed-types/should_fail/T16110_Fail2.stderr
new file mode 100644
index 0000000000..caa46af46a
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T16110_Fail2.stderr
@@ -0,0 +1,7 @@
+
+T16110_Fail2.hs:9:3: error:
+    • Illegal duplicate variable ‘b’ in:
+        ‘type T a b b = Int’
+        The arguments to ‘T’ must all be distinct type variables
+    • In the default type instance declaration for ‘T’
+      In the class declaration for ‘C’
diff --git a/testsuite/tests/indexed-types/should_fail/T16110_Fail3.hs b/testsuite/tests/indexed-types/should_fail/T16110_Fail3.hs
new file mode 100644
index 0000000000..89b1f27bb6
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T16110_Fail3.hs
@@ -0,0 +1,11 @@
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE TypeFamilies #-}
+module T16110_Fail3 where
+
+import Data.Kind
+
+-- Ensure that kind variables don't leak into error messages if they're not
+-- pertitent to the issue at hand
+class C (a :: j) where
+  type T (a :: j) (b :: Type)
+  type T a Int = Int
diff --git a/testsuite/tests/indexed-types/should_fail/T16110_Fail3.stderr b/testsuite/tests/indexed-types/should_fail/T16110_Fail3.stderr
new file mode 100644
index 0000000000..0fdea6a63a
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T16110_Fail3.stderr
@@ -0,0 +1,7 @@
+
+T16110_Fail3.hs:11:3: error:
+    • Illegal argument ‘Int’ in:
+        ‘type T a Int = Int’
+        The arguments to ‘T’ must all be distinct type variables
+    • In the default type instance declaration for ‘T’
+      In the class declaration for ‘C’
diff --git a/testsuite/tests/indexed-types/should_fail/T16356_Fail1.hs b/testsuite/tests/indexed-types/should_fail/T16356_Fail1.hs
new file mode 100644
index 0000000000..13a9cde2f3
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T16356_Fail1.hs
@@ -0,0 +1,10 @@
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE TypeFamilies #-}
+module T16356_Fail1 where
+
+import Data.Kind
+
+class C (a :: j) where
+  type T (a :: j)
+  type T @Type a = Maybe a
diff --git a/testsuite/tests/indexed-types/should_fail/T16356_Fail1.stderr b/testsuite/tests/indexed-types/should_fail/T16356_Fail1.stderr
new file mode 100644
index 0000000000..b354d1db00
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T16356_Fail1.stderr
@@ -0,0 +1,7 @@
+
+T16356_Fail1.hs:10:3: error:
+    • Illegal argument ‘*’ in:
+        ‘type T @* a = Maybe a’
+        The arguments to ‘T’ must all be distinct type variables
+    • In the default type instance declaration for ‘T’
+      In the class declaration for ‘C’
diff --git a/testsuite/tests/indexed-types/should_fail/T16356_Fail2.hs b/testsuite/tests/indexed-types/should_fail/T16356_Fail2.hs
new file mode 100644
index 0000000000..1ed53e02d9
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T16356_Fail2.hs
@@ -0,0 +1,8 @@
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE TypeFamilies #-}
+module T16356_Fail2 where
+
+class C (a :: j) where
+  type T (a :: j) (b :: k)
+  type T @k @k a b = k
diff --git a/testsuite/tests/indexed-types/should_fail/T16356_Fail2.stderr b/testsuite/tests/indexed-types/should_fail/T16356_Fail2.stderr
new file mode 100644
index 0000000000..37f8159ae0
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T16356_Fail2.stderr
@@ -0,0 +1,7 @@
+
+T16356_Fail2.hs:8:3: error:
+    • Illegal duplicate variable ‘k’ in:
+        ‘type T @k @k a b = k’
+        The arguments to ‘T’ must all be distinct type variables
+    • In the default type instance declaration for ‘T’
+      In the class declaration for ‘C’
diff --git a/testsuite/tests/indexed-types/should_fail/T16356_Fail3.hs b/testsuite/tests/indexed-types/should_fail/T16356_Fail3.hs
new file mode 100644
index 0000000000..da59f5399d
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T16356_Fail3.hs
@@ -0,0 +1,9 @@
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE TypeFamilies #-}
+module T16356_Fail3 where
+
+import Data.Kind
+
+class C a where
+  type T1 a
+  type T1 @Type a = a
diff --git a/testsuite/tests/indexed-types/should_fail/T16356_Fail3.stderr b/testsuite/tests/indexed-types/should_fail/T16356_Fail3.stderr
new file mode 100644
index 0000000000..e8b59175f4
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T16356_Fail3.stderr
@@ -0,0 +1,6 @@
+
+T16356_Fail3.hs:9:3: error:
+    • Cannot apply function of kind ‘* -> *’
+      to visible kind argument ‘Type’
+    • In the default type instance declaration for ‘T1’
+      In the class declaration for ‘C’
diff --git a/testsuite/tests/indexed-types/should_fail/all.T b/testsuite/tests/indexed-types/should_fail/all.T
index e154a31dd0..1ad9aa2504 100644
--- a/testsuite/tests/indexed-types/should_fail/all.T
+++ b/testsuite/tests/indexed-types/should_fail/all.T
@@ -153,3 +153,9 @@ test('T15870', normal, compile_fail, [''])
 test('T14887', normal, compile_fail, [''])
 test('T14230', normal, compile_fail, [''])
 test('T14230a', normal, compile_fail, [''])
+test('T16110_Fail1', normal, compile_fail, [''])
+test('T16110_Fail2', normal, compile_fail, [''])
+test('T16110_Fail3', normal, compile_fail, [''])
+test('T16356_Fail1', normal, compile_fail, [''])
+test('T16356_Fail2', normal, compile_fail, [''])
+test('T16356_Fail3', normal, compile_fail, [''])
diff --git a/testsuite/tests/typecheck/should_fail/AssocTyDef02.stderr b/testsuite/tests/typecheck/should_fail/AssocTyDef02.stderr
index b310a79a6f..e76e8a89e2 100644
--- a/testsuite/tests/typecheck/should_fail/AssocTyDef02.stderr
+++ b/testsuite/tests/typecheck/should_fail/AssocTyDef02.stderr
@@ -1,6 +1,7 @@
 
-AssocTyDef02.hs:6:14:
-    Unexpected type ‘[b]’
-    In the default declaration for ‘Typ’
-    A default declaration should have form
-      default Typ a = ...
+AssocTyDef02.hs:6:5: error:
+    • Illegal argument ‘[b]’ in:
+        ‘type Typ [b] = Int’
+        The arguments to ‘Typ’ must all be distinct type variables
+    • In the default type instance declaration for ‘Typ’
+      In the class declaration for ‘Cls’