Improve out-of-order inferred type variables

  Don't instantiate type variables for :type in
  `GHC.Tc.Gen.App.tcInstFun`, to avoid inconsistently instantianting
  `r1` but not `r2` in the type

    forall {r1} (a :: TYPE r1) {r2} (b :: TYPE r2). ...

  This fixes #21088.

  This patch also changes the primop pretty-printer to ensure
  that we put all the inferred type variables first. For example,
  the type of reallyUnsafePtrEquality# is now

    forall {l :: Levity} {k :: Levity}
           (a :: TYPE (BoxedRep l))
           (b :: TYPE (BoxedRep k)).
      a -> b -> Int#

  This means we avoid running into issue #21088 entirely with
  the types of primops. Users can still write a type signature where
  the inferred type variables don't come first, however.

  This change to primops had a knock-on consequence, revealing that
  we were sometimes performing eta reduction on keepAlive#.
  This patch updates tryEtaReduce to avoid eta reducing functions
  with no binding, bringing it in line with tryEtaReducePrep,
  and thus fixing #21090.

5 files changed, 62 insertions, 1 deletions

diff --git a/testsuite/tests/ghci/scripts/T12550.stdout b/testsuite/tests/ghci/scripts/T12550.stdout
index 48a1b8e11c..d753d4f666 100644
--- a/testsuite/tests/ghci/scripts/T12550.stdout
+++ b/testsuite/tests/ghci/scripts/T12550.stdout
@@ -62,7 +62,8 @@ instance Functor (URec Int) -- Defined in ‘GHC.Generics’
 instance Functor (URec Word) -- Defined in ‘GHC.Generics’
 instance Functor V1 -- Defined in ‘GHC.Generics’
 datatypeName
-  ∷ ∀ d k1 (t ∷ ★ → (k1 → ★) → k1 → ★) (f ∷ k1 → ★) (a ∷ k1).
+  ∷ ∀ {k} (d ∷ k) k1 (t ∷ k → (k1 → ★) → k1 → ★) (f ∷ k1 → ★)
+      (a ∷ k1).
     Datatype d ⇒
     t d f a → [Char]
 type Datatype :: ∀ {k}. k → Constraint
diff --git a/testsuite/tests/ghci/scripts/T21088.hs b/testsuite/tests/ghci/scripts/T21088.hs
new file mode 100644
index 0000000000..efe4e8ab93
--- /dev/null
+++ b/testsuite/tests/ghci/scripts/T21088.hs
@@ -0,0 +1,29 @@
+{-# LANGUAGE PolyKinds, DataKinds, ScopedTypeVariables #-}
+
+module T21088 where
+
+import Data.Proxy
+  ( Proxy(..) )
+import GHC.Exts
+  ( TYPE, RuntimeRep )
+
+-- We don't change the order of quantification,
+-- so we check we are not instantiating `r1` but not `r2`,
+-- which would be quite confusing.
+foo :: forall {r1 :: RuntimeRep} (a1 :: TYPE r1)
+              {r2 :: RuntimeRep} (a2 :: TYPE r2)
+    .  Proxy a1 -> Proxy a2
+foo _ = Proxy
+
+bar :: forall {r1 :: RuntimeRep} {r2 :: RuntimeRep}
+              (a1 :: TYPE r1)    (a2 :: TYPE r2)
+    .  Proxy a1 -> Proxy a2
+bar _ = Proxy
+
+baz :: forall {k1} (a1 :: k1) {k2} (a2 :: k2)
+    .  Proxy a1 -> Proxy a2
+baz _ = Proxy
+
+quux :: forall {k1} {k2} (a1 :: k1) (a2 :: k2)
+     .  Proxy a1 -> Proxy a2
+quux _ = Proxy
diff --git a/testsuite/tests/ghci/scripts/T21088.script b/testsuite/tests/ghci/scripts/T21088.script
new file mode 100644
index 0000000000..fe809b1970
--- /dev/null
+++ b/testsuite/tests/ghci/scripts/T21088.script
@@ -0,0 +1,11 @@
+:l T21088
+:type foo
+:type bar
+:type baz
+:type quux
+
+:set -fprint-explicit-kinds -fprint-explicit-runtime-reps -fprint-explicit-foralls
+:type foo
+:type bar
+:type baz
+:type quux
diff --git a/testsuite/tests/ghci/scripts/T21088.stdout b/testsuite/tests/ghci/scripts/T21088.stdout
new file mode 100644
index 0000000000..ca3c4ddcc7
--- /dev/null
+++ b/testsuite/tests/ghci/scripts/T21088.stdout
@@ -0,0 +1,19 @@
+foo :: Proxy a1 -> Proxy a2
+bar :: Proxy a1 -> Proxy a2
+baz :: forall {k1} (a1 :: k1) {k2} (a2 :: k2). Proxy a1 -> Proxy a2
+quux
+  :: forall {k1} {k2} (a1 :: k1) (a2 :: k2). Proxy a1 -> Proxy a2
+foo
+  :: forall {r1 :: RuntimeRep} (a1 :: TYPE r1) {r2 :: RuntimeRep}
+            (a2 :: TYPE r2).
+     Proxy @{TYPE r1} a1 -> Proxy @{TYPE r2} a2
+bar
+  :: forall {r1 :: RuntimeRep} {r2 :: RuntimeRep} (a1 :: TYPE r1)
+            (a2 :: TYPE r2).
+     Proxy @{TYPE r1} a1 -> Proxy @{TYPE r2} a2
+baz
+  :: forall {k1} (a1 :: k1) {k2} (a2 :: k2).
+     Proxy @{k1} a1 -> Proxy @{k2} a2
+quux
+  :: forall {k1} {k2} (a1 :: k1) (a2 :: k2).
+     Proxy @{k1} a1 -> Proxy @{k2} a2
diff --git a/testsuite/tests/ghci/scripts/all.T b/testsuite/tests/ghci/scripts/all.T
index bccfa977e5..0f6ed54ddb 100755
--- a/testsuite/tests/ghci/scripts/all.T
+++ b/testsuite/tests/ghci/scripts/all.T
@@ -363,3 +363,4 @@ test('T20587', [extra_files(['../shell.hs'])], ghci_script,
 test('T20909', normal, ghci_script, ['T20909.script'])
 test('T20150', normal, ghci_script, ['T20150.script'])
 test('T20974', normal, ghci_script, ['T20974.script'])
+test('T21088', normal, ghci_script, ['T21088.script'])