Fix #15845 by defining etaExpandFamInstLHS and using it

Summary:
Both #9692 and #14179 were caused by GHC being careless
about using eta-reduced data family instance axioms. Each of those
tickets were fixed by manually whipping up some code to eta-expand
the axioms. The same sort of issue has now caused #15845, so I
figured it was high time to factor out the code that each of these
fixes have in common.

This patch introduces the `etaExpandFamInstLHS` function, which takes
a family instance's type variables, LHS types, and RHS type, and
returns type variables and LHS types that have been eta-expanded if
necessary, in the case of a data family instance. (If it's a type
family instance, `etaExpandFamInstLHS` just returns the supplied type
variables and LHS types unchanged).

Along the way, I noticed that many references to
`Note [Eta reduction for data families]` (in `FamInstEnv`) had
slightly bitrotted (they either referred to a somewhat different
name, or claimed that the Note lived in a different module), so
I took the liberty of cleaning those up.

Test Plan: make test TEST="T9692 T15845"

Reviewers: goldfire, bgamari

Reviewed By: goldfire

Subscribers: rwbarton, carter

GHC Trac Issues: #15845

Differential Revision: https://phabricator.haskell.org/D5294

4 files changed, 25 insertions, 1 deletions

diff --git a/testsuite/tests/th/T15845.hs b/testsuite/tests/th/T15845.hs
new file mode 100644
index 0000000000..b44808f25a
--- /dev/null
+++ b/testsuite/tests/th/T15845.hs
@@ -0,0 +1,17 @@
+{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE TypeFamilies #-}
+module T15845 where
+
+import Language.Haskell.TH
+import System.IO
+
+data family F1 a b
+data instance F1 [a] b = MkF1
+
+data family F2 a
+data instance F2 a = MkF2
+
+$(do i1 <- reify ''F1
+     i2 <- reify ''F2
+     runIO $ mapM_ (hPutStrLn stderr . pprint) [i1, i2]
+     pure [])
diff --git a/testsuite/tests/th/T15845.stderr b/testsuite/tests/th/T15845.stderr
new file mode 100644
index 0000000000..2b6a37e453
--- /dev/null
+++ b/testsuite/tests/th/T15845.stderr
@@ -0,0 +1,5 @@
+data family T15845.F1 (a_0 :: *) (b_1 :: *) :: *
+data instance forall (a_2 :: *) (b_3 :: *). T15845.F1 ([a_2]) b_3
+    = T15845.MkF1
+data family T15845.F2 (a_0 :: *) :: *
+data instance forall (a_1 :: *). T15845.F2 a_1 = T15845.MkF2
diff --git a/testsuite/tests/th/T9692.stderr b/testsuite/tests/th/T9692.stderr
index ffa55365d4..070e4d3005 100644
--- a/testsuite/tests/th/T9692.stderr
+++ b/testsuite/tests/th/T9692.stderr
@@ -1,2 +1,3 @@
 data family T9692.F (a_0 :: k_1) (b_2 :: k_3) :: *
-data instance T9692.F GHC.Types.Int (x_4 :: *) = T9692.FInt x_4
+data instance forall (x_4 :: *). T9692.F GHC.Types.Int (x_4 :: *)
+    = T9692.FInt x_4
diff --git a/testsuite/tests/th/all.T b/testsuite/tests/th/all.T
index 2481a2ab12..adf897081b 100644
--- a/testsuite/tests/th/all.T
+++ b/testsuite/tests/th/all.T
@@ -448,3 +448,4 @@ test('T15783', normal, multimod_compile,
 test('T15792', normal, compile, ['-v0 -dsuppress-uniques'])
 test('T15815', normal, multimod_compile,
     ['T15815B', '-v0 ' + config.ghc_th_way_flags])
+test('T15845', normal, compile, ['-v0 -dsuppress-uniques'])