Clean up Coercible handling, and interaction of data families with newtypes

This patch fixes Trac #9580, in which the Coercible machinery succeeded
even though the relevant data constructor was not in scope.

As usual I got dragged into a raft of refactoring changes,
all for the better.

* Delete TcEvidence.coercionToTcCoercion (now unused)

* Move instNewTyConTF_maybe, instNewTyCon_maybe to FamInst,
  and rename them to tcInstNewTyConTF_maybe, tcInstNewTyCon
  (They both return TcCoercions.)

* tcInstNewTyConTF_maybe also gets more convenient type,
  which improves TcInteract.getCoercibleInst

* Define FamInst.tcLookupDataFamInst, and use it in TcDeriv,
  (as well as in tcInstNewTyConTF_maybe)

* Improve error report for Coercible errors, when data familes
  are involved  Another use of tcLookupDataFamInst

* In TcExpr.tcTagToEnum, use tcLookupDataFamInst to replace
  local hacky code

* Fix Coercion.instNewTyCon_maybe and Type.newTyConInstRhs to deal
  with eta-reduced newtypes, using
  (new) Type.unwrapNewTyConEtad_maybe and (new) Type.applyTysX

Some small refactoring of TcSMonad.matchFam.

5 files changed, 25 insertions, 1 deletions

diff --git a/testsuite/tests/indexed-types/should_fail/T9580.hs b/testsuite/tests/indexed-types/should_fail/T9580.hs
new file mode 100644
index 0000000000..de5455f493
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T9580.hs
@@ -0,0 +1,7 @@
+module T9580 where
+
+import T9580a
+import Data.Coerce
+
+foo :: Dimensional Int Double -> Double
+foo x = coerce x
diff --git a/testsuite/tests/indexed-types/should_fail/T9580.stderr b/testsuite/tests/indexed-types/should_fail/T9580.stderr
new file mode 100644
index 0000000000..19ceefb0ff
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T9580.stderr
@@ -0,0 +1,10 @@
+[1 of 2] Compiling T9580a           ( T9580a.hs, T9580a.o )
+[2 of 2] Compiling T9580            ( T9580.hs, T9580.o )
+
+T9580.hs:7:9:
+    Could not coerce from ‘Dimensional Int Double’ to ‘Double’
+    because the data constructor ‘T9580a.Quantity'’
+      of newtype ‘Dimensional Int v’ is not in scope
+      arising from a use of ‘coerce’
+    In the expression: coerce x
+    In an equation for ‘foo’: foo x = coerce x
diff --git a/testsuite/tests/indexed-types/should_fail/T9580a.hs b/testsuite/tests/indexed-types/should_fail/T9580a.hs
new file mode 100644
index 0000000000..578c8bafb8
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T9580a.hs
@@ -0,0 +1,5 @@
+{-# LANGUAGE KindSignatures, TypeFamilies #-}
+module T9580a( Dimensional ) where
+
+data family Dimensional var :: * -> *
+newtype instance Dimensional Int v = Quantity' v
diff --git a/testsuite/tests/indexed-types/should_fail/all.T b/testsuite/tests/indexed-types/should_fail/all.T
index 2862f1e854..f06060efd0 100644
--- a/testsuite/tests/indexed-types/should_fail/all.T
+++ b/testsuite/tests/indexed-types/should_fail/all.T
@@ -128,4 +128,5 @@ test('T9357', normal, compile_fail, [''])
 test('T9371', normal, compile_fail, [''])
 test('T9433', normal, compile_fail, [''])
 test('BadSock', normal, compile_fail, [''])
+test('T9580', normal, multimod_compile_fail, ['T9580', ''])
 
diff --git a/testsuite/tests/typecheck/should_fail/TcCoercibleFail.stderr b/testsuite/tests/typecheck/should_fail/TcCoercibleFail.stderr
index 5bb9210a13..b95b4cea67 100644
--- a/testsuite/tests/typecheck/should_fail/TcCoercibleFail.stderr
+++ b/testsuite/tests/typecheck/should_fail/TcCoercibleFail.stderr
@@ -33,7 +33,8 @@ TcCoercibleFail.hs:16:8:
 
 TcCoercibleFail.hs:18:8:
     Could not coerce from ‘Int’ to ‘Down Int’
-    because the constructor of ‘Down’ is not imported
+    because the data constructor ‘Data.Ord.Down’
+      of newtype ‘Down’ is not in scope
       arising from a use of ‘coerce’
     In the expression: coerce
     In the expression: coerce $ one :: Down Int