Slightly better `Coercible` errors.

This makes two real changes:
 - Equalities like (a ~R [a]) really *are* insoluble. Previously,
   GHC refused to give up when an occurs check bit on a representational
   equality. But for datatypes, it really should bail.

 - Now, GHC will sometimes report an occurs check error (in cases above)
   for representational equalities. Previously, it never did.

This "fixes" #10715, where by "fix", I mean clarifies the error message.
It's unclear how to do more to fix that ticket.

Test cases: typecheck/should_fail/T10715{,b}

5 files changed, 42 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T10715.hs b/testsuite/tests/typecheck/should_fail/T10715.hs
new file mode 100644
index 0000000000..969abd5cf2
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T10715.hs
@@ -0,0 +1,10 @@
+{-# LANGUAGE FlexibleContexts #-}
+module T10715 where
+
+import Data.Coerce (coerce, Coercible)
+import Data.Ord ( Down )  -- convenient newtype
+
+data X a
+
+doCoerce :: Coercible a (X a) => a -> X a
+doCoerce = coerce
diff --git a/testsuite/tests/typecheck/should_fail/T10715.stderr b/testsuite/tests/typecheck/should_fail/T10715.stderr
new file mode 100644
index 0000000000..e6f85a5256
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T10715.stderr
@@ -0,0 +1,15 @@
+
+T10715.hs:9:13: error:
+    Couldn't match representation of type ‘a’ with that of ‘X a’
+    ‘a’ is a rigid type variable bound by
+        the type signature for:
+          doCoerce :: Coercible a (X a) => a -> X a
+        at T10715.hs:9:13
+    Inaccessible code in
+      the type signature for:
+        doCoerce :: Coercible a (X a) => a -> X a
+    In the ambiguity check for the type signature for ‘doCoerce’:
+      doCoerce :: forall a. Coercible a (X a) => a -> X a
+    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
+    In the type signature for ‘doCoerce’:
+      doCoerce :: Coercible a (X a) => a -> X a
diff --git a/testsuite/tests/typecheck/should_fail/T10715b.hs b/testsuite/tests/typecheck/should_fail/T10715b.hs
new file mode 100644
index 0000000000..b2418b9b62
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T10715b.hs
@@ -0,0 +1,7 @@
+module T10715b where
+
+-- test error message: should complain about an occurs check
+
+import Data.Coerce
+
+foo = coerce `asTypeOf` head
diff --git a/testsuite/tests/typecheck/should_fail/T10715b.stderr b/testsuite/tests/typecheck/should_fail/T10715b.stderr
new file mode 100644
index 0000000000..47c85bb145
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T10715b.stderr
@@ -0,0 +1,8 @@
+
+T10715b.hs:7:7: error:
+    Occurs check: cannot construct the infinite type: b ~ [b]
+      arising from a use of ‘coerce’
+    Relevant bindings include foo :: [b] -> b (bound at T10715b.hs:7:1)
+    In the first argument of ‘asTypeOf’, namely ‘coerce’
+    In the expression: coerce `asTypeOf` head
+    In an equation for ‘foo’: foo = coerce `asTypeOf` head
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index 0a0281a587..a005bc5f29 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -386,3 +386,5 @@ test('ExpandSynsFail3', normal, compile_fail, ['-fprint-expanded-synonyms'])
 test('ExpandSynsFail4', normal, compile_fail, ['-fprint-expanded-synonyms'])
 test('T10698', expect_broken(10698), compile_fail, [''])
 test('T10836', normal, compile_fail, [''])
+test('T10715', normal, compile_fail, [''])
+test('T10715b', normal, compile_fail, [''])