Improve error message: Reduction stack overflow when using "coerce"
EDIT: Executive summary: The error messages below are confusing and not perspicuous to users. We should fix. ticket:15928#comment:163956 has a concrete suggestion to use as a starting point, and ticket:15928#comment:163991 suggests we print out the role signature of any tycons involved.
Compiling the following snippet results in a "Reduction stack overflow" error message:
{# Language ScopedTypeVariables #}
{# Language RankNTypes #}
import Data.Functor.Identity
import Data.Coerce
newtype Stream m a =
Stream {
unStream :: forall r. (Stream m a > m r) > m r
}
newtype SerialT m a = SerialT (Stream m a)
g :: SerialT Identity a > Identity Bool
g m = undefined
idSerial :: SerialT Identity a > SerialT Identity a
idSerial = id
f :: SerialT Identity a > Identity Bool
f = g . idSerial . coerce
main = undefined
The following error message is produced on compiling this with ghc8.6.2:
xy.hs:26:20: error:
• Reduction stack overflow; size = 201
When simplifying the following type:
Coercible
((Stream Identity a > Identity r) > Identity r)
((Stream Identity a0 > Identity r) > Identity r)
Use freductiondepth=0 to disable this check
(any upper bound you could choose might fail unpredictably with
minor updates to GHC, so disabling the check is recommended if
you're sure that type checking should terminate)
• In the second argument of ‘(.)’, namely ‘coerce’
In the second argument of ‘(.)’, namely ‘idSerial . coerce’
In the expression: g . idSerial . coerce

26  f = g . idSerial . coerce
 ^^^^^^
When I use an inline signature like this:
f :: SerialT Identity a > Identity Bool
f = g . (id :: SerialT Identity a > SerialT Identity a) . coerce
main = undefined
It again results in the same error:
xy.hs:18:60: error:
• Reduction stack overflow; size = 201
When simplifying the following type:
Coercible
((Stream Identity a > Identity r) > Identity r)
((Stream Identity a0 > Identity r) > Identity r)
Use freductiondepth=0 to disable this check
(any upper bound you could choose might fail unpredictably with
minor updates to GHC, so disabling the check is recommended if
you're sure that type checking should terminate)
• In the second argument of ‘(.)’, namely ‘coerce’
In the second argument of ‘(.)’, namely
‘(id :: SerialT Identity a > SerialT Identity a) . coerce’
In the expression:
g . (id :: SerialT Identity a > SerialT Identity a) . coerce

18  f = g . (id :: SerialT Identity a > SerialT Identity a) . coerce
 ^^^^^^
Everything works fine is I use an inline signature with a forall
keyword like this:
f :: forall a. SerialT Identity a > Identity Bool
f = g . (id :: SerialT Identity a > SerialT Identity a) . coerce
I have following questions:
 Why the first version results in a panic? Is that a bug?
 The second version might possibly be incorrect code because the types do not unify, but still it should not result in a panic, because of the panic I could not figure out what the problem is. It took a long time to isolate the code and then do some trial and error on it.