Support Typeable instances for types with coercions

If I say

{-# LANGUAGE GADTs, TypeApplications, TypeInType #-}

module Bug where

import Type.Reflection

data G a where
  MkG :: a ~ Bool => G a

rep = typeRep @MkG

I get

Bug.hs:10:7: error:
    • No instance for (Typeable <>) arising from a use of ‘typeRep’
    • In the expression: typeRep @MkG
      In an equation for ‘rep’: rep = typeRep @MkG
   |
10 | rep = typeRep @MkG
   |   

First off, the error message is confusing, mentioning the mysterious <>. But more importantly, it would be nice if the Typeable mechanism supported coercions.

Trac metadata

Trac field	Value
Version	8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

mentioned in issue #13872 (closed)

changed weight to 5

added Tbug Trac import Typeable labels

Trac metadata

Trac field	Value
Related	- → #13872 (closed)

mentioned in issue #15872 (closed)

See #15872 (closed) for a standalone ticket about the pretty-printing issue.

Trac metadata

Trac field	Value
Related	#13872 (closed) → #13872 (closed), #15872 (closed)

added TypeInType label

mentioned in issue #15862 (closed)

mentioned in issue #16835 (closed)

added Pnormal label

Now that constraints are no longer permitted in kinds (see #22298 (closed)), GHC no longer permits writing typeRep @MkG in the example above, which eliminates much of the motivation for this issue. Perhaps there are other reasons to desire allowing Typeable instances for coercions, but I'm not aware of any.