Compiler ignores Typeable constraint

Summary

Compiler allows passing types with no Typeable instance to functions declared with Typeable constraint.

Steps to reproduce

The following compiles (no pragmas needed)

import Data.Typeable 

brokenGhc :: Typeable a => a -> a 
brokenGhc = id  

tst = brokenGhc (Foo "bar") 

brokenGhc2 :: (Typeable a) => a -> String
brokenGhc2 = show . typeOf

tst2 = brokenGhc2 (Foo "Bar")
-- ghci> tst2
-- "Foo"

data Foo = Foo String

Possibly related to #10770?

Expected behavior

tst and tst2 should not compile. Note Foo has no instances!

Environment

• GHC version used: Tested with ghc-8.6.5/lts-14.0, ghc-8.2.2/lts-12.22

Optional:

• Operating System: Ubuntu 18.04 and Ubuntu 18.10

• System Architecture:

Suggested Labels:

Typeable ~bug program incorrectly accepted