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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