Change in behavior from GHC 7.10.3 to GHC 8.0.1?

[FranklinChen]

Upon upgrade from Stackage LTS 6.x (corresponding to GHC 7.10.3) to 7.0 (corresponding to GHC 8.0.1), tests that used to succeed now fail. A sample repo is here: https://github.com/FranklinChen/should-not-typecheck-example

$ stack test --resolver lts-6

shows that the tests used to pass.

$ stack test

will reproduce the new errors under lts-7.

The errors now:

  test/ShouldNotTypecheckExampleSpec.hs:21:
  1) ShouldNotTypecheckExample.should-not-typecheck you can run code even if it contains ill-typed parts
       uncaught exception: TypeError (src/ShouldNotTypecheckExample.hs:8:28: error:
           • Couldn't match expected type ‘[Char]’ with actual type ‘Bool’
           • In the second argument of ‘(/)’, namely ‘True’
             In the expression: "world" / True
             In the expression: ["world" / True, "!"]
       (deferred type error))

  test/ShouldNotTypecheckExampleSpec.hs:23:
  2) ShouldNotTypecheckExample.should-not-typecheck deferred type errors are only lazily reached
       uncaught exception: TypeError (src/ShouldNotTypecheckExample.hs:14:28: error:
           • Couldn't match expected type ‘[Char]’ with actual type ‘Bool’
           • In the second argument of ‘(/)’, namely ‘True’
             In the expression: "world" / True
             In the expression: ["world" / True, "!"]
       (deferred type error))