Poor type error messages (refer to generated code rather than the higher level problem)

[igloo]

With this module:

module Foo where

data Foo a = Foo
data Bar a = Bar (Foo a)
    deriving Eq

the HEAD says:

q.hs:5:0:
    Non type-variable argument in the constraint: Eq (Foo a)
    (Use -fglasgow-exts to permit this)
    In the context: (Eq (Foo a))
    While checking the context of an instance declaration
    In the derived instance: (Eq (Foo a)) => Eq (Bar a)

which isn't really the error message that we want, as it's refering to the generated instance.

6.6 gives the much better:

q.hs:5:5:
    No instance for (Eq (Foo a))
      arising from the 'deriving' clause of a data type declaration
      at q.hs:5:5
    Possible fix: add an instance declaration for (Eq (Foo a))
    When deriving the instance for `Eq (Bar a)'

This example is from tcfail046; tcfail169 is similar and tcfail118 is also a related issue.

[simonpj]

Fixed, thanks.

Simon