Boot file instances should imply superclasses

Currently, if I write the following program:

-- A.hs-boot
module A where
data T
instance Ord T

-- B.hs
module B where
import {-# SOURCE #-} A
f :: T -> T -> Bool
f x y = x == y 

I get:

B.hs:4:11:
    No instance for (Eq T) arising from a use of `=='
    Possible fix: add an instance declaration for (Eq T)
    In the expression: x == y
    In an equation for `f': f x y = x == y

This is a bit confusing, because Ord instances are supposed to imply Eq instances.

GHC should either:

1. Reject A.hs-boot (claiming that it could not find evidence that T had an Eq instance), or
2. Accept B.hs, since the instance requirement for Ord should imply Eq.

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture