Inconsistent reporting of missing instance docs

[crockeea]

Haddock reports missing documentation for instances based on whether there is an explicit export list or not.

Consider the following example:

Main.hs

class Foo a

instance Foo Int

-- | Main method
main :: IO ()
main = undefined

With GHC 7.10.3 and haddock 2.16.1, I get:

> haddock Main.hs
Haddock coverage:
  50% (  1 /  2) in 'Main'
  Missing documentation for:
    Module header
Warning: Main: could not find link destinations for:
    IO

If I add a module export line thusly:

module Main where

class Foo a

instance Foo Int

-- | Main method
main :: IO ()
main = undefined

> haddock Main.hs
Haddock coverage:
  25% (  1 /  4) in 'Main'
  Missing documentation for:
    Module header
    Foo (Main.hs:3)
     (Main.hs:5)
Warning: Main: could not find link destinations for:
    Int IO

So adding module Main where resulted in an unexpected "missing documentation" report for instance Foo Int. If I change the module line to module Main (main, Foo(..)) where (i.e., explicitly export everything), then haddock no longer complains about missing docs for the instance:

>haddock Main.hs
Haddock coverage:
  33% (  1 /  3) in 'Main'
  Missing documentation for:
    Module header
    Foo (Main.hs:3)
Warning: Main: could not find link destinations for:
    IO Int

When is haddock supposed to report missing docs for instances?
Since instances are always exported, why does haddock treat explicit export lists differently from implicit exports?

[brandonchinn178]

+1

[crockeea]

I just noticed another inconsistency in my example: with no module clause, Haddock does not complain about missing docs for class Foo, although with the module (with both explicit and implicit exports) Haddock does complain about missing docs for Foo.