fromInteger-related pattern match overlap warnings

[ashley-y]

The compiler incorrectly gives "Warning: Pattern match(es) are 
overlapped" for this file:

{-# OPTIONS -Werror #-}

module Buggy where

	instance (Num a) => Num (Maybe a) where
		(Just a) + (Just b) = Just (a + b)
		_ + _ = Nothing
		(Just a) - (Just b) = Just (a - b)
		_ - _ = Nothing
		(Just a) * (Just b) = Just (a * b)
		_ * _ = Nothing
		negate (Just a) = Just (negate a)
		negate _ = Nothing
		abs (Just a) = Just (abs a)
		abs _ = Nothing
		signum (Just a) = Just (signum a)
		signum _ = Nothing
		fromInteger = Just . fromInteger

	f :: Maybe Int -> Int
	f 1 = 1
	f Nothing = 2
	f _ = 3

[nobody]

Logged In: NO 

If we define the first line as:

f (Just 1) = 1

there is no problem. 

[simonmar]

See #595

[marcotmarcot]

I'm thinking about this bug, and the only way I can see to avoid it, is by knowing in compile-time, that is, when the compiler checks if the Patterns are complete, how the value (1 :: Integer) will be converted to (1 :: Maybe Int). This is defined in "fromInteger = Just . fromInteger", which is a user code. So the user code will have to be evaluated in compile time. Is this already done in any case? I don't think so, as this would create the possibility of the compilation not terminating.

[igloo]

You can't make the warning perfect, but in cases where we are not sure we have 3 choices:

• warn
• don't warn
• give a different warning, saying that we aren't sure (this warning could be en/disabled separately)