MultiWayIf makes it easy to write partial programs that are not catched by -Wall

The following partial function will pass -Wall

-- program 1
{-# LANGUAGE MultiWayIf #-}
foo :: Bool -> Int
foo b = if | b -> 23

While the following two alternatives will not:

-- program 2
foo :: Bool -> Int 
foo b | b = 23

-- program 3
foo :: Bool -> Int
foo b = case () of
  () | b -> 23

Note that the GHC User's Guide states that "program 1" and "program 3" are equivalent.

1. Is this a bug or by design?
2. I guess at the very least we would want to update the User's Guide

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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