PartialTypeSignatures, AllowAmbiguousTypes and ScopedTypeVariables don't play nicely together

*Motivation**: I was playing with something like this when I found this great confusion.

I was able to produce a minimal example. In each declaration/definition, the function body is the same, but the signature varies.

I expect every single one of them to compile, resulting in the same type as minimal4.

The actual results are shown in-line.

I think this sufficiently shows that this behavior is a bug.

(The line numbers in the following code aren't useful; I pasted them in after compiling)

(The question marks are there because CMD wasn't able to display bullets, I think.)

{-# LANGUAGE PartialTypeSignatures #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE KindSignatures #-}

minimal1_noksig :: forall m. _ => Bool
minimal1_noksig = (mempty :: m) == (mempty :: m)
{-
ambi.hs:17:30: error:
    ? Expected a type, but ‘m’ has kind ‘k0’
    ? In an expression type signature: m
      In the first argument of ‘(==)’, namely ‘(mempty :: m)’
      In the expression: (mempty :: m) == (mempty :: m)
-}

minimal1 :: forall (m :: *). _ => Bool
minimal1 = (mempty :: m) == (mempty :: m)
{-
ambi.hs:11:1: error:
    ? Ambiguous type variable ‘m0’
      prevents the constraint ‘(Monoid m0)’ from being solved.
    ? When checking that the inferred type
        minimal1 :: forall m. (Monoid m, Eq m) => Bool
      is as general as its (partial) signature
        minimal1 :: Bool
-}

minimal2 :: forall m. (Eq m, _) => Bool
minimal2 = (mempty :: m) == (mempty :: m)
{-
ambi.hs:14:1: error:
    ? Could not deduce (Monoid m1)
      from the context: (Eq m, Monoid m)
        bound by the inferred type for ‘minimal2’:
                   (Eq m, Monoid m) => Bool
        at ambi.hs:14:1-33
      The type variable ‘m1’ is ambiguous
    ? When checking that the inferred type
        minimal2 :: forall m. (Monoid m, Eq m) => Bool
      is as general as its (partial) signature
        minimal2 :: forall m. (Eq m, Monoid m) => Bool
-}

minimal3 :: forall m. (Monoid m, _) => Bool
minimal3 = (mempty :: m) == (mempty :: m)
-- Compiles

minimal4 :: forall m. (Monoid m, Eq m) => Bool
minimal4 = (mempty :: m) == (mempty :: m)
-- Compiles

The code was run in GHCi. GHC version is 8.0.2.

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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PartialTypeSignatures, AllowAmbiguousTypes and ScopedTypeVariables don't play nicely together

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