Induced `Eq` constraint on numeric literal + partial type signature = panic!

When I use a partial type signature in a non-top-level let-binding (or where clause), and the type is sufficiently ambiguous, and the binding in question uses a numeric literal, I get a panic from GHC instead of a report of the inferred type of the hole.

So, this breaks:

module Fails where

fails :: a
fails =
   let go :: _
       go 0 a = a
   in go (0 :: Int) undefined

Fails.hs:7:11:
    Couldn't match expected type ‘a’ with actual type ‘Int’
      ‘a’ is untouchable
        inside the constraints ()
        bound by the type signature for fails :: a1
        at Fails.hs:3:10ghc: panic! (the 'impossible' happened)
  (GHC version 7.10.2 for x86_64-apple-darwin):
   No skolem info: a_a1iD[sk]

1. ..but this succeeds:

module Succeeds where

succeeds :: a
succeeds = 
   let go :: _
       go _ a = a
   in go (0 :: Int) undefined

Succeeds.hs:13:14:
    Found hole ‘_’ with type: t -> t1 -> t1
    Where: ‘t’ is a rigid type variable bound by
               the inferred type of go :: t -> t1 -> t1 at Succeeds.hs:14:8
           ‘t1’ is a rigid type variable bound by
                the inferred type of go :: t -> t1 -> t1 at Succeeds.hs:14:8
    To use the inferred type, enable PartialTypeSignatures
    <snip>

The only difference between these two modules is the use of a numeric literal in a pattern match; that is, the troublesome line boils down to:

go 0 a = a

vs.

go _ a = a

Note that GHC gives us several pieces of feedback before talking about the panic. Before the above-quoted error, we get:

Fails.hs:5:14:
    Found hole ‘_’ with type: a2 -> t1 -> t1
    Where: ‘t1’ is a rigid type variable bound by
                the inferred type of go :: (Eq a2, Num a2) => a2 -> t1 -> t1
                at Fails.hs:6:8
           ‘a2’ is a rigid type variable bound by
                the inferred type of go :: (Eq a2, Num a2) => a2 -> t1 -> t1
                at Fails.hs:6:8
    To use the inferred type, enable PartialTypeSignatures
    <snip>

Fails.hs:6:8:
    No instance for (Eq a)
    When checking that ‘go’ has the specified type
      go :: forall t a. a -> t -> t
    Probable cause: the inferred type is ambiguous
    <snip>

Fails.hs:7:7:
    Couldn't match expected type ‘a1’ with actual type ‘t’
      because type variable ‘a1’ would escape its scope
    This (rigid, skolem) type variable is bound by
      the type signature for fails :: a1
      at Fails.hs:3:10
    <snip>

Trac metadata

Trac field	Value
Version	7.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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