GHCi :print produces variables that cause a panic

Using ghci installed by Stack, I noted some strange behaviour from the :sprint and :print commands:

λ> xs = [1,2,3]
λ> :sprint xs
xs = _
λ> head xs
1
λ> :sprint xs
xs = _
λ> :print xs
xs = (_t1::Num a => [a])
λ> _t1

<interactive>:6:1: error:
    • No instance for (Num a) arising from a use of ‘_t1’
    • In the expression: _t1
      In an equation for ‘it’: it = _t1
λ> :t _t1

<interactive>:1:1: error:
    No instance for (Num a) arising from a use of ‘_t1’
λ> :info _t1
_t1 :: Num a => [a]     -- Defined in ‘interactive:Ghci3’
λ> _t1 :: [Int]

<interactive>:9:1: error:ghc: panic! (the 'impossible' happened)
  (GHC version 8.2.2 for x86_64-unknown-linux):
        No skolem info:
  a_a1rR
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable
        pprPanic, called at compiler/typecheck/TcErrors.hs:2653:5 in ghc:TcErrors

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

I don't really understand the error, or why even the :type command fails, but it appears to be related to the way numeric types are polymorphic until a specific type is forced. Note that the both the :sprint and :print commands don't seem to recognise that we've evaluated any of xs, just printing an underscore no matter what.

The panic doesn't happen if I force a type when defining xs:

λ> xs = [1,2,3] :: [Int]
λ> :sp xs
xs = _
λ> head xs
1
λ> :sp xs
xs = 1 : _
λ> :print xs
xs = 1 : (_t2::[Int])
λ> _t2
[2,3]
λ> :t _t2
_t2 :: [Int]
λ> :in _t2
_t2 :: [Int]    -- Defined in ‘interactive:Ghci6’
λ> _t2 :: [Integer]

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture