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

mentioned in issue #9046 (closed)

mentioned in issue #12449 (closed)

changed milestone to %8.6.1

changed weight to 5

added GHCi Tbug Trac import labels

changed weight to 3

Trac metadata

Trac field	Value
Priority	normal → low

added debugger label

(tested and experienced the same behaviour in both 8.2.2 and 8.4.3)

Trac metadata

Trac field	Value
Version	8.2.2 → 8.4.3

closed

Thank you for the report. Unless I'm mistaken, this is a duplicate of #12449 (closed). Please reopen if you disagree.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedDuplicate

added Plow label

Yes, after MR !5649 (closed) fixed issue #12449 (closed) this issue can no longer be reproduced.