Panic; no skolem info (partial type signatures)

This rather minimal program

f :: _ => _f
f = const

results in this:

GHCi, version 7.10.1: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( Panic.hs, interpreted )

Panic.hs:1:6:
    Found hole ‘_’ with inferred constraints: ()
    To use the inferred type, enable PartialTypeSignatures
    In the type signature for ‘f’: _ => _f

Panic.hs:2:5:
    Couldn't match expected type ‘_f’ with actual type ‘a0 -> b0 -> a0’
      ‘_f’ is untouchable
        inside the constraints ()
        bound by the inferred type of f :: _f
        at Panic.hs:2:1-9ghc: panic! (the 'impossible' happened)
  (GHC version 7.10.1 for x86_64-apple-darwin):
	No skolem info: _f_alF[sk]

The newest open bugs (that reporter could find at the time of reporting) with "no skolem info" in their description are #10404 (closed) and #10432 (closed). The newest bug (which is closed) is #10503 (closed). It seems likely, though, that the above comes from the PartialTypeSignatures implementation (considering the simplicity of the repro).

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

BTW, actually turning on NamedWildCards gives the expected output:

GHCi, version 7.10.1: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Test             ( Test.hs, interpreted )

Test.hs:47:6: Warning:
    Found hole ‘_’ with inferred constraints: ()
    In the type signature for ‘f’: _ => _f

Test.hs:47:11: Warning:
    Found hole ‘_f’ with type: a -> b -> a
    Where: ‘a’ is a rigid type variable bound by
               the inferred type of f :: a -> b -> a at Test.hs:48:1
           ‘b’ is a rigid type variable bound by
               the inferred type of f :: a -> b -> a at Test.hs:48:1
    In the type signature for ‘f’: _ => _f
Ok, modules loaded: Test.

mentioned in commit 95364812

mentioned in commit 28096b27

closed

Thanks for the great bug report.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed
Test case	- → partial-sigs/should_fail/T10615

mentioned in issue #11122 (closed)

mentioned in issue #11171 (closed)

changed milestone to %8.0.1

added Pnormal label