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Summary

Compiling Example.hs causes ghc to panic.

Example.hs

The provided Example.hs source file contains a full set of notes regarding which changes reveal (or hide) the panic.

• The function step must be exported
  • -- NOTE: no error if step not exported
• Definition of datatype Point must have a context
  • -- NOTE: no error without 'RealFloat a` context; both examples fail
  • Example fails when using either of the GADT or ExistentialQuanitification extensions
    • ExistentialQuantification version is commented out
• step function where error occurs uses Arrow extensions and proc
• The error is depends on how the value is deconstructed
  • -- NOTE: no error without this arrow line; no error if not deconstructed.
    • (Point _) <- ptrPos -< gi
  • -- NOTE: this two-line code does work (in place of (Point _) <- ptrPos -< gi above)
    • pt <- ptrPos -< gi; let (Point _) = pt
• The returned object record must have a Point field
  • -- NOTE: no error without this 'pos' field
  • In the simplified Example.hs provided, this isn't as relevant because the signature of step is Arrow a => a Obj Obj; but, if it were Arrow a => a ObjA ObjB (similar to original version), I recall that if ObjB didn't have an existentially qualified field, the panic didn't occur.

Steps to reproduce

nix-shell -I nixpkgs=https://nixos.org/channels/nixos-unstable/nixexprs.tar.xz -p haskell.compiler.ghc901
ghc Example.hs

Result:

ghc: panic! (the 'impossible' happened)
  (GHC version 9.0.0.20200925:
	GHC.StgToCmm.Env: variable not found
  $dRealFloat_a1tn
  local binds for:
  pos
  Point
  Obj
  $dArrow_s1xr
  ds_s1xs
  ds1_s1xt
  ds2_s1ya
  ds3_s1yb
  ds4_s1yc
  ds6_s1ye
  sat_s1yf
  sat_s1yg
  sat_s1yh
  sat_s1yi
  sat_s1yj
  sat_s1yk
  sat_s1yl
  sat_s1ym
  sat_s1yn
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Outputable.hs:1230:37 in ghc:GHC.Utils.Outputable
        pprPanic, called at compiler/GHC/StgToCmm/Env.hs:152:9 in ghc:GHC.StgToCmm.Env

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

Expected behavior

Compile without error.

Environment

• GHC version used: 8.8.x, 8.10.x, and 9.0.x

Optional:

• Operating System: Ubuntu + Nix (Nixpkgs for GHC)
• System Architecture: x86_64
• Example.hs (duplicate): Example.hs