[TypeApplications] GHC internal error

Toying around, got

GHCi, version 8.1.20160503: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( /tmp/CatFail.hs, interpreted )

/tmp/CatFail.hs:55:26: error:
    • GHC internal error: ‘a’ is not in scope during type checking, but it passed the renamer
      tcl_env of environment: [a1Ej :-> Type variable ‘i’ = i,
                               a1Ek :-> Type variable ‘p’ = p, a1Et :-> Type variable ‘a’ = a,
                               a1Eu :-> Type variable ‘b’ = b]
    • In the first argument of ‘Vacuous’, namely ‘a’
      In the type ‘Vacuous a’
      In the expression: Dict @(Vacuous a)

/tmp/CatFail.hs:66:10: error:
    • Couldn't match type ‘Fun p q’ with ‘Vacuous’
        arising from a use of ‘Main.$dmsrc’
    • In the expression: Main.$dmsrc
      In an equation for ‘src’: src = Main.$dmsrc
      In the instance declaration for ‘Category (Nat p q)’
    • Relevant bindings include
        src :: Nat p q a b -> Dict (Ob (Nat p q) a)
          (bound at /tmp/CatFail.hs:66:10)
Failed, modules loaded: none.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
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Operating system
Architecture

changed weight to 5

added Tbug Trac import TypeApplications labels

Attached file CatFail.hs ($1198).

Attached file HaskellBug.hs ($1199).

Small example with similar behavior

I ran into this too, I'm not sure whether it is the same bug, but I managed to reproduce the same GHC internal error in a very small example. Based on what the two examples have in common, I would say this has something to do with the TypeFamilies switch (without it, ghc works as expected).

CatFail.hs fails with

T12033a.hs:45:31: error:
    Not in scope: type constructor or class ‘Dict’

so perhaps the test isn't right. Anyway, I think #12055 (closed), which does reproduce, is a simpler test case.

On the other hand HaskellBug.hs really is a new and quite different bug. Patch coming for that.

mentioned in commit 15b9bf4b

closed

With the above patch, HaskellBug problem is fixed. I've added it as a regression test.

That leaves #12055 (closed), which we'll handle on that ticket, so I'm closing this one.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed
Test case	- → partial-sigs/should_compile/T12033

added Pnormal label