Panic in rejigConRes

[goldfire]

When I compile the following erroneous code

{-# LANGUAGE DataKinds, TypeFamilies, GADTs #-}

import GHC.TypeLits

data Foo = Bar

data instance Sing (n :: Foo) where
  SBar :: Sing Foo

I get

*** Exception: compiler/typecheck/TcTyClsDecls.lhs:1266:5-62: Irrefutable pattern failed for pattern Data.Maybe.Just subst

I think this is my fault. I will fix.

[goldfire]

I have fixed this, but I am unable to validate, as per ​this email. If someone wants to validate and push, I'd be grateful. Or, suggest a way I can validate. :)

[monoidal]

@goldfire: It passes validate, except for some perf tests which failed previously. Do you want this trace? It is visible during normal build of ghc.

pprTrace "RAE1" (vcat [ppr tmpl_tvs, ppr res_tmpl, ppr dc_tvs, ppr res_ty]) $ 

[goldfire]

Yikes, no! Thanks for catching that -- searching for my initials is one of my internal steps during validation, which I obviously skipped. I just checked, and that should be the only one to remove.

Thanks!

[Krzysztof Gogolewski <krz.gogolewski@…>]

In 0c7d2d7507b9e4dca24c159c0bfcce9de7b233c4/ghc:

Fix Trac #8368.

Two different fixes were necessary here. First, we need to fail eagerly
in kcConDecl, to prevent the return-type error in tcConDecl from firing
twice. (This wasn't caught earlier because of the eager fail in the
datatype kind-checking code -- which isn't used for data instances!)
We also must check again in tcDataFamInstDecl, because it's possible for
a data instance return type to have the right head but the wrong body
(i.e., doesn't conform to the data instance type patterns). This check
is only possible *after* desugaring from HsType to Type, so it can't be
done in tcConRes with the first check.

This is documented in a comment at check_valid_data_con, a local
function within tcDataFamInstDecl.

[Krzysztof Gogolewski <krz.gogolewski@…>]

In e3e28cd65cea30a59e65a824b816f1a81ff46174/testsuite:

New tests for #8368.

[monoidal]

Done!

[goldfire]

Thanks, monoidal, for doing the last leg of the fix.