Breaking the staging restriction in type splices causes GHC panic

[reinerp]

Test case:

{-# LANGUAGE TemplateHaskell #-}
module B where

import Language.Haskell.TH

ty :: Q Type
ty = [t| Int |]

f :: $ty
f = undefined

The error message is:

B.hs:9:6:
    GHC internal error: `ty' is not in scope during type checking, but it passed the renamer
    tcl_env of environment: []
    In the expression: ty
    In the type signature for `f': f :: $ty

This is the same error as in #5792.

[igloo]

Thanks for the report.

[simonpj@…]

commit 6d46439c1c0e3765827dde26077ff45f85e210bd

Author: Simon Peyton Jones <simonpj@microsoft.com>
Date:   Thu Apr 5 11:37:02 2012 +0100

    Improve error reporting for out-of-scope variables
    
    Sometimes when a variable is out of scope in the type-checker,
    it's not a GHC bug, but rather a TH staging error.  See
    Note [Out of scope might be a staging error] in TcEnv.
    
    This showed up as Trac #5795.

 compiler/rename/RnEnv.lhs    |    4 +++-
 compiler/typecheck/TcEnv.lhs |   36 ++++++++++++++++++++++++++++--------
 2 files changed, 31 insertions(+), 9 deletions(-)

[simonpj]

Thanks. I've fixed this. Not perfectly but better

T5795.hs:9:6:
    GHC stage restriction:
      `ty' is used in a top-level splice or annotation,
      and must be imported, not defined locally
    In the expression: ty
    In the type signature for `f': f :: $ty