#7525: Compiler panic with ill-typed code using implicit parameters
-------------------------------+--------------------------------------------
Reporter: parcs | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.7 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: Compile-time crash | Blockedby:
Blocking: | Related:
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The following snippet produces a compiler panic with GHC 7.7.20121221:
{{{
{-# LANGUAGE ImplicitParams #-}
hmm :: ()
hmm = let ?a = True in ?a
main = print hmm
}}}
Which results in the error message:
{{{
[1 of 1] Compiling Main ( bug.hs, bug.o )
ghc-stage2: panic! (the 'impossible' happened)
(GHC version 7.7.20121221 for x86_64-unknown-linux):
ASSERT failed!
file compiler/typecheck/TcMType.lhs line 407
Double update of meta tyvar
t_ajp{tv} [tau[0]]
Indirect ghc-prim:GHC.Types.Bool{(w) tc 3c}
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
}}}
Note that the same snippet compiled with GHC 7.6.1 doesn't panic, and
instead gives a type error:
{{{
[1 of 1] Compiling Main ( bug.hs, bug.o )
bug.hs:4:24:
Couldn't match type `Bool' with `()'
When using functional dependencies to combine
?a::Bool,
arising from the implicit-parameter bindings for ?a
at bug.hs:4:7-25
?a::(),
arising from a use of implicit parameter `?a' at bug.hs:4:24-25
In the expression: ?a
In the expression: let ?a = True in ?a
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7525>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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