#2846: Impredicativity bug: GHC crash by type signature
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Reporter: mm_freak | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.12 branch
Component: Compiler | Version: 6.10.1
Severity: major | Resolution:
Keywords: crash, type | Difficulty: Unknown
Testcase: | Os: Unknown/Multiple
Architecture: Unknown/Multiple |
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Changes (by simonpj):
* summary: GHC crash by type signature => Impredicativity bug: GHC crash
by type signature
* milestone: 6.10.2 => 6.12 branch
Comment:
What's happening here is that `-fglasgow-exts` implies
`-XImpredicativeTypes` and `-XFlexibleContexts`. Then we get a constraint
`(Show [Num a => a])`, which should never happen. The impredicative
machinery should not allow polymorphism in class constraints, but that
check isn't implemented.
The impredicative stuff needs a proper overhaul (Dimitrios is working on
it) so I do not propose to tweak it now. But this should be fixed in due
course.
Meanwhile, I propose to '''stop''' `-fglasgow-exts` from implying
`-XImpredicativeTypes`. I don't want to encourage use of impredicativity
until it's working properly.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2846#comment:2>
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