Panic from combining generalized list comprehensions and record wildcards
Combining record wild cards with generalized list comprehension grouping causes the type-checker to mis-infer the type of the list comprehension, potentially leading to a panic. For example
data Rec = Rec {a :: Int} deriving (Show)
recs :: [[Int]]
recs = [a | Rec {..} <- [Rec 1], then group by a]results in a type error, because GHC thinks recs has type [Int]. On the other hand,
recs :: [[Int]]
recs = [a | Rec {a=a} <- [Rec 1], then group by a]compiles, correctly.
If the type annotation is omitted from the first example, GHC panics with
ghc: panic! (the 'impossible' happened)
(GHC version 6.12.1 for x86_64-unknown-linux):
expectJust rnStmtI've attached a test file that demonstrates the bug.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 6.12.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
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| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |