#3621: "No match in record selector Var.tcTyVarDetails" with incorrect multi-
parameter newtype derivation
-------------------------------------------+--------------------------------
Reporter: pwb | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler | Version: 6.10.4
Severity: minor | Resolution: fixed
Keywords: | Difficulty: Unknown
Testcase: deriving/should_fail/T3621 | Os: Unknown/Multiple
Architecture: Unknown/Multiple |
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Changes (by simonpj):
* testcase: => deriving/should_fail/T3621
* difficulty: => Unknown
* status: new => closed
* resolution: => fixed
Comment:
Actually it shouldn't be rejected. For example
{{{
class C a b
data S = MkS
instance C a S
newtype T = MkT S deriving( C a )
}}}
Here, the derived instance declaration looks like
{{{
instance C a T
}}}
which is fine, although 'a' is not a parameter of T.
However the compiler should not crash. And, happily, here's what the HEAD
says (and hence 6.12):
{{{
T3621.hs:11:21:
Couldn't match expected type `state' against inferred type `s'
`state' is a rigid type variable bound by
the instance declaration at T3621.hs:11:32
`s' is a rigid type variable bound by
the instance declaration at T3621.hs:10:21
When using functional dependencies to combine
MonadState s (State s),
arising from the dependency `m -> s'
in the instance declaration at T3621.hs:8:9
MonadState state (State s),
arising from the instance declaration at T3621.hs:11:21-36
When checking the super-classes of an instance declaration
In the instance declaration for `MonadState state (WrappedState s)'
}}}
Which seems plausible.
I'll add your program to our regression tests though.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3621#comment:1>
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