Constraint solver panic due to derived type function equality
{-# LANGUAGE MultiParamTypeClasses, TypeFamilies #-}
{-# OPTIONS_GHC -ddump-tc-trace #-}
type family F (x :: *) :: *
class (y ~ F x) => C x y
z = () :: C x y => ()
causes a panic while tracing the typechecker:
solveNestedImplications starting {
original inerts = Equalities:
Type-function equalities: [D] _ :: F x_aeH ~ y_aeI (CFunEqCan)
Dictionaries: [W] $dC_aeK :: C x_aeH y_aeI (CDictCan)
Irreds:
Insolubles = {}
Solved dicts 0
Solved funeqs 0
thinner_inerts = Equalities:ghc-stage2: panic! (the 'impossible' happened)
(GHC version 7.7.20130812 for i386-unknown-linux):
No match in record selector ctev_evar
If -ddump-tc-trace
is not used, the correct constraint solving error is generated:
Could not deduce (C x0 y0)
arising from the ambiguity check for an expression type signature
from the context (C x y)
bound by an expression type signature: C x y => ()
It seems that prepareInertsForImplications
in TcSMonad
assumes that all the
inert_funeqs
are givens or wanteds; if there are some deriveds, then the above panic results.
Trac metadata
Trac field | Value |
---|---|
Version | 7.7 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |