Failure to construct superclasses in instance

Consider

{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, ConstraintKinds #-}

module Foo where

class Eq a => C a b

type X a = (Eq a, Show a)

instance X a => C a [b]

HEAD gives

Foo.hs:8:10:
    Could not deduce (Eq a)
      arising from the superclasses of an instance declaration
    from the context: X a
      bound by the instance declaration at Foo.hs:8:10-23
    Possible fix: add (Eq a) to the context of the instance declaration
    In the instance declaration for ‘C a [b]’

This is fall-out from refactoring the silent-superclass story. Patch coming.

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

mentioned in commit 646866ff

Fixed, although the patch probably won't apply very nicely to 7.10.2, because it meddles with TcEvidence which has changed since 7.10.

But this patch effectively reverts one of the changes to TcEvidence so while it won't apply automatically, it won't be hard either. I'm inclined to think it's worth merging. I can help.

Simon

Trac metadata

Trac field	Value
Test case	→ typecheck/should_compile/T10335

added 1 deleted label

closed

changed milestone to %7.10.2

Merged to ghc-7.10.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

mentioned in issue #10491 (closed)

added Pnormal label