Bug in anyRewritableTyVar

The current implementation of TcType.anyRewritableTyVar has one wrong case:

    go_tc ReprEq bvs tc tys = foldr ((&&) . go_arg bvs) False $
                              (tyConRolesRepresentational tc `zip` tys)

Here foldr uses && in the function and a False as the initial value, which causes this case to always return False no matter what the inputs are.

This bug is introduced in [changeset:"3acd6164/ghc" 3acd6164/ghc], which is for fixing #14363 (closed).

The correct implementation (I think) should use || instead of &&:

    go_tc ReprEq bvs tc tys = foldr ((||) . go_arg bvs) False $
                              (tyConRolesRepresentational tc `zip` tys)
-- or just
    go_tc ReprEq bvs tc tys = any (go_arg bvs) (tyConRolesRepresentational tc `zip` tys)

We presume that there might be some program which loops because of this bug.

Patch incoming.

Trac metadata

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