UniqSet definition seems shady

Currently,

type UniqSet a = UniqFM a

The key invariant of UniqSet is expressed in the somewhat-poorly-named Note [Unsound mapUniqSet], and not enforced by the types. It seems likely that the clean thing is

newtype UniqSet a = US (UniqFM a)

Unfortunately there's an awful lot of code using UniqSet and assuming it's the same as UniqFM. To make this work, we'd need to expand the UniqSet API somewhat and figure out what to do at use sites using it interchangeably with UniqFM.

Trac metadata

Trac field	Value
Version	8.1
Type	Task
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Operating system
Architecture

changed milestone to %8.4.1

changed weight to 3

added Ttask Trac import labels

Trac metadata

Trac field	Value
Differential revisions	- → D3146

closed

changed milestone to %8.2.1

Fixed in cbe569a5

Upgrade UniqSet to a newtype

The fundamental problem with type UniqSet = UniqFM is that UniqSet
has a key invariant UniqFM does not. For example, fmap over
UniqSet will generally produce nonsense.

Upgrade UniqSet from a type synonym to a newtype.
Remove unused and shady extendVarSet_C and addOneToUniqSet_C.
Use cached unique in tyConsOfType by replacing unitNameEnv (tyConName tc) tc with unitUniqSet tc.
Reviewers: austin, hvr, goldfire, simonmar, niteria, bgamari

Reviewed By: niteria

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3146

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

mentioned in issue #15879 (closed)

mentioned in commit 5b98a38a

added Plow label