Make GHC handle orphan COMPLETE sets of conlikes better

Like orphan rewrite rules, it's possible to define orphan COMPLETE sets. For instance:

module Foo where

{-# COMPLETE False #-}

I suppose that we could define an orphan COMPLETE set as one that lives in a module where none of the conlikes are defined (or should we say "where one or more of the conlikes are not defined"? I'm not sure.)

Like orphan RULES, orphan COMPLETE sets are important to track properly when transitively reading from interface files, as failing to bring an orphan COMPLETE set into scope could affect pattern-match exhaustivness warnings that users see.

After discussing this with rwbarton and mpickering IRC, we decided that one of the two should happen:

1. Treat orphan COMPLETE sets like orphan RULES. That is, mark a module as an orphan if it defines an orphan COMPLETE set, and thread a "COMPLETE pragma visibility" state through various places.
2. Disallow orphan COMPLETE sets entirely.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	mpickering
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

changed milestone to %8.2.1

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Let's do 2 before COMPLETE pragmas are released into the wild, so that nobody starts to depend on orphan COMPLETE pragmas. I assume it is not difficult. Then we could consider whether to do 1 later; but that is certainly a fair amount of work and probably not worthwhile.

Trac metadata

Trac field	Value
Priority	normal → highest

assigned to @trac-rwbarton

I think there is just one place to change in the renamer.

Trac metadata

Trac field	Value
Differential revisions	- → D3243

I agree with ticket:13349#comment:132766. Let's adopt (2) for now.

mentioned in commit fce3d37c

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

added PatternSynonyms label

added Phighest label